D2300R11
std::execution

Draft Proposal,

Authors:
Source:
GitHub
Issue Tracking:
GitHub
Project:
ISO/IEC 14882 Programming Languages — C++, ISO/IEC JTC1/SC22/WG21
Audience:
SG1, LEWG

1. Introduction

This paper proposes a self-contained design for a Standard C++ framework for managing asynchronous execution on generic execution resources. It is based on the ideas in A Unified Executors Proposal for C++ and its companion papers.

1.1. Motivation

Today, C++ software is increasingly asynchronous and parallel, a trend that is likely to only continue going forward. Asynchrony and parallelism appears everywhere, from processor hardware interfaces, to networking, to file I/O, to GUIs, to accelerators. Every C++ domain and every platform needs to deal with asynchrony and parallelism, from scientific computing to video games to financial services, from the smallest mobile devices to your laptop to GPUs in the world’s fastest supercomputer.

While the C++ Standard Library has a rich set of concurrency primitives (std::atomic, std::mutex, std::counting_semaphore, etc) and lower level building blocks (std::thread, etc), we lack a Standard vocabulary and framework for asynchrony and parallelism that C++ programmers desperately need. std::async/std::future/std::promise, C++11’s intended exposure for asynchrony, is inefficient, hard to use correctly, and severely lacking in genericity, making it unusable in many contexts. We introduced parallel algorithms to the C++ Standard Library in C++17, and while they are an excellent start, they are all inherently synchronous and not composable.

This paper proposes a Standard C++ model for asynchrony based around three key abstractions: schedulers, senders, and receivers, and a set of customizable asynchronous algorithms.

1.2. Priorities

1.3. Examples: End User

In this section we demonstrate the end-user experience of asynchronous programming directly with the sender algorithms presented in this paper. See § 4.19 User-facing sender factories, § 4.20 User-facing sender adaptors, and § 4.21 User-facing sender consumers for short explanations of the algorithms used in these code examples.

1.3.1. Hello world

using namespace std::execution;

scheduler auto sch = thread_pool.scheduler();                                 // 1

sender auto begin = schedule(sch);                                            // 2
sender auto hi = then(begin, []{                                              // 3
    std::cout << "Hello world! Have an int.";                                 // 3
    return 13;                                                                // 3
});                                                                           // 3
sender auto add_42 = then(hi, [](int arg) { return arg + 42; });              // 4

auto [i] = this_thread::sync_wait(add_42).value();                            // 5

This example demonstrates the basics of schedulers, senders, and receivers:

  1. First we need to get a scheduler from somewhere, such as a thread pool. A scheduler is a lightweight handle to an execution resource.

  2. To start a chain of work on a scheduler, we call § 4.19.1 execution::schedule, which returns a sender that completes on the scheduler. A sender describes asynchronous work and sends a signal (value, error, or stopped) to some recipient(s) when that work completes.

  3. We use sender algorithms to produce senders and compose asynchronous work. § 4.20.2 execution::then is a sender adaptor that takes an input sender and a std::invocable, and calls the std::invocable on the signal sent by the input sender. The sender returned by then sends the result of that invocation. In this case, the input sender came from schedule, so its void, meaning it won’t send us a value, so our std::invocable takes no parameters. But we return an int, which will be sent to the next recipient.

  4. Now, we add another operation to the chain, again using § 4.20.2 execution::then. This time, we get sent a value - the int from the previous step. We add 42 to it, and then return the result.

  5. Finally, we’re ready to submit the entire asynchronous pipeline and wait for its completion. Everything up until this point has been completely asynchronous; the work may not have even started yet. To ensure the work has started and then block pending its completion, we use § 4.21.1 this_thread::sync_wait, which will either return a std::optional<std::tuple<...>> with the value sent by the last sender, or an empty std::optional if the last sender sent a stopped signal, or it throws an exception if the last sender sent an error.

1.3.2. Asynchronous inclusive scan

using namespace std::execution;

sender auto async_inclusive_scan(scheduler auto sch,                          // 2
                                 std::span<const double> input,               // 1
                                 std::span<double> output,                    // 1
                                 double init,                                 // 1
                                 std::size_t tile_count)                      // 3
{
  std::size_t const tile_size = (input.size() + tile_count - 1) / tile_count;

  std::vector<double> partials(tile_count + 1);                               // 4
  partials[0] = init;                                                         // 4

  return just(std::move(partials))                                            // 5
       | continues_on(sch)
       | bulk(tile_count,                                                     // 6
           [ = ](std::size_t i, std::vector<double>& partials) {              // 7
             auto start = i * tile_size;                                      // 8
             auto end   = std::min(input.size(), (i + 1) * tile_size);        // 8
             partials[i + 1] = *--std::inclusive_scan(begin(input) + start,   // 9
                                                      begin(input) + end,     // 9
                                                      begin(output) + start); // 9
           })                                                                 // 10
       | then(                                                                // 11
           [](std::vector<double>&& partials) {
             std::inclusive_scan(begin(partials), end(partials),              // 12
                                 begin(partials));                            // 12
             return std::move(partials);                                      // 13
           })
       | bulk(tile_count,                                                     // 14
           [ = ](std::size_t i, std::vector<double>& partials) {              // 14
             auto start = i * tile_size;                                      // 14
             auto end   = std::min(input.size(), (i + 1) * tile_size);        // 14
             std::for_each(begin(output) + start, begin(output) + end,        // 14
               [&] (double& e) { e = partials[i] + e; }                       // 14
             );
           })
       | then(                                                                // 15
           [ = ](std::vector<double>&& partials) {                            // 15
             return output;                                                   // 15
           });                                                                // 15
}

This example builds an asynchronous computation of an inclusive scan:

  1. It scans a sequence of doubles (represented as the std::span<const double> input) and stores the result in another sequence of doubles (represented as std::span<double> output).

  2. It takes a scheduler, which specifies what execution resource the scan should be launched on.

  3. It also takes a tile_count parameter that controls the number of execution agents that will be spawned.

  4. First we need to allocate temporary storage needed for the algorithm, which we’ll do with a std::vector, partials. We need one double of temporary storage for each execution agent we create.

  5. Next we’ll create our initial sender with § 4.19.2 execution::just and § 4.20.1 execution::continues_on. These senders will send the temporary storage, which we’ve moved into the sender. The sender has a completion scheduler of sch, which means the next item in the chain will use sch.

  6. Senders and sender adaptors support composition via operator|, similar to C++ ranges. We’ll use operator| to attach the next piece of work, which will spawn tile_count execution agents using § 4.20.9 execution::bulk (see § 4.12 Most sender adaptors are pipeable for details).

  7. Each agent will call a std::invocable, passing it two arguments. The first is the agent’s index (i) in the § 4.20.9 execution::bulk operation, in this case a unique integer in [0, tile_count). The second argument is what the input sender sent - the temporary storage.

  8. We start by computing the start and end of the range of input and output elements that this agent is responsible for, based on our agent index.

  9. Then we do a sequential std::inclusive_scan over our elements. We store the scan result for our last element, which is the sum of all of our elements, in our temporary storage partials.

  10. After all computation in that initial § 4.20.9 execution::bulk pass has completed, every one of the spawned execution agents will have written the sum of its elements into its slot in partials.

  11. Now we need to scan all of the values in partials. We’ll do that with a single execution agent which will execute after the § 4.20.9 execution::bulk completes. We create that execution agent with § 4.20.2 execution::then.

  12. § 4.20.2 execution::then takes an input sender and an std::invocable and calls the std::invocable with the value sent by the input sender. Inside our std::invocable, we call std::inclusive_scan on partials, which the input senders will send to us.

  13. Then we return partials, which the next phase will need.

  14. Finally we do another § 4.20.9 execution::bulk of the same shape as before. In this § 4.20.9 execution::bulk, we will use the scanned values in partials to integrate the sums from other tiles into our elements, completing the inclusive scan.

  15. async_inclusive_scan returns a sender that sends the output std::span<double>. A consumer of the algorithm can chain additional work that uses the scan result. At the point at which async_inclusive_scan returns, the computation may not have completed. In fact, it may not have even started.

1.3.3. Asynchronous dynamically-sized read

using namespace std::execution;

sender_of<std::size_t> auto async_read(                                       // 1
    sender_of<std::span<std::byte>> auto buffer,                              // 1
    auto handle);                                                             // 1

struct dynamic_buffer {                                                       // 3
  std::unique_ptr<std::byte[]> data;                                          // 3
  std::size_t size;                                                           // 3
};                                                                            // 3

sender_of<dynamic_buffer> auto async_read_array(auto handle) {                // 2
  return just(dynamic_buffer{})                                               // 4
       | let_value([handle] (dynamic_buffer& buf) {                           // 5
           return just(std::as_writable_bytes(std::span(&buf.size, 1)))       // 6
                | async_read(handle)                                          // 7
                | then(                                                       // 8
                    [&buf] (std::size_t bytes_read) {                         // 9
                      assert(bytes_read == sizeof(buf.size));                 // 10
                      buf.data = std::make_unique<std::byte[]>(buf.size);     // 11
                      return std::span(buf.data.get(), buf.size);             // 12
                    })
                | async_read(handle)                                          // 13
                | then(
                    [&buf] (std::size_t bytes_read) {
                      assert(bytes_read == buf.size);                         // 14
                      return std::move(buf);                                  // 15
                    });
       });
}

This example demonstrates a common asynchronous I/O pattern - reading a payload of a dynamic size by first reading the size, then reading the number of bytes specified by the size:

  1. async_read is a pipeable sender adaptor. It’s a customization point object, but this is what it’s call signature looks like. It takes a sender parameter which must send an input buffer in the form of a std::span<std::byte>, and a handle to an I/O context. It will asynchronously read into the input buffer, up to the size of the std::span. It returns a sender which will send the number of bytes read once the read completes.

  2. async_read_array takes an I/O handle and reads a size from it, and then a buffer of that many bytes. It returns a sender that sends a dynamic_buffer object that owns the data that was sent.

  3. dynamic_buffer is an aggregate struct that contains a std::unique_ptr<std::byte[]> and a size.

  4. The first thing we do inside of async_read_array is create a sender that will send a new, empty dynamic_array object using § 4.19.2 execution::just. We can attach more work to the pipeline using operator| composition (see § 4.12 Most sender adaptors are pipeable for details).

  5. We need the lifetime of this dynamic_array object to last for the entire pipeline. So, we use let_value, which takes an input sender and a std::invocable that must return a sender itself (see § 4.20.4 execution::let_* for details). let_value sends the value from the input sender to the std::invocable. Critically, the lifetime of the sent object will last until the sender returned by the std::invocable completes.

  6. Inside of the let_value std::invocable, we have the rest of our logic. First, we want to initiate an async_read of the buffer size. To do that, we need to send a std::span pointing to buf.size. We can do that with § 4.19.2 execution::just.

  7. We chain the async_read onto the § 4.19.2 execution::just sender with operator|.

  8. Next, we pipe a std::invocable that will be invoked after the async_read completes using § 4.20.2 execution::then.

  9. That std::invocable gets sent the number of bytes read.

  10. We need to check that the number of bytes read is what we expected.

  11. Now that we have read the size of the data, we can allocate storage for it.

  12. We return a std::span<std::byte> to the storage for the data from the std::invocable. This will be sent to the next recipient in the pipeline.

  13. And that recipient will be another async_read, which will read the data.

  14. Once the data has been read, in another § 4.20.2 execution::then, we confirm that we read the right number of bytes.

  15. Finally, we move out of and return our dynamic_buffer object. It will get sent by the sender returned by async_read_array. We can attach more things to that sender to use the data in the buffer.

1.4. Asynchronous Windows socket recv

To get a better feel for how this interface might be used by low-level operations see this example implementation of a cancellable async_recv() operation for a Windows Socket.

struct operation_base : WSAOVERALAPPED {
    using completion_fn = void(operation_base* op, DWORD bytesTransferred, int errorCode) noexcept;

    // Assume IOCP event loop will call this when this OVERLAPPED structure is dequeued.
    completion_fn* completed;
};

template<class Receiver>
struct recv_op : operation_base {
    using operation_state_concept = std::execution::operation_state_t;

    recv_op(SOCKET s, void* data, size_t len, Receiver r)
    : receiver(std::move(r))
    , sock(s) {
        this->Internal = 0;
        this->InternalHigh = 0;
        this->Offset = 0;
        this->OffsetHigh = 0;
        this->hEvent = NULL;
        this->completed = &recv_op::on_complete;
        buffer.len = len;
        buffer.buf = static_cast<CHAR*>(data);
    }

    void start() & noexcept {
        // Avoid even calling WSARecv() if operation already cancelled
        auto st = std::execution::get_stop_token(
          std::execution::get_env(receiver));
        if (st.stop_requested()) {
            std::execution::set_stopped(std::move(receiver));
            return;
        }

        // Store and cache result here in case it changes during execution
        const bool stopPossible = st.stop_possible();
        if (!stopPossible) {
            ready.store(true, std::memory_order_relaxed);
        }

        // Launch the operation
        DWORD bytesTransferred = 0;
        DWORD flags = 0;
        int result = WSARecv(sock, &buffer, 1, &bytesTransferred, &flags,
                             static_cast<WSAOVERLAPPED*>(this), NULL);
        if (result == SOCKET_ERROR) {
            int errorCode = WSAGetLastError();
            if (errorCode != WSA_IO_PENDING) {
                if (errorCode == WSA_OPERATION_ABORTED) {
                    std::execution::set_stopped(std::move(receiver));
                } else {
                    std::execution::set_error(std::move(receiver),
                                              std::error_code(errorCode, std::system_category()));
                }
                return;
            }
        } else {
            // Completed synchronously (assuming FILE_SKIP_COMPLETION_PORT_ON_SUCCESS has been set)
            execution::set_value(std::move(receiver), bytesTransferred);
            return;
        }

        // If we get here then operation has launched successfully and will complete asynchronously.
        // May be completing concurrently on another thread already.
        if (stopPossible) {
            // Register the stop callback
            stopCallback.emplace(std::move(st), cancel_cb{*this});

            // Mark as 'completed'
            if (ready.load(std::memory_order_acquire) ||
                ready.exchange(true, std::memory_order_acq_rel)) {
                // Already completed on another thread
                stopCallback.reset();

                BOOL ok = WSAGetOverlappedResult(sock, (WSAOVERLAPPED*)this, &bytesTransferred, FALSE, &flags);
                if (ok) {
                    std::execution::set_value(std::move(receiver), bytesTransferred);
                } else {
                    int errorCode = WSAGetLastError();
                    std::execution::set_error(std::move(receiver),
                                              std::error_code(errorCode, std::system_category()));
                }
            }
        }
    }

    struct cancel_cb {
        recv_op& op;

        void operator()() noexcept {
            CancelIoEx((HANDLE)op.sock, (OVERLAPPED*)(WSAOVERLAPPED*)&op);
        }
    };

    static void on_complete(operation_base* op, DWORD bytesTransferred, int errorCode) noexcept {
        recv_op& self = *static_cast<recv_op*>(op);

        if (self.ready.load(std::memory_order_acquire) ||
            self.ready.exchange(true, std::memory_order_acq_rel)) {
            // Unsubscribe any stop callback so we know that CancelIoEx() is not accessing 'op'
            // any more
            self.stopCallback.reset();

            if (errorCode == 0) {
                std::execution::set_value(std::move(self.receiver), bytesTransferred);
            } else {
                std::execution::set_error(std::move(self.receiver),
                                          std::error_code(errorCode, std::system_category()));
            }
        }
    }

    using stop_callback_t = stop_callback_of_t<stop_token_of_t<env_of_t<Receiver>>, cancel_cb>;

    Receiver receiver;
    SOCKET sock;
    WSABUF buffer;
    std::optional<stop_callback_t> stopCallback;
    std::atomic<bool> ready{false};
};

struct recv_sender {
    using sender_concept = std::execution::sender_t;
    SOCKET sock;
    void* data;
    size_t len;

    template<class Receiver>
    recv_op<Receiver> connect(Receiver r) const {
        return recv_op<Receiver>{sock, data, len, std::move(r)};
    }
};

recv_sender async_recv(SOCKET s, void* data, size_t len) {
    return recv_sender{s, data, len};
}

1.4.1. More end-user examples

1.4.1.1. Sudoku solver

This example comes from Kirk Shoop, who ported an example from TBB’s documentation to sender/receiver in his fork of the libunifex repo. It is a Sudoku solver that uses a configurable number of threads to explore the search space for solutions.

The sender/receiver-based Sudoku solver can be found here. Some things that are worth noting about Kirk’s solution:

  1. Although it schedules asynchronous work onto a thread pool, and each unit of work will schedule more work, its use of structured concurrency patterns make reference counting unnecessary. The solution does not make use of shared_ptr.

  2. In addition to eliminating the need for reference counting, the use of structured concurrency makes it easy to ensure that resources are cleaned up on all code paths. In contrast, the TBB example that inspired this one leaks memory.

For comparison, the TBB-based Sudoku solver can be found here.

1.4.1.2. File copy

This example also comes from Kirk Shoop which uses sender/receiver to recursively copy the files a directory tree. It demonstrates how sender/receiver can be used to do IO, using a scheduler that schedules work on Linux’s io_uring.

As with the Sudoku example, this example obviates the need for reference counting by employing structured concurrency. It uses iteration with an upper limit to avoid having too many open file handles.

You can find the example here.

1.4.1.3. Echo server

Dietmar Kuehl has proposed networking APIs that use the sender/receiver abstraction (see P2762). He has implemented an echo server as a demo. His echo server code can be found here.

Below, I show the part of the echo server code. This code is executed for each client that connects to the echo server. In a loop, it reads input from a socket and echos the input back to the same socket. All of this, including the loop, is implemented with generic async algorithms.

outstanding.start(
    EX::repeat_effect_until(
          EX::let_value(
              NN::async_read_some(ptr->d_socket,
                                  context.scheduler(),
                                  NN::buffer(ptr->d_buffer))
        | EX::then([ptr](::std::size_t n){
            ::std::cout << "read='" << ::std::string_view(ptr->d_buffer, n) << "'\n";
            ptr->d_done = n == 0;
            return n;
        }),
          [&context, ptr](::std::size_t n){
            return NN::async_write_some(ptr->d_socket,
                                        context.scheduler(),
                                        NN::buffer(ptr->d_buffer, n));
          })
        | EX::then([](auto&&...){})
        , [owner = ::std::move(owner)]{ return owner->d_done; }
    )
);

In this code, NN::async_read_some and NN::async_write_some are asynchronous socket-based networking APIs that return senders. EX::repeat_effect_until, EX::let_value, and EX::then are fully generic sender adaptor algorithms that accept and return senders.

This is a good example of seamless composition of async IO functions with non-IO operations. And by composing the senders in this structured way, all the state for the composite operation -- the repeat_effect_until expression and all its child operations -- is stored altogether in a single object.

1.5. Examples: Algorithms

In this section we show a few simple sender/receiver-based algorithm implementations.

1.5.1. then

namespace stdexec = std::execution;

template <class R, class F>
class _then_receiver : public R {
  F f_;

 public:
  _then_receiver(R r, F f) : R(std::move(r)), f_(std::move(f)) {}

  // Customize set_value by invoking the callable and passing the result to
  // the inner receiver
  template <class... As>
    requires std::invocable<F, As...>
  void set_value(As&&... as) && noexcept {
    try {
      stdexec::set_value(std::move(*this).base(), std::invoke((F&&) f_, (As&&) as...));
    } catch(...) {
      stdexec::set_error(std::move(*this).base(), std::current_exception());
    }
  }
};

template <stdexec::sender S, class F>
struct _then_sender {
  using sender_concept = stdexec::sender_t;
  S s_;
  F f_;

  template <class... Args>
    using _set_value_t = stdexec::completion_signatures<
      stdexec::set_value_t(std::invoke_result_t<F, Args...>)>;

  using _except_ptr_sig =
    stdexec::completion_signatures<stdexec::set_error_t(std::exception_ptr)>;

  // Compute the completion signatures
  template <class Env>
  auto get_completion_signatures(Env&& env) && noexcept
    -> stdexec::transform_completion_signatures_of<
        S, Env, _except_ptr_sig, _set_value_t> {
    return {};
  }

  // Connect:
  template <stdexec::receiver R>
  auto connect(R r) && -> stdexec::connect_result_t<S, _then_receiver<R, F>> {
    return stdexec::connect(
      (S&&) s_, _then_receiver{(R&&) r, (F&&) f_});
  }

  decltype(auto) get_env() const noexcept {
    return get_env(s_);
  }
};

template <stdexec::sender S, class F>
stdexec::sender auto then(S s, F f) {
  return _then_sender<S, F>{(S&&) s, (F&&) f};
}

This code builds a then algorithm that transforms the value(s) from the input sender with a transformation function. The result of the transformation becomes the new value. The other receiver functions (set_error and set_stopped), as well as all receiver queries, are passed through unchanged.

In detail, it does the following:

  1. Defines a receiver in terms of receiver and an invocable that:

    • Defines a constrained set_value member function for transforming the value channel.

    • Delegates set_error and set_stopped to the inner receiver.

  2. Defines a sender that aggregates another sender and the invocable, which defines a connect member function that wraps the incoming receiver in the receiver from (1) and passes it and the incoming sender to std::execution::connect, returning the result. It also defines a get_completion_signatures member function that declares the sender’s completion signatures when executed within a particular environment.

1.5.2. retry

using namespace std;
namespace stdexec = execution;

template<class From, class To>
concept _decays_to = same_as<decay_t<From>, To>;

// _conv needed so we can emplace construct non-movable types into
// a std::optional.
template<invocable F>
struct _conv {
  F f_;

  static_assert(is_nothrow_move_constructible_v<F>);
  explicit _conv(F f) noexcept : f_((F&&) f) {}

  operator invoke_result_t<F>() && {
    return ((F&&) f_)();
  }
};

template<class S, class R>
struct _retry_op;

// pass through all customizations except set_error, which retries
// the operation.
template<class S, class R>
struct _retry_receiver {
  _retry_op<S, R>* o_;

  void set_value(auto&&... as) && noexcept {
    stdexec::set_value(std::move(o_->r_), (decltype(as)&&) as...);
  }

  void set_error(auto&&) && noexcept {
    o_->_retry(); // This causes the op to be retried
  }

  void set_stopped() && noexcept {
    stdexec::set_stopped(std::move(o_->r_));
  }

  decltype(auto) get_env() const noexcept {
    return get_env(o_->r_);
  }
};

// Hold the nested operation state in an optional so we can
// re-construct and re-start it if the operation fails.
template<class S, class R>
struct _retry_op {
  using operation_state_concept = stdexec::operation_state_t;
  using _child_op_t =
    stdexec::connect_result_t<S&, _retry_receiver<S, R>>;

  S s_;
  R r_;
  optional<_child_op_t> o_;

  _op(_op&&) = delete;
  _op(S s, R r)
    : s_(std::move(s)), r_(std::move(r)), o_{_connect()} {}

  auto _connect() noexcept {
    return _conv{[this] {
      return stdexec::connect(s_, _retry_receiver<S, R>{this});
    }};
  }

  void _retry() noexcept {
    try {
      o_.emplace(_connect()); // potentially-throwing
      stdexec::start(*o_);
    } catch(...) {
      stdexec::set_error(std::move(r_), std::current_exception());
    }
  }

  void start() & noexcept {
    stdexec::start(*o_);
  }
};

// Helpers for computing the `then` sender's completion signatures:
template <class... Ts>
  using _value_t =
    stdexec::completion_signatures<stdexec::set_value_t(Ts...)>;

template <class>
  using _error_t = stdexec::completion_signatures<>;

using _except_sig =
  stdexec::completion_signatures<stdexec::set_error_t(std::exception_ptr)>;

template<class S>
struct _retry_sender {
  using sender_concept = stdexec::sender_t;
  S s_;
  explicit _retry_sender(S s) : s_(std::move(s)) {}

  // Declare the signatures with which this sender can complete
  template <class Env>
    using _compl_sigs =
      stdexec::transform_completion_signatures_of<
        S&, Env, _except_sig, _value_t, _error_t>;

  template <class Env>
  auto get_completion_signatures(Env&&) const noexcept -> _compl_sigs<Env> {
    return {};
  }

  template <stdexec::receiver R>
    requires stdexec::sender_to<S&, _retry_receiver<S, R>>
  _retry_op<S, R> connect(R r) && {
    return {std::move(s_), std::move(r)};
  }

  decltype(auto) get_env() const noexcept {
    return get_env(s_);
  }
};

template <stdexec::sender S>
stdexec::sender auto retry(S s) {
  return _retry_sender{std::move(s)};
}

The retry algorithm takes a multi-shot sender and causes it to repeat on error, passing through values and stopped signals. Each time the input sender is restarted, a new receiver is connected and the resulting operation state is stored in an optional, which allows us to reinitialize it multiple times.

This example does the following:

  1. Defines a _conv utility that takes advantage of C++17’s guaranteed copy elision to emplace a non-movable type in a std::optional.

  2. Defines a _retry_receiver that holds a pointer back to the operation state. It passes all customizations through unmodified to the inner receiver owned by the operation state except for set_error, which causes a _retry() function to be called instead.

  3. Defines an operation state that aggregates the input sender and receiver, and declares storage for the nested operation state in an optional. Constructing the operation state constructs a _retry_receiver with a pointer to the (under construction) operation state and uses it to connect to the input sender.

  4. Starting the operation state dispatches to start on the inner operation state.

  5. The _retry() function reinitializes the inner operation state by connecting the sender to a new receiver, holding a pointer back to the outer operation state as before.

  6. After reinitializing the inner operation state, _retry() calls start on it, causing the failed operation to be rescheduled.

  7. Defines a _retry_sender that implements a connect member function to return an operation state constructed from the passed-in sender and receiver.

  8. _retry_sender also implements a get_completion_signatures member function to describe the ways this sender may complete when executed in a particular execution resource.

1.6. Examples: Schedulers

In this section we look at some schedulers of varying complexity.

1.6.1. Inline scheduler

namespace stdexec = std::execution;

class inline_scheduler {
  template <class R>
  struct _op {
    using operation_state_concept = operation_state_t;
    R rec_;

    void start() & noexcept {
      stdexec::set_value(std::move(rec_));
    }
  };

  struct _env {
    template <class Tag>
    inline_scheduler query(stdexec::get_completion_scheduler_t<Tag>) const noexcept {
      return {};
    }
  };

  struct _sender {
    using sender_concept = stdexec::sender_t;
    using _compl_sigs = stdexec::completion_signatures<stdexec::set_value_t()>;
    using completion_signatures = _compl_sigs;

    template <stdexec::receiver_of<_compl_sigs> R>
    _op<R> connect(R rec) noexcept(std::is_nothrow_move_constructible_v<R>) {
      return {std::move(rec)};
    }

    _env get_env() const noexcept {
      return {};
    }
  };

 public:
  inline_scheduler() = default;

  _sender schedule() const noexcept {
    return {};
  }

  bool operator==(const inline_scheduler&) const noexcept = default;
};

The inline scheduler is a trivial scheduler that completes immediately and synchronously on the thread that calls std::execution::start on the operation state produced by its sender. In other words, start(connect(schedule(inline_scheduler()), receiver)) is just a fancy way of saying set_value(receiver), with the exception of the fact that start wants to be passed an lvalue.

Although not a particularly useful scheduler, it serves to illustrate the basics of implementing one. The inline_scheduler:

  1. Customizes execution::schedule to return an instance of the sender type _sender.

  2. The _sender type models the sender concept and provides the metadata needed to describe it as a sender of no values and that never calls set_error or set_stopped. This metadata is provided with the help of the execution::completion_signatures utility.

  3. The _sender type customizes execution::connect to accept a receiver of no values. It returns an instance of type _op that holds the receiver by value.

  4. The operation state customizes std::execution::start to call std::execution::set_value on the receiver.

1.6.2. Single thread scheduler

This example shows how to create a scheduler for an execution resource that consists of a single thread. It is implemented in terms of a lower-level execution resource called std::execution::run_loop.

class single_thread_context {
  std::execution::run_loop loop_;
  std::thread thread_;

public:
  single_thread_context()
    : loop_()
    , thread_([this] { loop_.run(); })
  {}
  single_thread_context(single_thread_context&&) = delete;

  ~single_thread_context() {
    loop_.finish();
    thread_.join();
  }

  auto get_scheduler() noexcept {
    return loop_.get_scheduler();
  }

  std::thread::id get_thread_id() const noexcept {
    return thread_.get_id();
  }
};

The single_thread_context owns an event loop and a thread to drive it. In the destructor, it tells the event loop to finish up what it’s doing and then joins the thread, blocking for the event loop to drain.

The interesting bits are in the execution::run_loop context implementation. It is slightly too long to include here, so we only provide a reference to it, but there is one noteworthy detail about its implementation: It uses space in its operation states to build an intrusive linked list of work items. In structured concurrency patterns, the operation states of nested operations compose statically, and in an algorithm like this_thread::sync_wait, the composite operation state lives on the stack for the duration of the operation. The end result is that work can be scheduled onto this thread with zero allocations.

1.7. Examples: Server theme

In this section we look at some examples of how one would use senders to implement an HTTP server. The examples ignore the low-level details of the HTTP server and looks at how senders can be combined to achieve the goals of the project.

General application context:

1.7.1. Composability with execution::let_*

Example context:

Goals:

namespace stdexec = std::execution;

// Returns a sender that yields an http_request object for an incoming request
stdexec::sender auto schedule_request_start(read_requests_ctx ctx) {...}

// Sends a response back to the client; yields a void signal on success
stdexec::sender auto send_response(const http_response& resp) {...}

// Validate that the HTTP request is well-formed; forwards the request on success
stdexec::sender auto validate_request(const http_request& req) {...}

// Handle the request; main application logic
stdexec::sender auto handle_request(const http_request& req) {
  //...
  return stdexec::just(http_response{200, result_body});
}

// Transforms server errors into responses to be sent to the client
stdexec::sender auto error_to_response(std::exception_ptr err) {
  try {
    std::rethrow_exception(err);
  } catch (const std::invalid_argument& e) {
    return stdexec::just(http_response{404, e.what()});
  } catch (const std::exception& e) {
    return stdexec::just(http_response{500, e.what()});
  } catch (...) {
    return stdexec::just(http_response{500, "Unknown server error"});
  }
}

// Transforms cancellation of the server into responses to be sent to the client
stdexec::sender auto stopped_to_response() {
  return stdexec::just(http_response{503, "Service temporarily unavailable"});
}

//...

// The whole flow for transforming incoming requests into responses
stdexec::sender auto snd =
    // get a sender when a new request comes
    schedule_request_start(the_read_requests_ctx)
    // make sure the request is valid; throw if not
    | stdexec::let_value(validate_request)
    // process the request in a function that may be using a different execution resource
    | stdexec::let_value(handle_request)
    // If there are errors transform them into proper responses
    | stdexec::let_error(error_to_response)
    // If the flow is cancelled, send back a proper response
    | stdexec::let_stopped(stopped_to_response)
    // write the result back to the client
    | stdexec::let_value(send_response)
    // done
    ;

// execute the whole flow asynchronously
stdexec::start_detached(std::move(snd));

The example shows how one can separate out the concerns for interpreting requests, validating requests, running the main logic for handling the request, generating error responses, handling cancellation and sending the response back to the client. They are all different phases in the application, and can be joined together with the let_* functions.

All our functions return execution::sender objects, so that they can all generate success, failure and cancellation paths. For example, regardless where an error is generated (reading request, validating request or handling the response), we would have one common block to handle the error, and following error flows is easy.

Also, because of using execution::sender objects at any step, we might expect any of these steps to be completely asynchronous; the overall flow doesn’t care. Regardless of the execution resource in which the steps, or part of the steps are executed in, the flow is still the same.

1.7.2. Moving between execution resources with execution::starts_on and execution::continues_on

Example context:

Goals:

namespace stdexec = std::execution;

size_t legacy_read_from_socket(int sock, char* buffer, size_t buffer_len);
void process_read_data(const char* read_data, size_t read_len);
//...

// A sender that just calls the legacy read function
auto snd_read = stdexec::just(sock, buf, buf_len)
              | stdexec::then(legacy_read_from_socket);

// The entire flow
auto snd =
    // start by reading data on the I/O thread
    stdexec::starts_on(io_sched, std::move(snd_read))
    // do the processing on the worker threads pool
    | stdexec::continues_on(work_sched)
    // process the incoming data (on worker threads)
    | stdexec::then([buf](int read_len) { process_read_data(buf, read_len); })
    // done
    ;

// execute the whole flow asynchronously
stdexec::start_detached(std::move(snd));

The example assume that we need to wrap some legacy code of reading sockets, and handle execution resource switching. (This style of reading from socket may not be the most efficient one, but it’s working for our purposes.) For performance reasons, the reading from the socket needs to be done on the I/O thread, and all the processing needs to happen on a work-specific execution resource (i.e., thread pool).

Calling execution::starts_on will ensure that the given sender will be started on the given scheduler. In our example, snd_read is going to be started on the I/O scheduler. This sender will just call the legacy code.

The completion-signal will be issued in the I/O execution resource, so we have to move it to the work thread pool. This is achieved with the help of the execution::continues_on algorithm. The rest of the processing (in our case, the last call to then) will happen in the work thread pool.

The reader should notice the difference between execution::starts_on and execution::continues_on. The execution::starts_on algorithm will ensure that the given sender will start in the specified context, and doesn’t care where the completion-signal for that sender is sent. The execution::continues_on algorithm will not care where the given sender is going to be started, but will ensure that the completion-signal of will be transferred to the given context.

1.8. Design changes from P0443

  1. The executor concept has been removed and all of its proposed functionality is now based on schedulers and senders, as per SG1 direction.

  2. Properties are not included in this paper. We see them as a possible future extension, if the committee gets more comfortable with them.

  3. Senders now advertise what scheduler, if any, their evaluation will complete on.

  4. The places of execution of user code in P0443 weren’t precisely defined, whereas they are in this paper. See § 4.5 Senders can propagate completion schedulers.

  5. P0443 did not propose a suite of sender algorithms necessary for writing sender code; this paper does. See § 4.19 User-facing sender factories, § 4.20 User-facing sender adaptors, and § 4.21 User-facing sender consumers.

  6. P0443 did not specify the semantics of variously qualified connect overloads; this paper does. See § 4.7 Senders can be either multi-shot or single-shot.

  7. This paper extends the sender traits/typed sender design to support typed senders whose value/error types depend on type information provided late via the receiver.

  8. Support for untyped senders is dropped; the typed_sender concept is renamed sender; sender_traits is replaced with completion_signatures_of_t.

  9. Specific type erasure facilities are omitted, as per LEWG direction. Type erasure facilities can be built on top of this proposal, as discussed in § 5.9 Customization points.

  10. A specific thread pool implementation is omitted, as per LEWG direction.

  11. Some additional utilities are added:

    • run_loop: An execution resource that provides a multi-producer, single-consumer, first-in-first-out work queue.

    • completion_signatures and transform_completion_signatures: Utilities for describing the ways in which a sender can complete in a declarative syntax.

1.9. Prior art

This proposal builds upon and learns from years of prior art with asynchronous and parallel programming frameworks in C++. In this section, we discuss async abstractions that have previously been suggested as a possible basis for asynchronous algorithms and why they fall short.

1.9.1. Futures

A future is a handle to work that has already been scheduled for execution. It is one end of a communication channel; the other end is a promise, used to receive the result from the concurrent operation and to communicate it to the future.

Futures, as traditionally realized, require the dynamic allocation and management of a shared state, synchronization, and typically type-erasure of work and continuation. Many of these costs are inherent in the nature of "future" as a handle to work that is already scheduled for execution. These expenses rule out the future abstraction for many uses and makes it a poor choice for a basis of a generic mechanism.

1.9.2. Coroutines

C++20 coroutines are frequently suggested as a basis for asynchronous algorithms. It’s fair to ask why, if we added coroutines to C++, are we suggesting the addition of a library-based abstraction for asynchrony. Certainly, coroutines come with huge syntactic and semantic advantages over the alternatives.

Although coroutines are lighter weight than futures, coroutines suffer many of the same problems. Since they typically start suspended, they can avoid synchronizing the chaining of dependent work. However in many cases, coroutine frames require an unavoidable dynamic allocation and indirect function calls. This is done to hide the layout of the coroutine frame from the C++ type system, which in turn makes possible the separate compilation of coroutines and certain compiler optimizations, such as optimization of the coroutine frame size.

Those advantages come at a cost, though. Because of the dynamic allocation of coroutine frames, coroutines in embedded or heterogeneous environments, which often lack support for dynamic allocation, require great attention to detail. And the allocations and indirections tend to complicate the job of the inliner, often resulting in sub-optimal codegen.

The coroutine language feature mitigates these shortcomings somewhat with the HALO optimization Halo: coroutine Heap Allocation eLision Optimization: the joint response, which leverages existing compiler optimizations such as allocation elision and devirtualization to inline the coroutine, completely eliminating the runtime overhead. However, HALO requires a sophisiticated compiler, and a fair number of stars need to align for the optimization to kick in. In our experience, more often than not in real-world code today’s compilers are not able to inline the coroutine, resulting in allocations and indirections in the generated code.

In a suite of generic async algorithms that are expected to be callable from hot code paths, the extra allocations and indirections are a deal-breaker. It is for these reasons that we consider coroutines a poor choice for a basis of all standard async.

1.9.3. Callbacks

Callbacks are the oldest, simplest, most powerful, and most efficient mechanism for creating chains of work, but suffer problems of their own. Callbacks must propagate either errors or values. This simple requirement yields many different interface possibilities. The lack of a standard callback shape obstructs generic design.

Additionally, few of these possibilities accommodate cancellation signals when the user requests upstream work to stop and clean up.

1.10. Field experience

1.10.1. libunifex

This proposal draws heavily from our field experience with libunifex. Libunifex implements all of the concepts and customization points defined in this paper (with slight variations -- the design of P2300 has evolved due to LEWG feedback), many of this paper’s algorithms (some under different names), and much more besides.

Libunifex has several concrete schedulers in addition to the run_loop suggested here (where it is called manual_event_loop). It has schedulers that dispatch efficiently to epoll and io_uring on Linux and the Windows Thread Pool on Windows.

In addition to the proposed interfaces and the additional schedulers, it has several important extensions to the facilities described in this paper, which demonstrate directions in which these abstractions may be evolved over time, including:

Libunifex has seen heavy production use at Meta. An employee summarizes it as follows:

As of June, 2023, Unifex is still used in production at Meta. It’s used to express the asynchrony in rsys, and is therefore serving video calling to billions of people every month on Meta’s social networking apps on iOS, Android, Windows, and macOS. It’s also serving the Virtual Desktop experience on Oculus Quest devices, and some internal uses that run on Linux.

One team at Meta has migrated from folly::Future to unifex::task and seen significant developer efficiency improvements. Coroutines are easier to understand than chained futures so the team was able to meet requirements for certain constrained environments that would have been too complicated to maintain with futures.

In all the cases mentioned above, developers mix-and-match between the sender algorithms in Unifex and Unifex’s coroutine type, unifex::task. We also rely on unifex::task’s scheduler affinity to minimize surprise when programming with coroutines.

1.10.2. stdexec

stdexec is the reference implementation of this proposal. It is a complete implementation, written from the specification in this paper, and is current with \R8.

The original purpose of stdexec was to help find specification bugs and to harden the wording of the proposal, but it has since become one of NVIDIA’s core C++ libraries for high-performance computing. In addition to the facilities proposed in this paper, stdexec has schedulers for CUDA, Intel TBB, and MacOS. Like libunifex, its scope has also expanded to include a streaming abstraction and stream algorithms, and time-based schedulers and algorithms.

The stdexec project has seen lots of community interest and contributions. At the time of writing (March, 2024), the GitHub repository has 1.2k stars, 130 forks, and 50 contributors.

stdexec is fit for broad use and for ultimate contribution to libc++.

1.10.3. Other implementations

The authors are aware of a number of other implementations of sender/receiver from this paper. These are presented here in perceived order of maturity and field experience.

1.10.4. Inspirations

This proposal also draws heavily from our experience with Thrust and Agency. It is also inspired by the needs of countless other C++ frameworks for asynchrony, parallelism, and concurrency, including:

2. Revision history

2.1. R10

The changes since R9 are as follows:

Fixes:

Enhancements:

2.2. R9

The changes since R8 are as follows:

Fixes:

Enhancements:

2.3. R8

The changes since R7 are as follows:

Fixes:

Enhancements:

2.4. R7

The changes since R6 are as follows:

Fixes:

Enhancements:

2.5. R6

The changes since R5 are as follows:

Fixes:

Enhancements:

2.5.1. Environments and attributes

In earlier revisions, receivers, senders, and schedulers all were directly queryable. In R4, receiver queries were moved into a separate "environment" object, obtainable from a receiver with a get_env accessor. In R6, the sender queries are given similar treatment, relocating to a "attributes" object obtainable from a sender with a get_attrs accessor. This was done to solve a number of design problems with the split and ensure_started algorithms; e.g., see NVIDIA/stdexec#466.

Schedulers, however, remain directly queryable. As lightweight handles that are required to be movable and copyable, there is little reason to want to dispose of a scheduler and yet persist the scheduler’s queries.

This revision also makes operation states directly queryable, even though there isn’t yet a use for such. Some early prototypes of cooperative bulk parallel sender algorithms done at NVIDIA suggest the utility of forwardable operation state queries. The authors chose to make opstates directly queryable since the opstate object is itself required to be kept alive for the duration of asynchronous operation.

2.6. R5

The changes since R4 are as follows:

Fixes:

Enhancements:

2.7. R4

The changes since R3 are as follows:

Fixes:

Enhancements:

2.7.1. Dependently-typed senders

Background:

In the sender/receiver model, as with coroutines, contextual information about the current execution is most naturally propagated from the consumer to the producer. In coroutines, that means information like stop tokens, allocators and schedulers are propagated from the calling coroutine to the callee. In sender/receiver, that means that that contextual information is associated with the receiver and is queried by the sender and/or operation state after the sender and the receiver are connect-ed.

Problem:

The implication of the above is that the sender alone does not have all the information about the async computation it will ultimately initiate; some of that information is provided late via the receiver. However, the sender_traits mechanism, by which an algorithm can introspect the value and error types the sender will propagate, only accepts a sender parameter. It does not take into consideration the type information that will come in late via the receiver. The effect of this is that some senders cannot be typed senders when they otherwise could be.

Example:

To get concrete, consider the case of the "get_scheduler()" sender: when connect-ed and start-ed, it queries the receiver for its associated scheduler and passes it back to the receiver through the value channel. That sender’s "value type" is the type of the receiver’s scheduler. What then should sender_traits<get_scheduler_sender>::value_types report for the get_scheduler()’s value type? It can’t answer because it doesn’t know.

This causes knock-on problems since some important algorithms require a typed sender, such as sync_wait. To illustrate the problem, consider the following code:

namespace ex = std::execution;

ex::sender auto task =
  ex::let_value(
    ex::get_scheduler(), // Fetches scheduler from receiver.
    [](auto current_sched) {
      // Lauch some nested work on the current scheduler:
      return ex::starts_on(current_sched, nested work...);
    });

std::this_thread::sync_wait(std::move(task));

The code above is attempting to schedule some work onto the sync_wait’s run_loop execution resource. But let_value only returns a typed sender when the input sender is typed. As we explained above, get_scheduler() is not typed, so task is likewise not typed. Since task isn’t typed, it cannot be passed to sync_wait which is expecting a typed sender. The above code would fail to compile.

Solution:

The solution is conceptually quite simple: extend the sender_traits mechanism to optionally accept a receiver in addition to the sender. The algorithms can use sender_traits<Sender, Receiver> to inspect the async operation’s completion-signals. The typed_sender concept would also need to take an optional receiver parameter. This is the simplest change, and it would solve the immediate problem.

Design:

Using the receiver type to compute the sender traits turns out to have pitfalls in practice. Many receivers make use of that type information in their implementation. It is very easy to create cycles in the type system, leading to inscrutible errors. The design pursued in R4 is to give receivers an associated environment object -- a bag of key/value pairs -- and to move the contextual information (schedulers, etc) out of the receiver and into the environment. The sender_traits template and the typed_sender concept, rather than taking a receiver, take an environment. This is a much more robust design.

A further refinement of this design would be to separate the receiver and the environment entirely, passing then as separate arguments along with the sender to connect. This paper does not propose that change.

Impact:

This change, apart from increasing the expressive power of the sender/receiver abstraction, has the following impact:

"Has it been implemented?"

Yes, the reference implementation, which can be found at https://github.com/NVIDIA/stdexec, has implemented this design as well as some dependently-typed senders to confirm that it works.

Implementation experience

Although this change has not yet been made in libunifex, the most widely adopted sender/receiver implementation, a similar design can be found in Folly’s coroutine support library. In Folly.Coro, it is possible to await a special awaitable to obtain the current coroutine’s associated scheduler (called an executor in Folly).

For instance, the following Folly code grabs the current executor, schedules a task for execution on that executor, and starts the resulting (scheduled) task by enqueueing it for execution.

// From Facebook's Folly open source library:
template <class T>
folly::coro::Task<void> CancellableAsyncScope::co_schedule(folly::coro::Task<T>&& task) {
  this->add(std::move(task).scheduleOn(co_await co_current_executor));
  co_return;
}

Facebook relies heavily on this pattern in its coroutine code. But as described above, this pattern doesn’t work with R3 of std::execution because of the lack of dependently-typed schedulers. The change to sender_traits in R4 rectifies that.

Why now?

The authors are loathe to make any changes to the design, however small, at this stage of the C++23 release cycle. But we feel that, for a relatively minor design change -- adding an extra template parameter to sender_traits and typed_sender -- the returns are large enough to justify the change. And there is no better time to make this change than as early as possible.

One might wonder why this missing feature not been added to sender/receiver before now. The designers of sender/receiver have long been aware of the need. What was missing was a clean, robust, and simple design for the change, which we now have.

Drive-by:

We took the opportunity to make an additional drive-by change: Rather than providing the sender traits via a class template for users to specialize, we changed it into a sender query: get_completion_signatures(sender, env). That function’s return type is used as the sender’s traits. The authors feel this leads to a more uniform design and gives sender authors a straightforward way to make the value/error types dependent on the cv- and ref-qualification of the sender if need be.

Details:

Below are the salient parts of the new support for dependently-typed senders in R4:

2.8. R3

The changes since R2 are as follows:

Fixes:

Enhancements:

2.9. R2

The changes since R1 are as follows:

2.10. R1

The changes since R0 are as follows:

2.11. R0

Initial revision.

3. Design - introduction

The following three sections describe the entirety of the proposed design.

3.1. Conventions

The following conventions are used throughout the design section:

  1. The namespace proposed in this paper is the same as in A Unified Executors Proposal for C++: std::execution; however, for brevity, the std:: part of this name is omitted. When you see execution::foo, treat that as std::execution::foo.

  2. Universal references and explicit calls to std::move/std::forward are omitted in code samples and signatures for simplicity; assume universal references and perfect forwarding unless stated otherwise.

  3. None of the names proposed here are names that we are particularly attached to; consider the names to be reasonable placeholders that can freely be changed, should the committee want to do so.

3.2. Queries and algorithms

A query is a callable that takes some set of objects (usually one) as parameters and returns facts about those objects without modifying them. Queries are usually customization point objects, but in some cases may be functions.

An algorithm is a callable that takes some set of objects as parameters and causes those objects to do something. Algorithms are usually customization point objects, but in some cases may be functions.

4. Design - user side

4.1. Execution resources describe the place of execution

An execution resource is a resource that represents the place where execution will happen. This could be a concrete resource - like a specific thread pool object, or a GPU - or a more abstract one, like the current thread of execution. Execution contexts don’t need to have a representation in code; they are simply a term describing certain properties of execution of a function.

4.2. Schedulers represent execution resources

A scheduler is a lightweight handle that represents a strategy for scheduling work onto an execution resource. Since execution resources don’t necessarily manifest in C++ code, it’s not possible to program directly against their API. A scheduler is a solution to that problem: the scheduler concept is defined by a single sender algorithm, schedule, which returns a sender that will complete on an execution resource determined by the scheduler. Logic that you want to run on that context can be placed in the receiver’s completion-signalling method.

execution::scheduler auto sch = thread_pool.scheduler();
execution::sender auto snd = execution::schedule(sch);
// snd is a sender (see below) describing the creation of a new execution resource
// on the execution resource associated with sch

Note that a particular scheduler type may provide other kinds of scheduling operations which are supported by its associated execution resource. It is not limited to scheduling purely using the execution::schedule API.

Future papers will propose additional scheduler concepts that extend scheduler to add other capabilities. For example:

4.3. Senders describe work

A sender is an object that describes work. Senders are similar to futures in existing asynchrony designs, but unlike futures, the work that is being done to arrive at the values they will send is also directly described by the sender object itself. A sender is said to send some values if a receiver connected (see § 5.3 execution::connect) to that sender will eventually receive said values.

The primary defining sender algorithm is § 5.3 execution::connect; this function, however, is not a user-facing API; it is used to facilitate communication between senders and various sender algorithms, but end user code is not expected to invoke it directly.

The way user code is expected to interact with senders is by using sender algorithms. This paper proposes an initial set of such sender algorithms, which are described in § 4.4 Senders are composable through sender algorithms, § 4.19 User-facing sender factories, § 4.20 User-facing sender adaptors, and § 4.21 User-facing sender consumers. For example, here is how a user can create a new sender on a scheduler, attach a continuation to it, and then wait for execution of the continuation to complete:

execution::scheduler auto sch = thread_pool.scheduler();
execution::sender auto snd = execution::schedule(sch);
execution::sender auto cont = execution::then(snd, []{
    std::fstream file{ "result.txt" };
    file << compute_result;
});

this_thread::sync_wait(cont);
// at this point, cont has completed execution

4.4. Senders are composable through sender algorithms

Asynchronous programming often departs from traditional code structure and control flow that we are familiar with. A successful asynchronous framework must provide an intuitive story for composition of asynchronous work: expressing dependencies, passing objects, managing object lifetimes, etc.

The true power and utility of senders is in their composability. With senders, users can describe generic execution pipelines and graphs, and then run them on and across a variety of different schedulers. Senders are composed using sender algorithms:

4.5. Senders can propagate completion schedulers

One of the goals of executors is to support a diverse set of execution resources, including traditional thread pools, task and fiber frameworks (like HPX Legion), and GPUs and other accelerators (managed by runtimes such as CUDA or SYCL). On many of these systems, not all execution agents are created equal and not all functions can be run on all execution agents. Having precise control over the execution resource used for any given function call being submitted is important on such systems, and the users of standard execution facilities will expect to be able to express such requirements.

A Unified Executors Proposal for C++ was not always clear about the place of execution of any given piece of code. Precise control was present in the two-way execution API present in earlier executor designs, but it has so far been missing from the senders design. There has been a proposal (Towards C++23 executors: A proposal for an initial set of algorithms) to provide a number of sender algorithms that would enforce certain rules on the places of execution of the work described by a sender, but we have found those sender algorithms to be insufficient for achieving the best performance on all platforms that are of interest to us. The implementation strategies that we are aware of result in one of the following situations:

  1. trying to submit work to one execution resource (such as a CPU thread pool) from another execution resource (such as a GPU or a task framework), which assumes that all execution agents are as capable as a std::thread (which they aren’t).

  2. forcibly interleaving two adjacent execution graph nodes that are both executing on one execution resource (such as a GPU) with glue code that runs on another execution resource (such as a CPU), which is prohibitively expensive for some execution resources (such as CUDA or SYCL).

  3. having to customise most or all sender algorithms to support an execution resource, so that you can avoid problems described in 1. and 2, which we believe is impractical and brittle based on months of field experience attempting this in Agency.

None of these implementation strategies are acceptable for many classes of parallel runtimes, such as task frameworks (like HPX) or accelerator runtimes (like CUDA or SYCL).

Therefore, in addition to the starts_on sender algorithm from Towards C++23 executors: A proposal for an initial set of algorithms, we are proposing a way for senders to advertise what scheduler (and by extension what execution resource) they will complete on. Any given sender may have completion schedulers for some or all of the signals (value, error, or stopped) it completes with (for more detail on the completion-signals, see § 5.1 Receivers serve as glue between senders). When further work is attached to that sender by invoking sender algorithms, that work will also complete on an appropriate completion scheduler.

4.5.1. execution::get_completion_scheduler

get_completion_scheduler is a query that retrieves the completion scheduler for a specific completion-signal from a sender’s environment. For a sender that lacks a completion scheduler query for a given signal, calling get_completion_scheduler is ill-formed. If a sender advertises a completion scheduler for a signal in this way, that sender must ensure that it sends that signal on an execution agent belonging to an execution resource represented by a scheduler returned from this function. See § 4.5 Senders can propagate completion schedulers for more details.

execution::scheduler auto cpu_sched = new_thread_scheduler{};
execution::scheduler auto gpu_sched = cuda::scheduler();

execution::sender auto snd0 = execution::schedule(cpu_sched);
execution::scheduler auto completion_sch0 =
  execution::get_completion_scheduler<execution::set_value_t>(get_env(snd0));
// completion_sch0 is equivalent to cpu_sched

execution::sender auto snd1 = execution::then(snd0, []{
    std::cout << "I am running on cpu_sched!\n";
});
execution::scheduler auto completion_sch1 =
  execution::get_completion_scheduler<execution::set_value_t>(get_env(snd1));
// completion_sch1 is equivalent to cpu_sched

execution::sender auto snd2 = execution::continues_on(snd1, gpu_sched);
execution::sender auto snd3 = execution::then(snd2, []{
    std::cout << "I am running on gpu_sched!\n";
});
execution::scheduler auto completion_sch3 =
  execution::get_completion_scheduler<execution::set_value_t>(get_env(snd3));
// completion_sch3 is equivalent to gpu_sched

4.6. Execution resource transitions are explicit

A Unified Executors Proposal for C++ does not contain any mechanisms for performing an execution resource transition. The only sender algorithm that can create a sender that will move execution to a specific execution resource is execution::schedule, which does not take an input sender. That means that there’s no way to construct sender chains that traverse different execution resources. This is necessary to fulfill the promise of senders being able to replace two-way executors, which had this capability.

We propose that, for senders advertising their completion scheduler, all execution resource transitions must be explicit; running user code anywhere but where they defined it to run must be considered a bug.

The execution::continues_on sender adaptor performs a transition from one execution resource to another:

execution::scheduler auto sch1 = ...;
execution::scheduler auto sch2 = ...;

execution::sender auto snd1 = execution::schedule(sch1);
execution::sender auto then1 = execution::then(snd1, []{
    std::cout << "I am running on sch1!\n";
});

execution::sender auto snd2 = execution::continues_on(then1, sch2);
execution::sender auto then2 = execution::then(snd2, []{
    std::cout << "I am running on sch2!\n";
});

this_thread::sync_wait(then2);

4.7. Senders can be either multi-shot or single-shot

Some senders may only support launching their operation a single time, while others may be repeatable and support being launched multiple times. Executing the operation may consume resources owned by the sender.

For example, a sender may contain a std::unique_ptr that it will be transferring ownership of to the operation-state returned by a call to execution::connect so that the operation has access to this resource. In such a sender, calling execution::connect consumes the sender such that after the call the input sender is no longer valid. Such a sender will also typically be move-only so that it can maintain unique ownership of that resource.

A single-shot sender can only be connected to a receiver at most once. Its implementation of execution::connect only has overloads for an rvalue-qualified sender. Callers must pass the sender as an rvalue to the call to execution::connect, indicating that the call consumes the sender.

A multi-shot sender can be connected to multiple receivers and can be launched multiple times. Multi-shot senders customise execution::connect to accept an lvalue reference to the sender. Callers can indicate that they want the sender to remain valid after the call to execution::connect by passing an lvalue reference to the sender to call these overloads. Multi-shot senders should also define overloads of execution::connect that accept rvalue-qualified senders to allow the sender to be also used in places where only a single-shot sender is required.

If the user of a sender does not require the sender to remain valid after connecting it to a receiver then it can pass an rvalue-reference to the sender to the call to execution::connect. Such usages should be able to accept either single-shot or multi-shot senders.

If the caller does wish for the sender to remain valid after the call then it can pass an lvalue-qualified sender to the call to execution::connect. Such usages will only accept multi-shot senders.

Algorithms that accept senders will typically either decay-copy an input sender and store it somewhere for later usage (for example as a data-member of the returned sender) or will immediately call execution::connect on the input sender, such as in this_thread::sync_wait.

Some multi-use sender algorithms may require that an input sender be copy-constructible but will only call execution::connect on an rvalue of each copy, which still results in effectively executing the operation multiple times. Other multi-use sender algorithms may require that the sender is move-constructible but will invoke execution::connect on an lvalue reference to the sender.

For a sender to be usable in both multi-use scenarios, it will generally be required to be both copy-constructible and lvalue-connectable.

4.8. Senders are forkable

Any non-trivial program will eventually want to fork a chain of senders into independent streams of work, regardless of whether they are single-shot or multi-shot. For instance, an incoming event to a middleware system may be required to trigger events on more than one downstream system. This requires that we provide well defined mechanisms for making sure that connecting a sender multiple times is possible and correct.

The split sender adaptor facilitates connecting to a sender multiple times, regardless of whether it is single-shot or multi-shot:

auto some_algorithm(execution::sender auto&& input) {
    execution::sender auto multi_shot = split(input);
    // "multi_shot" is guaranteed to be multi-shot,
    // regardless of whether "input" was multi-shot or not

    return when_all(
      then(multi_shot, [] { std::cout << "First continuation\n"; }),
      then(multi_shot, [] { std::cout << "Second continuation\n"; })
    );
}

4.9. Senders support cancellation

Senders are often used in scenarios where the application may be concurrently executing multiple strategies for achieving some program goal. When one of these strategies succeeds (or fails) it may not make sense to continue pursuing the other strategies as their results are no longer useful.

For example, we may want to try to simultaneously connect to multiple network servers and use whichever server responds first. Once the first server responds we no longer need to continue trying to connect to the other servers.

Ideally, in these scenarios, we would somehow be able to request that those other strategies stop executing promptly so that their resources (e.g. cpu, memory, I/O bandwidth) can be released and used for other work.

While the design of senders has support for cancelling an operation before it starts by simply destroying the sender or the operation-state returned from execution::connect() before calling execution::start(), there also needs to be a standard, generic mechanism to ask for an already-started operation to complete early.

The ability to be able to cancel in-flight operations is fundamental to supporting some kinds of generic concurrency algorithms.

For example:

The mechanism used for communcating cancellation-requests, or stop-requests, needs to have a uniform interface so that generic algorithms that compose sender-based operations, such as the ones listed above, are able to communicate these cancellation requests to senders that they don’t know anything about.

The design is intended to be composable so that cancellation of higher-level operations can propagate those cancellation requests through intermediate layers to lower-level operations that need to actually respond to the cancellation requests.

For example, we can compose the algorithms mentioned above so that child operations are cancelled when any one of the multiple cancellation conditions occurs:

sender auto composed_cancellation_example(auto query) {
  return stop_when(
    timeout(
      when_all(
        first_successful(
          query_server_a(query),
          query_server_b(query)),
        load_file("some_file.jpg")),
      5s),
    cancelButton.on_click());
}

In this example, if we take the operation returned by query_server_b(query), this operation will receive a stop-request when any of the following happens:

Note that within this code there is no explicit mention of cancellation, stop-tokens, callbacks, etc. yet the example fully supports and responds to the various cancellation sources.

The intent of the design is that the common usage of cancellation in sender/receiver-based code is primarily through use of concurrency algorithms that manage the detailed plumbing of cancellation for you. Much like algorithms that compose senders relieve the user from having to write their own receiver types, algorithms that introduce concurrency and provide higher-level cancellation semantics relieve the user from having to deal with low-level details of cancellation.

4.9.1. Cancellation design summary

The design of cancellation described in this paper is built on top of and extends the std::stop_token-based cancellation facilities added in C++20, first proposed in Composable cancellation for sender-based async operations.

At a high-level, the facilities proposed by this paper for supporting cancellation include:

In addition, there are requirements added to some of the algorithms to specify what their cancellation behaviour is and what the requirements of customisations of those algorithms are with respect to cancellation.

The key component that enables generic cancellation within sender-based operations is the execution::get_stop_token() CPO. This CPO takes a single parameter, which is the execution environment of the receiver passed to execution::connect, and returns a std::stoppable_token that the operation can use to check for stop-requests for that operation.

As the caller of execution::connect typically has control over the receiver type it passes, it is able to customise the std::execution::get_env() CPO for that receiver to return an execution environment that hooks the execution::get_stop_token() CPO to return a stop-token that the receiver has control over and that it can use to communicate a stop-request to the operation once it has started.

4.9.2. Support for cancellation is optional

Support for cancellation is optional, both on part of the author of the receiver and on part of the author of the sender.

If the receiver’s execution environment does not customise the execution::get_stop_token() CPO then invoking the CPO on that receiver’s environment will invoke the default implementation which returns std::never_stop_token. This is a special stoppable_token type that is statically known to always return false from the stop_possible() method.

Sender code that tries to use this stop-token will in general result in code that handles stop-requests being compiled out and having little to no run-time overhead.

If the sender doesn’t call execution::get_stop_token(), for example because the operation does not support cancellation, then it will simply not respond to stop-requests from the caller.

Note that stop-requests are generally racy in nature as there is often a race betwen an operation completing naturally and the stop-request being made. If the operation has already completed or past the point at which it can be cancelled when the stop-request is sent then the stop-request may just be ignored. An application will typically need to be able to cope with senders that might ignore a stop-request anyway.

4.9.3. Cancellation is inherently racy

Usually, an operation will attach a stop callback at some point inside the call to execution::start() so that a subsequent stop-request will interrupt the logic.

A stop-request can be issued concurrently from another thread. This means the implementation of execution::start() needs to be careful to ensure that, once a stop callback has been registered, that there are no data-races between a potentially concurrently-executing stop callback and the rest of the execution::start() implementation.

An implementation of execution::start() that supports cancellation will generally need to perform (at least) two separate steps: launch the operation, subscribe a stop callback to the receiver’s stop-token. Care needs to be taken depending on the order in which these two steps are performed.

If the stop callback is subscribed first and then the operation is launched, care needs to be taken to ensure that a stop-request that invokes the stop callback on another thread after the stop callback is registered but before the operation finishes launching does not either result in a missed cancellation request or a data-race. e.g. by performing an atomic write after the launch has finished executing

If the operation is launched first and then the stop callback is subscribed, care needs to be taken to ensure that if the launched operation completes concurrently on another thread that it does not destroy the operation-state until after the stop callback has been registered. e.g. by having the execution::start implementation write to an atomic variable once it has finished registering the stop callback and having the concurrent completion handler check that variable and either call the completion-signalling operation or store the result and defer calling the receiver’s completion-signalling operation to the execution::start() call (which is still executing).

For an example of an implementation strategy for solving these data-races see § 1.4 Asynchronous Windows socket recv.

4.9.4. Cancellation design status

This paper currently includes the design for cancellation as proposed in Composable cancellation for sender-based async operations - "Composable cancellation for sender-based async operations". P2175R0 contains more details on the background motivation and prior-art and design rationale of this design.

It is important to note, however, that initial review of this design in the SG1 concurrency subgroup raised some concerns related to runtime overhead of the design in single-threaded scenarios and these concerns are still being investigated.

The design of P2175R0 has been included in this paper for now, despite its potential to change, as we believe that support for cancellation is a fundamental requirement for an async model and is required in some form to be able to talk about the semantics of some of the algorithms proposed in this paper.

This paper will be updated in the future with any changes that arise from the investigations into P2175R0.

4.10. Sender factories and adaptors are lazy

In an earlier revision of this paper, some of the proposed algorithms supported executing their logic eagerly; i.e., before the returned sender has been connected to a receiver and started. These algorithms were removed because eager execution has a number of negative semantic and performance implications.

We have originally included this functionality in the paper because of a long-standing belief that eager execution is a mandatory feature to be included in the standard Executors facility for that facility to be acceptable for accelerator vendors. A particular concern was that we must be able to write generic algorithms that can run either eagerly or lazily, depending on the kind of an input sender or scheduler that have been passed into them as arguments. We considered this a requirement, because the _latency_ of launching work on an accelerator can sometimes be considerable.

However, in the process of working on this paper and implementations of the features proposed within, our set of requirements has shifted, as we understood the different implementation strategies that are available for the feature set of this paper better, and, after weighing the earlier concerns against the points presented below, we have arrived at the conclusion that a purely lazy model is enough for most algorithms, and users who intend to launch work earlier may write an algorithm to achieve that goal. We have also come to deeply appreciate the fact that a purely lazy model allows both the implementation and the compiler to have a much better understanding of what the complete graph of tasks looks like, allowing them to better optimize the code - also when targetting accelerators.

4.10.1. Eager execution leads to detached work or worse

One of the questions that arises with APIs that can potentially return eagerly-executing senders is "What happens when those senders are destructed without a call to execution::connect?" or similarly, "What happens if a call to execution::connect is made, but the returned operation state is destroyed before execution::start is called on that operation state"?

In these cases, the operation represented by the sender is potentially executing concurrently in another thread at the time that the destructor of the sender and/or operation-state is running. In the case that the operation has not completed executing by the time that the destructor is run we need to decide what the semantics of the destructor is.

There are three main strategies that can be adopted here, none of which is particularly satisfactory:

  1. Make this undefined-behaviour - the caller must ensure that any eagerly-executing sender is always joined by connecting and starting that sender. This approach is generally pretty hostile to programmers, particularly in the presence of exceptions, since it complicates the ability to compose these operations.

    Eager operations typically need to acquire resources when they are first called in order to start the operation early. This makes eager algorithms prone to failure. Consider, then, what might happen in an expression such as when_all(eager_op_1(), eager_op_2()). Imagine eager_op_1() starts an asynchronous operation successfully, but then eager_op_2() throws. For lazy senders, that failure happens in the context of the when_all algorithm, which handles the failure and ensures that async work joins on all code paths. In this case though -- the eager case -- the child operation has failed even before when_all has been called.

    It then becomes the responsibility, not of the algorithm, but of the end user to handle the exception and ensure that eager_op_1() is joined before allowing the exception to propagate. If they fail to do that, they incur undefined behavior.

  2. Detach from the computation - let the operation continue in the background - like an implicit call to std::thread::detach(). While this approach can work in some circumstances for some kinds of applications, in general it is also pretty user-hostile; it makes it difficult to reason about the safe destruction of resources used by these eager operations. In general, detached work necessitates some kind of garbage collection; e.g., std::shared_ptr, to ensure resources are kept alive until the operations complete, and can make clean shutdown nigh impossible.

  3. Block in the destructor until the operation completes. This approach is probably the safest to use as it preserves the structured nature of the concurrent operations, but also introduces the potential for deadlocking the application if the completion of the operation depends on the current thread making forward progress.

    The risk of deadlock might occur, for example, if a thread-pool with a small number of threads is executing code that creates a sender representing an eagerly-executing operation and then calls the destructor of that sender without joining it (e.g. because an exception was thrown). If the current thread blocks waiting for that eager operation to complete and that eager operation cannot complete until some entry enqueued to the thread-pool’s queue of work is run then the thread may wait for an indefinite amount of time. If all threads of the thread-pool are simultaneously performing such blocking operations then deadlock can result.

There are also minor variations on each of these choices. For example:

  1. A variation of (1): Call std::terminate if an eager sender is destructed without joining it. This is the approach that std::thread destructor takes.

  2. A variation of (2): Request cancellation of the operation before detaching. This reduces the chances of operations continuing to run indefinitely in the background once they have been detached but does not solve the lifetime- or shutdown-related challenges.

  3. A variation of (3): Request cancellation of the operation before blocking on its completion. This is the strategy that std::jthread uses for its destructor. It reduces the risk of deadlock but does not eliminate it.

4.10.2. Eager senders complicate algorithm implementations

Algorithms that can assume they are operating on senders with strictly lazy semantics are able to make certain optimizations that are not available if senders can be potentially eager. With lazy senders, an algorithm can safely assume that a call to execution::start on an operation state strictly happens before the execution of that async operation. This frees the algorithm from needing to resolve potential race conditions. For example, consider an algorithm sequence that puts async operations in sequence by starting an operation only after the preceding one has completed. In an expression like sequence(a(), then(src, [] { b(); }), c()), one may reasonably assume that a(), b() and c() are sequenced and therefore do not need synchronisation. Eager algorithms break that assumption.

When an algorithm needs to deal with potentially eager senders, the potential race conditions can be resolved one of two ways, neither of which is desirable:

  1. Assume the worst and implement the algorithm defensively, assuming all senders are eager. This obviously has overheads both at runtime and in algorithm complexity. Resolving race conditions is hard.

  2. Require senders to declare whether they are eager or not with a query. Algorithms can then implement two different implementation strategies, one for strictly lazy senders and one for potentially eager senders. This addresses the performance problem of (1) while compounding the complexity problem.

4.10.3. Eager senders incur cancellation-related overhead

Another implication of the use of eager operations is with regards to cancellation. The eagerly executing operation will not have access to the caller’s stop token until the sender is connected to a receiver. If we still want to be able to cancel the eager operation then it will need to create a new stop source and pass its associated stop token down to child operations. Then when the returned sender is eventually connected it will register a stop callback with the receiver’s stop token that will request stop on the eager sender’s stop source.

As the eager operation does not know at the time that it is launched what the type of the receiver is going to be, and thus whether or not the stop token returned from execution::get_stop_token is an std::unstoppable_token or not, the eager operation is going to need to assume it might be later connected to a receiver with a stop token that might actually issue a stop request. Thus it needs to declare space in the operation state for a type-erased stop callback and incur the runtime overhead of supporting cancellation, even if cancellation will never be requested by the caller.

The eager operation will also need to do this to support sending a stop request to the eager operation in the case that the sender representing the eager work is destroyed before it has been joined (assuming strategy (5) or (6) listed above is chosen).

4.10.4. Eager senders cannot access execution resource from the receiver

In sender/receiver, contextual information is passed from parent operations to their children by way of receivers. Information like stop tokens, allocators, current scheduler, priority, and deadline are propagated to child operations with custom receivers at the time the operation is connected. That way, each operation has the contextual information it needs before it is started.

But if the operation is started before it is connected to a receiver, then there isn’t a way for a parent operation to communicate contextual information to its child operations, which may complete before a receiver is ever attached.

4.11. Schedulers advertise their forward progress guarantees

To decide whether a scheduler (and its associated execution resource) is sufficient for a specific task, it may be necessary to know what kind of forward progress guarantees it provides for the execution agents it creates. The C++ Standard defines the following forward progress guarantees:

This paper introduces a scheduler query function, get_forward_progress_guarantee, which returns one of the enumerators of a new enum type, forward_progress_guarantee. Each enumerator of forward_progress_guarantee corresponds to one of the aforementioned guarantees.

4.12. Most sender adaptors are pipeable

To facilitate an intuitive syntax for composition, most sender adaptors are pipeable; they can be composed (piped) together with operator|. This mechanism is similar to the operator| composition that C++ range adaptors support and draws inspiration from piping in *nix shells. Pipeable sender adaptors take a sender as their first parameter and have no other sender parameters.

a | b will pass the sender a as the first argument to the pipeable sender adaptor b. Pipeable sender adaptors support partial application of the parameters after the first. For example, all of the following are equivalent:

execution::bulk(snd, N, [] (std::size_t i, auto d) {});
execution::bulk(N, [] (std::size_t i, auto d) {})(snd);
snd | execution::bulk(N, [] (std::size_t i, auto d) {});

Piping enables you to compose together senders with a linear syntax. Without it, you’d have to use either nested function call syntax, which would cause a syntactic inversion of the direction of control flow, or you’d have to introduce a temporary variable for each stage of the pipeline. Consider the following example where we want to execute first on a CPU thread pool, then on a CUDA GPU, then back on the CPU thread pool:

Syntax Style Example
Function call
(nested)
auto snd = execution::then(
             execution::continues_on(
               execution::then(
                 execution::continues_on(
                   execution::then(
                     execution::schedule(thread_pool.scheduler())
                     []{ return 123; }),
                   cuda::new_stream_scheduler()),
                 [](int i){ return 123 * 5; }),
               thread_pool.scheduler()),
             [](int i){ return i - 5; });
auto [result] = this_thread::sync_wait(snd).value();
// result == 610
Function call
(named temporaries)
auto snd0 = execution::schedule(thread_pool.scheduler());
auto snd1 = execution::then(snd0, []{ return 123; });
auto snd2 = execution::continues_on(snd1, cuda::new_stream_scheduler());
auto snd3 = execution::then(snd2, [](int i){ return 123 * 5; })
auto snd4 = execution::continues_on(snd3, thread_pool.scheduler())
auto snd5 = execution::then(snd4, [](int i){ return i - 5; });
auto [result] = *this_thread::sync_wait(snd4);
// result == 610
Pipe
auto snd = execution::schedule(thread_pool.scheduler())
         | execution::then([]{ return 123; })
         | execution::continues_on(cuda::new_stream_scheduler())
         | execution::then([](int i){ return 123 * 5; })
         | execution::continues_on(thread_pool.scheduler())
         | execution::then([](int i){ return i - 5; });
auto [result] = this_thread::sync_wait(snd).value();
// result == 610

Certain sender adaptors are not pipeable, because using the pipeline syntax can result in confusion of the semantics of the adaptors involved. Specifically, the following sender adaptors are not pipeable.

Sender consumers could be made pipeable, but we have chosen to not do so. However, since these are terminal nodes in a pipeline and nothing can be piped after them, we believe a pipe syntax may be confusing as well as unnecessary, as consumers cannot be chained. We believe sender consumers read better with function call syntax.

4.13. A range of senders represents an async sequence of data

Senders represent a single unit of asynchronous work. In many cases though, what is being modeled is a sequence of data arriving asynchronously, and you want computation to happen on demand, when each element arrives. This requires nothing more than what is in this paper and the range support in C++20. A range of senders would allow you to model such input as keystrikes, mouse movements, sensor readings, or network requests.

Given some expression R that is a range of senders, consider the following in a coroutine that returns an async generator type:

for (auto snd : R) {
  if (auto opt = co_await execution::stopped_as_optional(std::move(snd)))
    co_yield fn(*std::move(opt));
  else
    break;
}

This transforms each element of the asynchronous sequence R with the function fn on demand, as the data arrives. The result is a new asynchronous sequence of the transformed values.

Now imagine that R is the simple expression views::iota(0) | views::transform(execution::just). This creates a lazy range of senders, each of which completes immediately with monotonically increasing integers. The above code churns through the range, generating a new infine asynchronous range of values [fn(0), fn(1), fn(2), ...].

Far more interesting would be if R were a range of senders representing, say, user actions in a UI. The above code gives a simple way to respond to user actions on demand.

4.14. Senders can represent partial success

Receivers have three ways they can complete: with success, failure, or cancellation. This begs the question of how they can be used to represent async operations that partially succeed. For example, consider an API that reads from a socket. The connection could drop after the API has filled in some of the buffer. In cases like that, it makes sense to want to report both that the connection dropped and that some data has been successfully read.

Often in the case of partial success, the error condition is not fatal nor does it mean the API has failed to satisfy its post-conditions. It is merely an extra piece of information about the nature of the completion. In those cases, "partial success" is another way of saying "success". As a result, it is sensible to pass both the error code and the result (if any) through the value channel, as shown below:

// Capture a buffer for read_socket_async to fill in
execution::just(array<byte, 1024>{})
  | execution::let_value([socket](array<byte, 1024>& buff) {
      // read_socket_async completes with two values: an error_code and
      // a count of bytes:
      return read_socket_async(socket, span{buff})
          // For success (partial and full), specify the next action:
        | execution::let_value([](error_code err, size_t bytes_read) {
            if (err != 0) {
              // OK, partial success. Decide how to deal with the partial results
            } else {
              // OK, full success here.
            }
          });
    })

In other cases, the partial success is more of a partial failure. That happens when the error condition indicates that in some way the function failed to satisfy its post-conditions. In those cases, sending the error through the value channel loses valuable contextual information. It’s possible that bundling the error and the incomplete results into an object and passing it through the error channel makes more sense. In that way, generic algorithms will not miss the fact that a post-condition has not been met and react inappropriately.

Another possibility is for an async API to return a range of senders: if the API completes with full success, full error, or cancellation, the returned range contains just one sender with the result. Otherwise, if the API partially fails (doesn’t satisfy its post-conditions, but some incomplete result is available), the returned range would have two senders: the first containing the partial result, and the second containing the error. Such an API might be used in a coroutine as follows:

// Declare a buffer for read_socket_async to fill in
array<byte, 1024> buff;

for (auto snd : read_socket_async(socket, span{buff})) {
  try {
    if (optional<size_t> bytes_read =
          co_await execution::stopped_as_optional(std::move(snd))) {
      // OK, we read some bytes into buff. Process them here....
    } else {
      // The socket read was cancelled and returned no data. React
      // appropriately.
    }
  } catch (...) {
    // read_socket_async failed to meet its post-conditions.
    // Do some cleanup and propagate the error...
  }
}

Finally, it’s possible to combine these two approaches when the API can both partially succeed (meeting its post-conditions) and partially fail (not meeting its post-conditions).

4.15. All awaitables are senders

Since C++20 added coroutines to the standard, we expect that coroutines and awaitables will be how a great many will choose to express their asynchronous code. However, in this paper, we are proposing to add a suite of asynchronous algorithms that accept senders, not awaitables. One might wonder whether and how these algorithms will be accessible to those who choose coroutines instead of senders.

In truth there will be no problem because all generally awaitable types automatically model the sender concept. The adaptation is transparent and happens in the sender customization points, which are aware of awaitables. (By "generally awaitable" we mean types that don’t require custom await_transform trickery from a promise type to make them awaitable.)

For an example, imagine a coroutine type called task<T> that knows nothing about senders. It doesn’t implement any of the sender customization points. Despite that fact, and despite the fact that the this_thread::sync_wait algorithm is constrained with the sender concept, the following would compile and do what the user wants:

task<int> doSomeAsyncWork();

int main() {
  // OK, awaitable types satisfy the requirements for senders:
  auto o = this_thread::sync_wait(doSomeAsyncWork());
}

Since awaitables are senders, writing a sender-based asynchronous algorithm is trivial if you have a coroutine task type: implement the algorithm as a coroutine. If you are not bothered by the possibility of allocations and indirections as a result of using coroutines, then there is no need to ever write a sender, a receiver, or an operation state.

4.16. Many senders can be trivially made awaitable

If you choose to implement your sender-based algorithms as coroutines, you’ll run into the issue of how to retrieve results from a passed-in sender. This is not a problem. If the coroutine type opts in to sender support -- trivial with the execution::with_awaitable_senders utility -- then a large class of senders are transparently awaitable from within the coroutine.

For example, consider the following trivial implementation of the sender-based retry algorithm:

template<class S>
  requires single-sender<S&> // see [exec.as.awaitable]
task<single-sender-value-type<S>> retry(S s) {
  for (;;) {
    try {
      co_return co_await s;
    } catch(...) {
    }
  }
}

Only some senders can be made awaitable directly because of the fact that callbacks are more expressive than coroutines. An awaitable expression has a single type: the result value of the async operation. In contrast, a callback can accept multiple arguments as the result of an operation. What’s more, the callback can have overloaded function call signatures that take different sets of arguments. There is no way to automatically map such senders into awaitables. The with_awaitable_senders utility recognizes as awaitables those senders that send a single value of a single type. To await another kind of sender, a user would have to first map its value channel into a single value of a single type -- say, with the into_variant sender algorithm -- before co_await-ing that sender.

4.17. Cancellation of a sender can unwind a stack of coroutines

When looking at the sender-based retry algorithm in the previous section, we can see that the value and error cases are correctly handled. But what about cancellation? What happens to a coroutine that is suspended awaiting a sender that completes by calling execution::set_stopped?

When your task type’s promise inherits from with_awaitable_senders, what happens is this: the coroutine behaves as if an uncatchable exception had been thrown from the co_await expression. (It is not really an exception, but it’s helpful to think of it that way.) Provided that the promise types of the calling coroutines also inherit from with_awaitable_senders, or more generally implement a member function called unhandled_stopped, the exception unwinds the chain of coroutines as if an exception were thrown except that it bypasses catch(...) clauses.

In order to "catch" this uncatchable stopped exception, one of the calling coroutines in the stack would have to await a sender that maps the stopped channel into either a value or an error. That is achievable with the execution::let_stopped, execution::upon_stopped, execution::stopped_as_optional, or execution::stopped_as_error sender adaptors. For instance, we can use execution::stopped_as_optional to "catch" the stopped signal and map it into an empty optional as shown below:

if (auto opt = co_await execution::stopped_as_optional(some_sender)) {
  // OK, some_sender completed successfully, and opt contains the result.
} else {
  // some_sender completed with a cancellation signal.
}

As described in the section "All awaitables are senders", the sender customization points recognize awaitables and adapt them transparently to model the sender concept. When connect-ing an awaitable and a receiver, the adaptation layer awaits the awaitable within a coroutine that implements unhandled_stopped in its promise type. The effect of this is that an "uncatchable" stopped exception propagates seamlessly out of awaitables, causing execution::set_stopped to be called on the receiver.

Obviously, unhandled_stopped is a library extension of the coroutine promise interface. Many promise types will not implement unhandled_stopped. When an uncatchable stopped exception tries to propagate through such a coroutine, it is treated as an unhandled exception and terminate is called. The solution, as described above, is to use a sender adaptor to handle the stopped exception before awaiting it. It goes without saying that any future Standard Library coroutine types ought to implement unhandled_stopped. The author of Add lazy coroutine (coroutine task) type, which proposes a standard coroutine task type, is in agreement.

4.18. Composition with parallel algorithms

The C++ Standard Library provides a large number of algorithms that offer the potential for non-sequential execution via the use of execution policies. The set of algorithms with execution policy overloads are often referred to as "parallel algorithms", although additional policies are available.

Existing policies, such as execution::par, give the implementation permission to execute the algorithm in parallel. However, the choice of execution resources used to perform the work is left to the implementation.

We will propose a customization point for combining schedulers with policies in order to provide control over where work will execute.

template<class ExecutionPolicy>
unspecified executing_on(
    execution::scheduler auto scheduler,
    ExecutionPolicy && policy
);

This function would return an object of an unspecified type which can be used in place of an execution policy as the first argument to one of the parallel algorithms. The overload selected by that object should execute its computation as requested by policy while using scheduler to create any work to be run. The expression may be ill-formed if scheduler is not able to support the given policy.

The existing parallel algorithms are synchronous; all of the effects performed by the computation are complete before the algorithm returns to its caller. This remains unchanged with the executing_on customization point.

In the future, we expect additional papers will propose asynchronous forms of the parallel algorithms which (1) return senders rather than values or void and (2) where a customization point pairing a sender with an execution policy would similarly be used to obtain an object of unspecified type to be provided as the first argument to the algorithm.

4.19. User-facing sender factories

A sender factory is an algorithm that takes no senders as parameters and returns a sender.

4.19.1. execution::schedule

execution::sender auto schedule(
    execution::scheduler auto scheduler
);

Returns a sender describing the start of a task graph on the provided scheduler. See § 4.2 Schedulers represent execution resources.

execution::scheduler auto sch1 = get_system_thread_pool().scheduler();

execution::sender auto snd1 = execution::schedule(sch1);
// snd1 describes the creation of a new task on the system thread pool

4.19.2. execution::just

execution::sender auto just(
    auto ...&& values
);

Returns a sender with no completion schedulers, which sends the provided values. The input values are decay-copied into the returned sender. When the returned sender is connected to a receiver, the values are moved into the operation state if the sender is an rvalue; otherwise, they are copied. Then xvalues referencing the values in the operation state are passed to the receiver’s set_value.

execution::sender auto snd1 = execution::just(3.14);
execution::sender auto then1 = execution::then(snd1, [] (double d) {
  std::cout << d << "\n";
});

execution::sender auto snd2 = execution::just(3.14, 42);
execution::sender auto then2 = execution::then(snd2, [] (double d, int i) {
  std::cout << d << ", " << i << "\n";
});

std::vector v3{1, 2, 3, 4, 5};
execution::sender auto snd3 = execution::just(v3);
execution::sender auto then3 = execution::then(snd3, [] (std::vector<int>&& v3copy) {
  for (auto&& e : v3copy) { e *= 2; }
  return std::move(v3copy);
}
auto&& [v3copy] = this_thread::sync_wait(then3).value();
// v3 contains {1, 2, 3, 4, 5}; v3copy will contain {2, 4, 6, 8, 10}.

execution::sender auto snd4 = execution::just(std::vector{1, 2, 3, 4, 5});
execution::sender auto then4 = execution::then(std::move(snd4), [] (std::vector<int>&& v4) {
  for (auto&& e : v4) { e *= 2; }
  return std::move(v4);
});
auto&& [v4] = this_thread::sync_wait(std::move(then4)).value();
// v4 contains {2, 4, 6, 8, 10}. No vectors were copied in this example.

4.19.3. execution::just_error

execution::sender auto just_error(
    auto && error
);

Returns a sender with no completion schedulers, which completes with the specified error. If the provided error is an lvalue reference, a copy is made inside the returned sender and a non-const lvalue reference to the copy is sent to the receiver’s set_error. If the provided value is an rvalue reference, it is moved into the returned sender and an rvalue reference to it is sent to the receiver’s set_error.

4.19.4. execution::just_stopped

execution::sender auto just_stopped();

Returns a sender with no completion schedulers, which completes immediately by calling the receiver’s set_stopped.

4.19.5. execution::read_env

execution::sender auto read_env(auto tag);

Returns a sender that reaches into a receiver’s environment and pulls out the current value associated with the customization point denoted by Tag. It then sends the value read back to the receiver through the value channel. For instance, read_env(get_scheduler) is a sender that asks the receiver for the currently suggested scheduler and passes it to the receiver’s set_value completion-signal.

This can be useful when scheduling nested dependent work. The following sender pulls the current schduler into the value channel and then schedules more work onto it.

execution::sender auto task =
  execution::read_env(get_scheduler)
    | execution::let_value([](auto sched) {
        return execution::starts_on(sched, some nested work here);
    });

this_thread::sync_wait( std::move(task) ); // wait for it to finish

This code uses the fact that sync_wait associates a scheduler with the receiver that it connects with task. read_env(get_scheduler) reads that scheduler out of the receiver, and passes it to let_value’s receiver’s set_value function, which in turn passes it to the lambda. That lambda returns a new sender that uses the scheduler to schedule some nested work onto sync_wait’s scheduler.

4.20. User-facing sender adaptors

A sender adaptor is an algorithm that takes one or more senders, which it may execution::connect, as parameters, and returns a sender, whose completion is related to the sender arguments it has received.

Sender adaptors are lazy, that is, they are never allowed to submit any work for execution prior to the returned sender being started later on, and are also guaranteed to not start any input senders passed into them. Sender consumers such as § 4.21.1 this_thread::sync_wait start senders.

For more implementer-centric description of starting senders, see § 5.5 Sender adaptors are lazy.

4.20.1. execution::continues_on

execution::sender auto continues_on(
    execution::sender auto input,
    execution::scheduler auto scheduler
);

Returns a sender describing the transition from the execution agent of the input sender to the execution agent of the target scheduler. See § 4.6 Execution resource transitions are explicit.

execution::scheduler auto cpu_sched = get_system_thread_pool().scheduler();
execution::scheduler auto gpu_sched = cuda::scheduler();

execution::sender auto cpu_task = execution::schedule(cpu_sched);
// cpu_task describes the creation of a new task on the system thread pool

execution::sender auto gpu_task = execution::continues_on(cpu_task, gpu_sched);
// gpu_task describes the transition of the task graph described by cpu_task to the gpu

4.20.2. execution::then

execution::sender auto then(
    execution::sender auto input,
    std::invocable<values-sent-by(input)...> function
);

then returns a sender describing the task graph described by the input sender, with an added node of invoking the provided function with the values sent by the input sender as arguments.

then is guaranteed to not begin executing function until the returned sender is started.

execution::sender auto input = get_input();
execution::sender auto snd = execution::then(input, [](auto... args) {
    std::print(args...);
});
// snd describes the work described by pred
// followed by printing all of the values sent by pred

This adaptor is included as it is necessary for writing any sender code that actually performs a useful function.

4.20.3. execution::upon_*

execution::sender auto upon_error(
    execution::sender auto input,
    std::invocable<errors-sent-by(input)...> function
);

execution::sender auto upon_stopped(
    execution::sender auto input,
    std::invocable auto function
);

upon_error and upon_stopped are similar to then, but where then works with values sent by the input sender, upon_error works with errors, and upon_stopped is invoked when the "stopped" signal is sent.

4.20.4. execution::let_*

execution::sender auto let_value(
    execution::sender auto input,
    std::invocable<values-sent-by(input)...> function
);

execution::sender auto let_error(
    execution::sender auto input,
    std::invocable<errors-sent-by(input)...> function
);

execution::sender auto let_stopped(
    execution::sender auto input,
    std::invocable auto function
);

let_value is very similar to then: when it is started, it invokes the provided function with the values sent by the input sender as arguments. However, where the sender returned from then sends exactly what that function ends up returning - let_value requires that the function return a sender, and the sender returned by let_value sends the values sent by the sender returned from the callback. This is similar to the notion of "future unwrapping" in future/promise-based frameworks.

let_value is guaranteed to not begin executing function until the returned sender is started.

let_error and let_stopped are similar to let_value, but where let_value works with values sent by the input sender, let_error works with errors, and let_stopped is invoked when the "stopped" signal is sent.

4.20.5. execution::starts_on

execution::sender auto starts_on(
    execution::scheduler auto sched,
    execution::sender auto snd
);

Returns a sender which, when started, will start the provided sender on an execution agent belonging to the execution resource associated with the provided scheduler. This returned sender has no completion schedulers.

4.20.6. execution::into_variant

execution::sender auto into_variant(
    execution::sender auto snd
);

Returns a sender which sends a variant of tuples of all the possible sets of types sent by the input sender. Senders can send multiple sets of values depending on runtime conditions; this is a helper function that turns them into a single variant value.

4.20.7. execution::stopped_as_optional

execution::sender auto stopped_as_optional(
    single-sender auto snd
);

Returns a sender that maps the value channel from a T to an optional<decay_t<T>>, and maps the stopped channel to a value of an empty optional<decay_t<T>>.

4.20.8. execution::stopped_as_error

template<move_constructible Error>
execution::sender auto stopped_as_error(
    execution::sender auto snd,
    Error err
);

Returns a sender that maps the stopped channel to an error of err.

4.20.9. execution::bulk

execution::sender auto bulk(
    execution::sender auto input,
    std::integral auto shape,
    invocable<decltype(size), values-sent-by(input)...> function
);

Returns a sender describing the task of invoking the provided function with every index in the provided shape along with the values sent by the input sender. The returned sender completes once all invocations have completed, or an error has occurred. If it completes by sending values, they are equivalent to those sent by the input sender.

No instance of function will begin executing until the returned sender is started. Each invocation of function runs in an execution agent whose forward progress guarantees are determined by the scheduler on which they are run. All agents created by a single use of bulk execute with the same guarantee. The number of execution agents used by bulk is not specified. This allows a scheduler to execute some invocations of the function in parallel.

In this proposal, only integral types are used to specify the shape of the bulk section. We expect that future papers may wish to explore extensions of the interface to explore additional kinds of shapes, such as multi-dimensional grids, that are commonly used for parallel computing tasks.

4.20.10. execution::split

execution::sender auto split(execution::sender auto sender);

If the provided sender is a multi-shot sender, returns that sender. Otherwise, returns a multi-shot sender which sends values equivalent to the values sent by the provided sender. See § 4.7 Senders can be either multi-shot or single-shot.

4.20.11. execution::when_all

execution::sender auto when_all(
    execution::sender auto ...inputs
);

execution::sender auto when_all_with_variant(
    execution::sender auto ...inputs
);

when_all returns a sender that completes once all of the input senders have completed. It is constrained to only accept senders that can complete with a single set of values (_i.e._, it only calls one overload of set_value on its receiver). The values sent by this sender are the values sent by each of the input senders, in order of the arguments passed to when_all. It completes inline on the execution resource on which the last input sender completes, unless stop is requested before when_all is started, in which case it completes inline within the call to start.

when_all_with_variant does the same, but it adapts all the input senders using into_variant, and so it does not constrain the input arguments as when_all does.

The returned sender has no completion schedulers.

execution::scheduler auto sched = thread_pool.scheduler();

execution::sender auto sends_1 = ...;
execution::sender auto sends_abc = ...;

execution::sender auto both = execution::when_all(
    sends_1,
    sends_abc
);

execution::sender auto final = execution::then(both, [](auto... args){
    std::cout << std::format("the two args: {}, {}", args...);
});
// when final executes, it will print "the two args: 1, abc"

4.21. User-facing sender consumers

A sender consumer is an algorithm that takes one or more senders, which it may execution::connect, as parameters, and does not return a sender.

4.21.1. this_thread::sync_wait

auto sync_wait(
    execution::sender auto sender
) requires (always-sends-same-values(sender))
    -> std::optional<std::tuple<values-sent-by(sender)>>;

this_thread::sync_wait is a sender consumer that submits the work described by the provided sender for execution, blocking the current std::thread or thread of main until the work is completed, and returns an optional tuple of values that were sent by the provided sender on its completion of work. Where § 4.19.1 execution::schedule and § 4.19.2 execution::just are meant to enter the domain of senders, sync_wait is one way to exit the domain of senders, retrieving the result of the task graph.

If the provided sender sends an error instead of values, sync_wait throws that error as an exception, or rethrows the original exception if the error is of type std::exception_ptr.

If the provided sender sends the "stopped" signal instead of values, sync_wait returns an empty optional.

For an explanation of the requires clause, see § 5.8 All senders are typed. That clause also explains another sender consumer, built on top of sync_wait: sync_wait_with_variant.

Note: This function is specified inside std::this_thread, and not inside execution. This is because sync_wait has to block the current execution agent, but determining what the current execution agent is is not reliable. Since the standard does not specify any functions on the current execution agent other than those in std::this_thread, this is the flavor of this function that is being proposed. If C++ ever obtains fibers, for instance, we expect that a variant of this function called std::this_fiber::sync_wait would be provided. We also expect that runtimes with execution agents that use different synchronization mechanisms than std::thread’s will provide their own flavors of sync_wait as well (assuming their execution agents have the means to block in a non-deadlock manner).

5. Design - implementer side

5.1. Receivers serve as glue between senders

A receiver is a callback that supports more than one channel. In fact, it supports three of them:

Once an async operation has been started exactly one of these functions must be invoked on a receiver before it is destroyed.

While the receiver interface may look novel, it is in fact very similar to the interface of std::promise, which provides the first two signals as set_value and set_exception, and it’s possible to emulate the third channel with lifetime management of the promise.

Receivers are not a part of the end-user-facing API of this proposal; they are necessary to allow unrelated senders communicate with each other, but the only users who will interact with receivers directly are authors of senders.

Receivers are what is passed as the second argument to § 5.3 execution::connect.

5.2. Operation states represent work

An operation state is an object that represents work. Unlike senders, it is not a chaining mechanism; instead, it is a concrete object that packages the work described by a full sender chain, ready to be executed. An operation state is neither movable nor copyable, and its interface consists of a single algorithm: start, which serves as the submission point of the work represented by a given operation state.

Operation states are not a part of the user-facing API of this proposal; they are necessary for implementing sender consumers like this_thread::sync_wait, and the knowledge of them is necessary to implement senders, so the only users who will interact with operation states directly are authors of senders and authors of sender algorithms.

The return value of § 5.3 execution::connect must satisfy the operation state concept.

5.3. execution::connect

execution::connect is a customization point which connects senders with receivers, resulting in an operation state that will ensure that if start is called that one of the completion operations will be called on the receiver passed to connect.

execution::sender auto snd = some input sender;
execution::receiver auto rcv = some receiver;
execution::operation_state auto state = execution::connect(snd, rcv);

execution::start(state);
// at this point, it is guaranteed that the work represented by state has been submitted
// to an execution resource, and that execution resource will eventually call one of the
// completion operations on rcv

// operation states are not movable, and therefore this operation state object must be
// kept alive until the operation finishes

5.4. Sender algorithms are customizable

Senders being able to advertise what their completion schedulers are fulfills one of the promises of senders: that of being able to customize an implementation of a sender algorithm based on what scheduler any work it depends on will complete on.

The simple way to provide customizations for functions like then, that is for sender adaptors and sender consumers, is to follow the customization scheme that has been adopted for C++20 ranges library; to do that, we would define the expression execution::then(sender, invocable) to be equivalent to:

  1. sender.then(invocable), if that expression is well-formed; otherwise

  2. then(sender, invocable), performed in a context where this call always performs ADL, if that expression is well-formed; otherwise

  3. a default implementation of then, which returns a sender adaptor, and then define the exact semantics of said adaptor.

However, this definition is problematic. Imagine another sender adaptor, bulk, which is a structured abstraction for a loop over an index space. Its default implementation is just a for loop. However, for accelerator runtimes like CUDA, we would like sender algorithms like bulk to have specialized behavior, which invokes a kernel of more than one thread (with its size defined by the call to bulk); therefore, we would like to customize bulk for CUDA senders to achieve this. However, there’s no reason for CUDA kernels to necessarily customize the then sender adaptor, as the generic implementation is perfectly sufficient. This creates a problem, though; consider the following snippet:

execution::scheduler auto cuda_sch = cuda_scheduler{};

execution::sender auto initial = execution::schedule(cuda_sch);
// the type of initial is a type defined by the cuda_scheduler
// let’s call it cuda::schedule_sender<>

execution::sender auto next = execution::then(cuda_sch, []{ return 1; });
// the type of next is a standard-library unspecified sender adaptor
// that wraps the cuda sender
// let’s call it execution::then_sender_adaptor<cuda::schedule_sender<>>

execution::sender auto kernel_sender = execution::bulk(next, shape, [](int i){ ... });

How can we specialize the bulk sender adaptor for our wrapped schedule_sender? Well, here’s one possible approach, taking advantage of ADL (and the fact that the definition of "associated namespace" also recursively enumerates the associated namespaces of all template parameters of a type):

namespace cuda::for_adl_purposes {
template<typename... SentValues>
class schedule_sender {
    execution::operation_state auto connect(execution::receiver auto rcv);
    execution::scheduler auto get_completion_scheduler() const;
};

execution::sender auto bulk(
    execution::sender auto && input,
    execution::shape auto && shape,
    invocable%lt;sender-values(input)> auto && fn)
{
    // return a cuda sender representing a bulk kernel launch
}
} // namespace cuda::for_adl_purposes

However, if the input sender is not just a then_sender_adaptor like in the example above, but another sender that overrides bulk by itself, as a member function, because its author believes they know an optimization for bulk - the specialization above will no longer be selected, because a member function of the first argument is a better match than the ADL-found overload.

This means that well-meant specialization of sender algorithms that are entirely scheduler-agnostic can have negative consequences. The scheduler-specific specialization - which is essential for good performance on platforms providing specialized ways to launch certain sender algorithms - would not be selected in such cases. But it’s really the scheduler that should control the behavior of sender algorithms when a non-default implementation exists, not the sender. Senders merely describe work; schedulers, however, are the handle to the runtime that will eventually execute said work, and should thus have the final say in how the work is going to be executed.

Therefore, we are proposing the following customization scheme: the expression execution::<sender-algorithm>(sender, args...), for any given sender algorithm that accepts a sender as its first argument, should do the following:

  1. Create a sender that implements the default implementation of the sender algorithm. That sender is tuple-like; it can be destructured into its constituent parts: algorithm tag, data, and child sender(s).

  2. We query the child sender for its domain. A domain is a tag type associated with the scheduler that the child sender will complete on. If there are multiple child senders, we query all of them for their domains and require that they all be the same.

  3. We use the domain to dispatch to a transform_sender customization, which accepts the sender and optionally performs a domain-specific transformation on it. This customization is expected to return a new sender, which will be returned from <sender-algorithm> in place of the original sender.

5.5. Sender adaptors are lazy

Contrary to early revisions of this paper, we propose to make all sender adaptors perform strictly lazy submission, unless specified otherwise.

Strictly lazy submission means that there is a guarantee that no work is submitted to an execution resource before a receiver is connected to a sender, and execution::start is called on the resulting operation state.

5.6. Lazy senders provide optimization opportunities

Because lazy senders fundamentally describe work, instead of describing or representing the submission of said work to an execution resource, and thanks to the flexibility of the customization of most sender algorithms, they provide an opportunity for fusing multiple algorithms in a sender chain together, into a single function that can later be submitted for execution by an execution resource. There are two ways this can happen.

The first (and most common) way for such optimizations to happen is thanks to the structure of the implementation: because all the work is done within callbacks invoked on the completion of an earlier sender, recursively up to the original source of computation, the compiler is able to see a chain of work described using senders as a tree of tail calls, allowing for inlining and removal of most of the sender machinery. In fact, when work is not submitted to execution resources outside of the current thread of execution, compilers are capable of removing the senders abstraction entirely, while still allowing for composition of functions across different parts of a program.

The second way for this to occur is when a sender algorithm is specialized for a specific set of arguments. For instance, an implementation could recognize two subsequent § 4.20.9 execution::bulks of compatible shapes, and merge them together into a single submission of a GPU kernel.

5.7. Execution resource transitions are two-step

Because execution::continues_on takes a sender as its first argument, it is not actually directly customizable by the target scheduler. This is by design: the target scheduler may not know how to transition from a scheduler such as a CUDA scheduler; transitioning away from a GPU in an efficient manner requires making runtime calls that are specific to the GPU in question, and the same is usually true for other kinds of accelerators too (or for scheduler running on remote systems). To avoid this problem, specialized schedulers like the ones mentioned here can still hook into the transition mechanism, and inject a sender which will perform a transition to the regular CPU execution resource, so that any sender can be attached to it.

This, however, is a problem: because customization of sender algorithms must be controlled by the scheduler they will run on (see § 5.4 Sender algorithms are customizable), the type of the sender returned from continues_on must be controllable by the target scheduler. Besides, the target scheduler may itself represent a specialized execution resource, which requires additional work to be performed to transition to it. GPUs and remote node schedulers are once again good examples of such schedulers: executing code on their execution resources requires making runtime API calls for work submission, and quite possibly for the data movement of the values being sent by the input sender passed into continues_on.

To allow for such customization from both ends, we propose the inclusion of a secondary transitioning sender adaptor, called schedule_from. This adaptor is a form of schedule, but takes an additional, second argument: the input sender. This adaptor is not meant to be invoked manually by the end users; they are always supposed to invoke continues_on, to ensure that both schedulers have a say in how the transitions are made. Any scheduler that specializes continues_on(snd, sch) shall ensure that the return value of their customization is equivalent to schedule_from(sch, snd2), where snd2 is a successor of snd that sends values equivalent to those sent by snd.

The default implementation of continues_on(snd, sched) is schedule_from(sched, snd).

5.8. All senders are typed

All senders must advertise the types they will send when they complete. There are many sender adaptors that need this information. Even just transitioning from one execution context to another requires temporarily storing the async result data so it can be propagated in the new execution context. Doing that efficiently requires knowing the type of the data.

The mechanism a sender uses to advertise its completions is the get_completion_signatures customization point, which takes an environment and must return a specialization of the execution::completion_signatures class template. The template parameters of execution::completion_signatures is a list of function types that represent the completion operations of the sender. for example, the type execution::set_value_t(size_t, const char*) indicates that the sender can complete successfully by passing a size_t and a const char* to the receiver’s set_value function.

This proposal includes utilities for parsing and manipulating the list of a sender’s completion signatures. For instance, values_of_t is a template alias for accessing a sender’s value completions. It takes a sender, an environment, and two variadic template template parameters: a tuple-like template and a variant-like template. You can get the value completions of S and Env with value_types_of_t<S, Env, tuple-like, variant-like>. For example, for a sender that can complete successfully with either Ts... or Us..., value_types_of_t<S, Env, std::tuple, std::variant> would name the type std::variant<std::tuple<Ts...>, std::tuple<Us...>>.

5.9. Customization points

Earlier versions of this paper used a dispatching technique known as tag_invoke (see tag_invoke: A general pattern for supporting customisable functions) to allow for customization of basis operations and sender algorithms. This technique used private friend functions named "tag_invoke" that are found by argument-dependent look-up. The tag_invoke overloads are distinguished from each other by their first argument, which is the type of the customization point object being customized. For instance, to customize the execution::set_value operation, a receiver type might do the following:

struct my_receiver {
  friend void tag_invoke(execution::set_value_t, my_receiver&& self, int value) noexcept {
    std::cout << "received value: " << value;
  }
  //...
};

The tag_invoke technique, although it had its strengths, has been replaced with a new (or rather, a very old) technique that uses explicit concept opt-ins and named member functions. For instance, the execution::set_value operation is now customized by defining a member function named set_value in the receiver type. This technique is more explicit and easier to understand than tag_invoke. This is what a receiver author would do to customize execution::set_value now:

struct my_receiver {
  using receiver_concept = execution::receiver_t;

  void set_value(int value) && noexcept {
    std::cout << "received value: " << value;
  }
  //...
};

The only exception to this is the customization of queries. There is a need to build queryable adaptors that can forward an open and unknowable set of queries to some wrapped object. This is done by defining a member function named query in the adaptor type that takes the query CPO object as its first (and usually only) argument. A queryable adaptor might look like this:

template<class Query, class Queryable, class... Args>
concept query_for =
  requires (const Queryable& o, Args&&... args) {
    o.query(Query(), (Args&&) args...);
  };

template<class Allocator = std::allocator<>,
        class Base = execution::empty_env>
struct with_allocator {
  Allocator alloc{};
  Base base{};

  // Forward unknown queries to the wrapped object:
  template<query_for<Base> Query>
  decltype(auto) query(Query q) const {
    return base.query(q);
  }

  // Specialize the query for the allocator:
  Allocator query(execution::get_allocator_t) const {
    return alloc;
  }
};

Customization of sender algorithms such as execution::then and execution::bulk are handled differently because they must dispatch based on where the sender is executing. See the section on § 5.4 Sender algorithms are customizable for more information.

6. Specification

Much of this wording follows the wording of A Unified Executors Proposal for C++.

§ 22 General utilities library [utilities] is meant to be a diff relative to the wording of the [utilities] clause of Working Draft, Standard for Programming Language C++.

§ 33 Concurrency support library [thread] is meant to be a diff relative to the wording of the [thread] clause of Working Draft, Standard for Programming Language C++. This diff applies changes from Composable cancellation for sender-based async operations.

§ 34 Execution control library [exec] is meant to be added as a new library clause to the working draft of C++.

7.

8.

9.

10.

11.

12.

13.

14. Exception handling [except]

14.6. Special functions [except.special]

14.6.2. The std::terminate function [except.terminate]

At the end of the bulleted list in the Note in paragraph 1, add a new bullet as follows:

  • when a call to a wait(), wait_until(), or wait_for() function on a condition variable (33.7.4, 33.7.5) fails to meet a postcondition.

  • when a callback invocation exits via an exception when requesting stop on a std::stop_source or a std::inplace_stop_source ([stopsource.mem], [stopsource.inplace.mem]), or in the constructor of std::stop_callback or std::inplace_stop_callback ([stopcallback.cons], [stopcallback.inplace.cons]) when a callback invocation exits via an exception.

  • when a run_loop object is destroyed that is still in the running state ([exec.run.loop]).

  • when unhandled_stopped() is called on a with_awaitable_senders<T> object ([exec.with.awaitable.senders]) whose continuation is not a handle to a coroutine whose promise type has an unhandled_stopped() member function.

16. Library introduction [library]

At the end of [expos.only.entity], add the following:

  1. The following are defined for exposition only to aid in the specification of the library:

    namespace std {
      // ...as before...
    }
    
  1. An object dst is said to be decay-copied from a subexpression src if the type of dst is decay_t<decltype((src))>, and dst is copy-initialized from src.

16.4.4.6.1. General [allocator.requirements.general]

At the end of [allocator.requirements.general], add the following new paragraph:

  1. [Example 2: The following is an allocator class template supporting the minimal interface that meets the requirements of [allocator.requirements.general]:

    template<class T>
    struct SimpleAllocator {
      using value_type = T;
      SimpleAllocator(ctor args);
    
      template<class U> SimpleAllocator(const SimpleAllocator<U>& other);
    
      T* allocate(std::size_t n);
      void deallocate(T* p, std::size_t n);
    
      template<class U> bool operator==(const SimpleAllocator<U>& rhs) const;
    };
    

    -- end example]

  1. The following exposition-only concept defines the minimal requirements on an Allocator type.

    template<class Alloc>
    concept simple-allocator =
      requires(Alloc alloc, size_t n) {
        { *alloc.allocate(n) } -> same_as<typename Alloc::value_type&>;
        { alloc.deallocate(alloc.allocate(n), n) };
      } &&
      copy_constructible<Alloc> &&
      equality_comparable<Alloc>;
    
    1. A type Alloc models simple-allocator if it meets the requirements of [allocator.requirements.general].

17. Language support library [cpp]

17.3. Implementation properties [support.limits]

17.3.2. Header <version> synopsis [version.syn]

To the <version> synopsis, add the following:

#define __cpp_lib_semaphore       201907L         // also in <semaphore>
#define __cpp_lib_senders         2024XXL         // also in <execution>
#define __cpp_lib_shared_mutex    201505L         // also in <shared_mutex>

22. General utilities library [utilities]

22.10. Function objects [function.objects]

22.10.2. Header <functional> synopsis [functional.syn]

At the end of this subclause, insert the following declarations into the synopsis within namespace std:

namespace std {
  // ...as before...

  namespace ranges {
    // 22.10.9, concept-constrained comparisons
    struct equal_to;                                    // freestanding
    struct not_equal_to;                                // freestanding
    struct greater;                                     // freestanding
    struct less;                                        // freestanding
    struct greater_equal;                               // freestanding
    struct less_equal;                                  // freestanding
  }

template<class Fn, class... Args> concept callable = // exposition only requires (Fn&& fn, Args&&... args) { std::forward<Fn>(fn)(std::forward<Args>(args)...); }; template<class Fn, class... Args> concept nothrow-callable = // exposition only callable<Fn, Args...> && requires (Fn&& fn, Args&&... args) { { std::forward<Fn>(fn)(std::forward<Args>(args)...) } noexcept; }; // exposition only: template<class Fn, class... Args> using call-result-t = decltype(declval<Fn>()(declval<Args>()...)); template<const auto& Tag> using decayed-typeof = decltype(auto(Tag)); // exposition only
}

33. Concurrency support library [thread]

33.3. Stop tokens [thread.stoptoken]

33.3.1. Introduction [thread.stoptoken.intro]

  1. Subclause [thread.stoptoken] describes components that can be used to asynchronously request that an operation stops execution in a timely manner, typically because the result is no longer required. Such a request is called a stop request.

  2. stop_source, stop_token, and stop_callback implement stoppable-source, stoppable_token, and stoppable-callback-for are concepts that specify the required syntax and semantics of shared ownership of access to a stop state. Any stop_source, stop_token, or stop_callback object that shares ownership of the same stop state is an associated stop_source, stop_token, or stop_callback, respectively. Any object modeling stoppable-source, stoppable_token, or stoppable-callback-for that refers to the same stop state is an associated stoppable-source, stoppable_token, or stoppable-callback-for, respectively. The last remaining owner of the stop state automatically releases the resources associated with the stop state.
  3. A n object of a type that models stoppable_token can be passed to an operation which that can either

    • actively poll the token to check if there has been a stop request, or

    • register a callback using the stop_callback class template which that will be called in the event that a stop request is made.

    A stop request made via a stop_source an object whose type models stoppable-source will be visible to all associated stoppable_token and stop_source stoppable-source objects. Once a stop request has been made it cannot be withdrawn (a subsequent stop request has no effect).

  4. Callbacks registered via a stop_callback object an object whose type models stoppable-callback-for are called when a stop request is first made by any associated stop_source stoppable-source object.

The following paragraph is moved to the specification of the new stoppable-source concept.

  1. Calls to the functions request_stop, stop_requested, and stop_possible do not introduce data races. A call to request_stop that returns true synchronizes with a call to stop_requested on an associated stop_token or stop_source object that returns true. Registration of a callback synchronizes with the invocation of that callback.

  1. The types stop_source and stop_token and the class template stop_callback implement the semantics of shared ownership of a stop state. The last remaining owner of the stop state automatically releases the resources associated with the stop state.

  2. An object of type inplace_stop_source is the sole owner of its stop state. An object of type inplace_stop_token or of a specialization of the class template inplace_stop_callback does not participate in ownership of its associated stop state. They are for use when all uses of the associated token and callback objects are known to nest within the lifetime of the inplace_stop_source object.

33.3.2. Header <stop_token> synopsis [thread.stoptoken.syn]

In this subclause, insert the following declarations into the <stop_token> synopsis:

namespace std {
// [stoptoken.concepts], stop token concepts template<class CallbackFn, class Token, class Initializer = CallbackFn> concept stoppable-callback-for = see below; // exposition only template<class Token> concept stoppable_token = see below; template<class Token> concept unstoppable_token = see below; template<class Source> concept stoppable-source = see below; // exposition only
// 33.3.3, class stop_token class stop_token; // 33.3.4, class stop_source class stop_source; // no-shared-stop-state indicator struct nostopstate_t { explicit nostopstate_t() = default; }; inline constexpr nostopstate_t nostopstate{}; // 33.3.5, class template stop_callback template<class CallbackFn> class stop_callback;
// [stoptoken.never], class never_stop_token class never_stop_token; // [stoptoken.inplace], class inplace_stop_token class inplace_stop_token; // [stopsource.inplace], class inplace_stop_source class inplace_stop_source; // [stopcallback.inplace], class template inplace_stop_callback template<class CallbackFn> class inplace_stop_callback; template<class T, class CallbackFn> using stop_callback_for_t = T::template callback_type<CallbackFn>;
}

Insert the following subclause as a new subclause between Header <stop_token> synopsis [thread.stoptoken.syn] and Class stop_token [stoptoken].

33.3.3. Stop token concepts [stoptoken.concepts]

  1. The exposition-only stoppable-callback-for concept checks for a callback compatible with a given Token type.

    template<class CallbackFn, class Token, class Initializer = CallbackFn>
      concept stoppable-callback-for = // exposition only
        invocable<CallbackFn> &&
        constructible_from<CallbackFn, Initializer> &&
        requires { typename stop_callback_for_t<Token, CallbackFn>; } &&
        constructible_from<stop_callback_for_t<Token, CallbackFn>, const Token&, Initializer>;
    
  2. Let t and u be distinct, valid objects of type Token that reference the same logical stop state; let init be an expression such that same_as<decltype(init), Initializer> is true; and let SCB denote the type stop_callback_for_t<Token, CallbackFn>.

  3. The concept stoppable-callback-for<CallbackFn, Token, Initializer> is modeled only if:

    1. The following concepts are modeled:

      • constructible_from<SCB, Token, Initializer>

      • constructible_from<SCB, Token&, Initializer>

      • constructible_from<SCB, const Token, Initializer>

    2. An object of type SCB has an associated callback function of type CallbackFn. Let scb be an object of type SCB and let callback_fn denote scb’s associated callback function. Direct-non-list-initializing scb from arguments t and init shall execute a stoppable callback registration as follows:

      1. If t.stop_possible() is true:

        1. callback_fn shall be direct-initialized with init.

        2. Construction of scb shall only throw exceptions thrown by the initialization of callback_fn from init.

        3. The callback invocation std::forward<CallbackFn>(callback_fn)() shall be registered with t’s associated stop state as follows:

          1. If t.stop_requested() evaluates to false at the time of registration, the callback invocation is added to the stop state’s list of callbacks such that std::forward<CallbackFn>(callback_fn)() is evaluated if a stop request is made on the stop state.

          2. Otherwise, std::forward<CallbackFn>(callback_fn)() shall be immediately evaluated on the thread executing scb’s constructor, and the callback invocation shall not be added to the list of callback invocations.

        4. If the callback invocation was added to stop state’s list of callbacks, scb shall be associated with the stop state.

      2. If t.stop_possible() is false, there is no requirement that the initialization of scb causes the initialization of callback_fn.

    3. Destruction of scb shall execute a stoppable callback deregistration as follows (in order):

      1. If the constructor of scb did not register a callback invocation with t’s stop state, then the stoppable callback deregistration shall have no effect other than destroying callback_fn if it was constructed.

      2. Otherwise, the invocation of callback_fn shall be removed from the associated stop state.

      3. If callback_fn is concurrently executing on another thread then the stoppable callback deregistration shall block ([defns.block]) until the invocation of callback_fn returns such that the return from the invocation of callback_fn strongly happens before ([intro.races]) the destruction of callback_fn.

      4. If callback_fn is executing on the current thread, then the destructor shall not block waiting for the return from the invocation of callback_fn.

      5. A stoppable callback deregistration shall not block on the completion of the invocation of some other callback registered with the same logical stop state.

      6. The stoppable callback deregistration shall destroy callback_fn.

  4. The stoppable_token concept checks for the basic interface of a stop token that is copyable and allows polling to see if stop has been requested and also whether a stop request is possible. The unstoppable_token concept checks for a stoppable_token type that does not allow stopping.

    template<template<class> class>
      struct check-type-alias-exists; // exposition-only
    
    template<class Token>
      concept stoppable_token =
        requires (const Token tok) {
          typename check-type-alias-exists<Token::template callback_type>;
          { tok.stop_requested() } noexcept -> same_as<bool>;
          { tok.stop_possible() } noexcept -> same_as<bool>;
          { Token(tok) } noexcept; // see implicit expression variations
                                   // ([concepts.equality])
        } &&
        copyable<Token> &&
        equality_comparable<Token> &&
        swappable<Token>;
    
    template<class Token>
      concept unstoppable_token =
        stoppable_token<Token> &&
        requires (const Token tok) {
          requires bool_constant<(!tok.stop_possible())>::value;
        };
    
  5. An object whose type models stoppable_token has at most one associated logical stop state. A stoppable_token object with no associated stop state is said to be disengaged.

  6. Let SP be an evaluation of t.stop_possible() that is false, and let SR be an evaluation of t.stop_requested() that is true.

  7. The type Token models stoppable_token only if:

    1. Any evaluation of u.stop_possible() or u.stop_requested() that happens after ([intro.races]) SP is false.

    2. Any evaluation of u.stop_possible() or u.stop_requested() that happens after SR is true.

    3. For any types CallbackFn and Initializer such that stoppable-callback-for<CallbackFn, Token, Initializer> is satisfied, stoppable-callback-for<CallbackFn, Token, Initializer> is modeled.

    4. If t is disengaged, evaluations of t.stop_possible() and t.stop_requested() are false.

    5. If t and u reference the same stop state, or if both t and u are disengaged, t == u is true; otherwise, it is false.

  8. An object whose type models the exposition-only stoppable-source concept can be queried whether stop has been requested (stop_requested) and whether stop is possible (stop_possible). It is a factory for associated stop tokens (get_token), and a stop request can be made on it (request_stop). It maintains a list of registered stop callback invocations that it executes when a stop request is first made.

    template<class Source>
      concept stoppable-source = // exposition only
        requires (Source& src, const Source csrc) { // see implicit expression variations
                                                    // ([concepts.equality])
          { csrc.get_token() } -> stoppable_token;
          { csrc.stop_possible() } noexcept -> same_as<bool>;
          { csrc.stop_requested() } noexcept -> same_as<bool>;
          { src.request_stop() } -> same_as<bool>;
        };
    
    1. An object whose type models stoppable-source has at most one associated logical stop state. If it has no associated stop state, it is said to be disengaged. Let s be an object whose type models stoppable-source and that is disengaged. s.stop_possible() and s.stop_requested() shall return false.

    2. Let t be an object whose type models stoppable-source. If t is disengaged, t.get_token() shall return a disengaged stop token; otherwise, it shall return a stop token that is associated with the stop state of t.

    The following paragraph is moved from the introduction, with minor modifications (underlined in green).

    1. Calls to the member functions request_stop, stop_requested, and stop_possible and similarly named member functions on associated stoppable_token objects do not introduce data races. A call to request_stop that returns true synchronizes with a call to stop_requested on an associated stoppable_token or stop_source stoppable-source object that returns true. Registration of a callback synchronizes with the invocation of that callback.

    The following paragraph is taken from § 33.3.5.3 Member functions [stopsource.mem] and modified.

    1. If the stoppable-source is disengaged, request_stop shall have no effect and return false. Otherwise, it shall execute a stop request operation on the associated stop state. A stop request operation determines whether the stop state has received a stop request, and if not, makes a stop request. The determination and making of the stop request shall happen atomically, as-if by a read-modify-write operation ([intro.races]). If the request was made, the stop state’s registered callback invocations shall be synchronously executed. If an invocation of a callback exits via an exception then terminate shall be invoked ([except.terminate]). No constraint is placed on the order in which the callback invocations are executed. request_stop shall return true if a stop request was made, and false otherwise. After a call to request_stop either a call to stop_possible shall return false or a call to stop_requested shall return true.

      A stop request includes notifying all condition variables of type condition_variable_any temporarily registered during an interruptible wait ([thread.condvarany.intwait]).

Modify subclause [stoptoken] as follows:

33.3.4. Class stop_token [stoptoken]

33.3.4.1. General [stoptoken.general]
  1. The class stop_token provides an interface for querying whether a stop request has been made (stop_requested) or can ever be made (stop_possible) using an associated stop_source object ([stopsource]). A stop_token can also be passed to a stop_callback ([stopcallback]) constructor to register a callback to be called when a stop request has been made from an associated stop_source. The class stop_token models the concept stoppable_token. It shares ownership of its stop state, if any, with its associated stop_source object ([stopsource]) and any stop_token objects to which it compares equal.
namespace std {
  class stop_token {
  public:
template<class CallbackFn> using callback_type = stop_callback<CallbackFn>;
// [stoptoken.cons], constructors, copy, and assignment stop_token() noexcept = default;
stop_token(const stop_token&) noexcept; stop_token(stop_token&&) noexcept; stop_token& operator=(const stop_token&) noexcept; stop_token& operator=(stop_token&&) noexcept; ~stop_token();
// [stoptoken.mem], Member functions void swap(stop_token&) noexcept; // [stoptoken.mem], stop handling [[nodiscard]] bool stop_requested() const noexcept; [[nodiscard]] bool stop_possible() const noexcept; bool operator==(const stop_token& rhs) const noexcept = default; [[nodiscard]] friend bool operator==(const stop_token& lhs, const stop_token& rhs) noexcept; friend void swap(stop_token& lhs, stop_token& rhs) noexcept; private: shared_ptr<unspecified> stop-state{}; // exposition only }; }
  1. stop-state refers to the stop_token’s associated stop state. A stop_token object is disengaged when stop-state is empty.

33.3.4.2. Constructors, copy, and assignment [stoptoken.cons]
stop_token() noexcept;
  1. Postconditions: stop_possible() is false and stop_requested() is false. Because the created stop_token object can never receive a stop request, no resources are allocated for a stop state.

stop_token(const stop_token& rhs) noexcept;
  1. Postconditions: *this == rhs is true. *this and rhs share the ownership of the same stop state, if any.

stop_token(stop_token&& rhs) noexcept;
  1. Postconditions: *this contains the value of rhs prior to the start of construction and rhs.stop_possible() is false.

~stop_token();
  1. Effects: Releases ownership of the stop state, if any.

stop_token& operator=(const stop_token& rhs) noexcept;
  1. Effects: Equivalent to: stop_token(rhs).swap(*this).

  2. Returns: *this.

stop_token& operator=(stop_token&& rhs) noexcept;
  1. Effects: Equivalent to: stop_token(std::move(rhs)).swap(*this).

  2. Returns: *this.

Move swap into [stoptoken.mem]:

33.3.4.3. Member functions [stoptoken.mem]
void swap(stop_token& rhs) noexcept;
  1. Effects: Exchanges the values of *this and rhs. Equivalent to: stop-state.swap(rhs.stop-state).

[[nodiscard]] bool stop_requested() const noexcept;
  1. Returns: true if *this has ownership of stop-state refers to a stop state that has received a stop request; otherwise, false.

[[nodiscard]] bool stop_possible() const noexcept;
  1. Returns: false if:

    • *this does not have ownership of a stop state is disengaged , or

    • a stop request was not made and there are no associated stop_source objects; otherwise, true.

The following are covered by the equality_comparable and swappable concepts.

33.3.4.4. Non-member functions [stoptoken.nonmembers]
[[nodiscard]] bool operator==(const stop_token& lhs, const stop_token& rhs) noexcept;
  1. Returns: true if lhs and rhs have ownership of the same stop state or if both lhs and rhs do not have ownership of a stop state; otherwise false.

friend void swap(stop_token& x, stop_token& y) noexcept;
  1. Effects: Equivalent to: x.swap(y).

33.3.5. Class stop_source [stopsource]

33.3.5.1. General [stopsource.general]
  1. The class stop_source implements the semantics of making a stop request. A stop request made on a stop_source object is visible to all associated stop_source and stop_token ([thread.stoptoken]) objects. Once a stop request has been made it cannot be withdrawn (a subsequent stop request has no effect).
namespace std {
  The following definitions are already specified in the <stop_token> synopsis:
// no-shared-stop-state indicator struct nostopstate_t { explicit nostopstate_t() = default; }; inline constexpr nostopstate_t nostopstate{};
class stop_source { public: // 33.3.4.2, constructors, copy, and assignment stop_source(); explicit stop_source(nostopstate_t) noexcept; {}
stop_source(const stop_source&) noexcept; stop_source(stop_source&&) noexcept; stop_source& operator=(const stop_source&) noexcept; stop_source& operator=(stop_source&&) noexcept; ~stop_source();
// [stopsource.mem], Member functions void swap(stop_source&) noexcept; // 33.3.4.3, stop handling [[nodiscard]] stop_token get_token() const noexcept; [[nodiscard]] bool stop_possible() const noexcept; [[nodiscard]] bool stop_requested() const noexcept; bool request_stop() noexcept; bool operator==(const stop_source& rhs) const noexcept = default;
[[nodiscard]] friend bool operator==(const stop_source& lhs, const stop_source& rhs) noexcept; friend void swap(stop_source& lhs, stop_source& rhs) noexcept;
private: shared_ptr<unspecified> stop-state{}; // exposition only }; }
  1. stop-state refers to the stop_source’s associated stop state. A stop_source object is disengaged when stop-state is empty.

  2. stop_source models stoppable-source, copyable, equality_comparable, and swappable.

33.3.5.2. Constructors, copy, and assignment [stopsource.cons]
stop_source();
  1. Effects: Initialises *this to have ownership of stop-state with a pointer to a new stop state.

  2. Postconditions: stop_possible() is true and stop_requested() is false.

  3. Throws: bad_alloc if memory cannot be allocated for the stop state.

explicit stop_source(nostopstate_t) noexcept;
  1. Postconditions: stop_possible() is false and stop_requested() is false. No resources are allocated for the state.

stop_source(const stop_source& rhs) noexcept;
  1. Postconditions: *this == rhs is true. *this and rhs share the ownership of the same stop state, if any.

stop_source(stop_source&& rhs) noexcept;
  1. Postconditions: *this contains the value of rhs prior to the start of construction and rhs.stop_possible() is false.

~stop_source();
  1. Effects: Releases ownership of the stop state, if any.

stop_source& operator=(const stop_source& rhs) noexcept;
  1. Effects: Equivalent to: stop_source(rhs).swap(*this).

  2. Returns: *this.

stop_source& operator=(stop_source&& rhs) noexcept;
  1. Effects: Equivalent to: stop_source(std::move(rhs)).swap(*this).

  2. Returns: *this.

Move swap into [stopsource.mem]:

33.3.5.3. Member functions [stopsource.mem]
void swap(stop_source& rhs) noexcept;
  1. Effects: Exchanges the values of *this and rhs Equivalent to: stop-state.swap(rhs.stop-state) .

[[nodiscard]] stop_token get_token() const noexcept;
  1. Returns: stop_token() if stop_possible() is false; otherwise a new associated stop_token object ; i.e., its stop-state member is equal to the stop-state member of *this .

[[nodiscard]] bool stop_possible() const noexcept;
  1. Returns: true if *this has ownership of a stop state; otherwise, false stop-state != nullptr .

[[nodiscard]] bool stop_requested() const noexcept;
  1. Returns: true if *this has ownership of stop-state refers to a stop state that has received a stop request; otherwise, false.

bool request_stop() noexcept;
  1. Effects: Executes a stop request operation ([stoptoken.concepts]) on the associated stop state, if any.

  1. Effects: If *this does not have ownership of a stop state, returns false. Otherwise, atomically determines whether the owned stop state has received a stop request, and if not, makes a stop request. The determination and making of the stop request are an atomic read-modify-write operation ([intro.races]). If the request was made, the callbacks registered by associated stop_callback objects are synchronously called. If an invocation of a callback exits via an exception then terminate is invoked ([except.terminate]).

    A stop request includes notifying all condition variables of type condition_variable_any temporarily registered during an interruptible wait ([thread.condvarany.intwait]).

  2. Postconditions: stop_possible() is false or stop_requested() is true.

  3. Returns: true if this call made a stop request; otherwise false.

33.3.5.4. Non-member functions [stopsource.nonmembers]
[[nodiscard]] friend bool
  operator==(const stop_source& lhs, const stop_source& rhs) noexcept;
  1. Returns: true if lhs and rhs have ownership of the same stop state or if both lhs and rhs do not have ownership of a stop state; otherwise false.

friend void swap(stop_source& x, stop_source& y) noexcept;
  1. Effects: Equivalent to: x.swap(y).

33.3.6. Class template stop_callback [stopcallback]

33.3.6.1. General [stopcallback.general]
namespace std {
  template<class CallbackFn>
  class stop_callback {
  public:
    using callback_type = CallbackFn;

    // 33.3.5.2, constructors and destructor
    template<class CInitializer>
      explicit stop_callback(const stop_token& st, CInitializer&& cbinit)
        noexcept(is_nothrow_constructible_v<CallbackFn, CInitializer>);
    template<class CInitializer>
      explicit stop_callback(stop_token&& st, CInitializer&& cbinit)
        noexcept(is_nothrow_constructible_v<CallbackFn, CInitializer>);
    ~stop_callback();

    stop_callback(const stop_callback&) = delete;
    stop_callback(stop_callback&&) = delete;
    stop_callback& operator=(const stop_callback&) = delete;
    stop_callback& operator=(stop_callback&&) = delete;

  private:
    CallbackFn callbackcallback-fn; // exposition only
  };

  template<class CallbackFn>
    stop_callback(stop_token, CallbackFn) -> stop_callback<CallbackFn>;
}
  1. Mandates: stop_callback is instantiated with an argument for the template parameter CallbackFn that satisfies both invocable and destructible.

  1. Preconditions: stop_callback is instantiated with an argument for the template parameter Callback that models both invocable and destructible.

  1. Remarks: For a type Initializer, if stoppable-callback-for<CallbackFn, stop_token, Initializer> is satisfied, then stoppable-callback-for<CallbackFn, stop_token, Initializer> is modeled. The exposition-only callback-fn member is the associated callback function ([stoptoken.concepts]) of stop_callback<CallbackFn> objects.

33.3.6.2. Constructors and destructor [stopcallback.cons]
template<class CInitializer>
explicit stop_callback(const stop_token& st, CInitializer&& cbinit)
  noexcept(is_nothrow_constructible_v<CallbackFn, CInitializer>);
template<class CInitializer>
explicit stop_callback(stop_token&& st, CInitializer&& cbinit)
  noexcept(is_nothrow_constructible_v<CallbackFn, CInitializer>);
  1. Constraints: CallbackFn and CInitializer satisfy constructible_from<CallbackFn, CInitializer>.

  1. Preconditions: Callback and C model constructible_from<Callback, C>.

  1. Effects: Initializes callbackcallback-fn with std::forward<CInitializer>(cbinit) and executes a stoppable callback registration ([stoptoken.concepts]) . If st.stop_requested() is true, then std::forward&lt;Callback>(callback)() is evaluated in the current thread before the constructor returns. Otherwise, if st has ownership of a stop state, acquires shared ownership of that stop state and registers the callback with that stop state such that std::forward&lt;Callback>(callback)() is evaluated by the first call to request_stop() on an associated stop_source. If a callback is registered with st’s shared stop state, then *this acquires shared ownership of that stop state.

  1. Throws: Any exception thrown by the initialization of callback.

  2. Remarks: If evaluating std::forward<Callback>(callback)() exits via an exception, then terminate is invoked ([except.terminate]).

~stop_callback();
  1. Effects: Unregisters the callback from the owned stop state, if any. The destructor does not block waiting for the execution of another callback registered by an associated stop_callback. If callback is concurrently executing on another thread, then the return from the invocation of callback strongly happens before ([intro.races]) callback is destroyed. If callback is executing on the current thread, then the destructor does not block ([defns.block]) waiting for the return from the invocation of callback. Releases Executes a stoppable callback deregistration ([stoptoken.concepts]) and releases ownership of the stop state, if any.

Insert a new subclause, Class never_stop_token [stoptoken.never], after subclause Class template stop_callback [stopcallback], as a new subclause of Stop tokens [thread.stoptoken].

33.3.7. Class never_stop_token [stoptoken.never]

33.3.7.1. General [stoptoken.never.general]
  1. The class never_stop_token models the unstoppable_token concept. It provides a stop token interface, but also provides static information that a stop is never possible nor requested.

    namespace std {
      class never_stop_token {
        struct callback-type { // exposition only
          explicit callback-type(never_stop_token, auto&&) noexcept {}
        };
      public:
        template<class>
          using callback_type = callback-type;
    
        static constexpr bool stop_requested() noexcept { return false; }
        static constexpr bool stop_possible() noexcept { return false; }
    
        bool operator==(const never_stop_token&) const = default;
      };
    }
    

Insert a new subclause, Class inplace_stop_token [stoptoken.inplace], after the subclause added above, as a new subclause of Stop tokens [thread.stoptoken].

33.3.8. Class inplace_stop_token [stoptoken.inplace]

33.3.8.1. General [stoptoken.inplace.general]
  1. The class inplace_stop_token models the concept stoppable_token. It references the stop state of its associated inplace_stop_source object ([stopsource.inplace]), if any.

    namespace std {
      class inplace_stop_token {
      public:
        template<class CallbackFn>
          using callback_type = inplace_stop_callback<CallbackFn>;
    
        inplace_stop_token() = default;
        bool operator==(const inplace_stop_token&) const = default;
    
        // [stoptoken.inplace.mem], member functions
        bool stop_requested() const noexcept;
        bool stop_possible() const noexcept;
        void swap(inplace_stop_token&) noexcept;
    
      private:
        const inplace_stop_source* stop-source = nullptr; // exposition only
      };
    }
    
33.3.8.2. Member functions [stoptoken.inplace.members]
void swap(inplace_stop_token& rhs) noexcept;
  1. Effects: Exchanges the values of stop-source and rhs.stop-source.

bool stop_requested() const noexcept;
  1. Effects: Equivalent to: return stop-source != nullptr && stop-source->stop_requested();

  2. As specified in [basic.life], the behavior of stop_requested() is undefined unless the call strongly happens before the start of the destructor of the associated inplace_stop_source, if any.

bool stop_possible() const noexcept;
  1. Returns: stop-source != nullptr.

  2. As specified in [basic.stc.general], the behavior of stop_possible() is implementation-defined unless the call strongly happens before the end of the storage duration of the associated inplace_stop_source object, if any.

Insert a new subclause, Class inplace_stop_source [stopsource.inplace], after the subclause added above, as a new subclause of Stop tokens [thread.stoptoken].

33.3.9. Class inplace_stop_source [stopsource.inplace]

33.3.9.1. General [stopsource.inplace.general]
  1. The class inplace_stop_source models stoppable-source.

    namespace std {
      class inplace_stop_source {
      public:
        // [stopsource.inplace.cons], constructors, copy, and assignment
        constexpr inplace_stop_source() noexcept;
    
        inplace_stop_source(inplace_stop_source&&) = delete;
        inplace_stop_source(const inplace_stop_source&) = delete;
        inplace_stop_source& operator=(inplace_stop_source&&) = delete;
        inplace_stop_source& operator=(const inplace_stop_source&) = delete;
        ~inplace_stop_source();
    
        //[stopsource.inplace.mem], stop handling
        constexpr inplace_stop_token get_token() const noexcept;
        static constexpr bool stop_possible() noexcept { return true; }
        bool stop_requested() const noexcept;
        bool request_stop() noexcept;
      };
    }
    
33.3.9.2. Constructors, copy, and assignment [stopsource.inplace.cons]
constexpr inplace_stop_source() noexcept;
  1. Effects: Initializes a new stop state inside *this.

  2. Postconditions: stop_requested() is false.

33.3.9.3. Members [stopsource.inplace.mem]
constexpr inplace_stop_token get_token() const noexcept;
  1. Returns: A new associated inplace_stop_token object. The inplace_stop_token object’s stop-source member is equal to this.

bool stop_requested() const noexcept;
  1. Returns: true if the stop state inside *this has received a stop request; otherwise, false.

bool request_stop() noexcept;
  1. Effects: Executes a stop request operation ([stoptoken.concepts]).

  2. Postconditions: stop_requested() is true.

Insert a new subclause, Class template inplace_stop_callback [stopcallback.inplace], after the subclause added above, as a new subclause of Stop tokens [thread.stoptoken].

33.3.10. Class template inplace_stop_callback [stopcallback.inplace]

33.3.10.1. General [stopcallback.inplace.general]
namespace std {
  template<class CallbackFn>
  class inplace_stop_callback {
  public:
    using callback_type = CallbackFn;

    // [stopcallback.inplace.cons], constructors and destructor
    template<class Initializer>
      explicit inplace_stop_callback(inplace_stop_token st, Initializer&& init)
        noexcept(is_nothrow_constructible_v<CallbackFn, Initializer>);
    ~inplace_stop_callback();

    inplace_stop_callback(inplace_stop_callback&&) = delete;
    inplace_stop_callback(const inplace_stop_callback&) = delete;
    inplace_stop_callback& operator=(inplace_stop_callback&&) = delete;
    inplace_stop_callback& operator=(const inplace_stop_callback&) = delete;

  private:
    CallbackFn callback-fn;      // exposition only
  };

  template<class CallbackFn>
    inplace_stop_callback(inplace_stop_token, CallbackFn)
      -> inplace_stop_callback<CallbackFn>;
}
  1. Mandates: CallbackFn satisfies both invocable and destructible.

  2. Remarks: For a type Initializer, if stoppable-callback-for<CallbackFn, inplace_stop_token, Initializer> is satisfied, then stoppable-callback-for<CallbackFn, inplace_stop_token, Initializer> is modeled. For an inplace_stop_callback<CallbackFn> object, the exposition-only callback-fn member is its associated callback function ([stoptoken.concepts]).

33.3.10.2. Constructors and destructor [stopcallback.inplace.cons]
template<class Initializer>
  explicit inplace_stop_callback(inplace_stop_token st, Initializer&& init)
    noexcept(is_nothrow_constructible_v<CallbackFn, Initializer>);
  1. Constraints: constructible_from<CallbackFn, Initializer> is satisfied.

  2. Effects: Initializes callback-fn with std::forward<Initializer>(init) and executes a stoppable callback registration ([stoptoken.concepts]).

~inplace_stop_callback();
  1. Effects: Executes a stoppable callback deregistration ([stoptoken.concepts]).

Insert a new top-level clause

34. Execution control library [exec]

34.1. General [exec.general]

  1. This Clause describes components supporting execution of function objects [function.objects].

  2. The following subclauses describe the requirements, concepts, and components for execution control primitives as summarized in Table 1.

Table N: Execution control library summary [tab:execution.summary]
Subclause Header
[exec.sched] Schedulers <execution>
[exec.recv] Receivers
[exec.opstate] Operation states
[exec.snd] Senders
  1. Table 2 shows the types of customization point objects [customization.point.object] used in the execution control library:

Table N+1: Types of customization point objects in the execution control library [tab:execution.cpos]
Customization point object type Purpose Examples
core provide core execution functionality, and connection between core components e.g., connect, start
completion functions called by senders to announce the completion of the work (success, error, or cancellation) set_value, set_error, set_stopped
senders allow the specialization of the provided sender algorithms
  • sender factories (e.g., schedule, just, read_env)
  • sender adaptors (e.g., continues_on, then, let_value)
  • sender consumers (e.g., sync_wait)
queries allow querying different properties of objects
  • general queries (e.g., get_allocator, get_stop_token)
  • environment queries (e.g., get_scheduler, get_delegation_scheduler)
  • scheduler queries (e.g., get_forward_progress_guarantee)
  • sender attribute queries (e.g., get_completion_scheduler)
  1. This clause makes use of the following exposition-only entities:

    1. For a subexpression expr, let MANDATE-NOTHROW(expr) be expression-equivalent to expr.

      • Mandates: noexcept(expr) is true.

    2. namespace std {
        template<class T>
          concept movable-value = // exposition only
            move_constructible<decay_t<T>> &&
            constructible_from<decay_t<T>, T> &&
            (!is_array_v<remove_reference_t<T>>);
      }
      
    3. For function types F1 and F2 denoting R1(Args1...) and R2(Args2...) respectively, MATCHING-SIG(F1, F2) is true if and only if same_as<R1(Args1&&...), R2(Args2&&...)> is true.

    4. For a subexpression err, let Err be decltype((err)) and let AS-EXCEPT-PTR(err) be:

      1. err if decay_t<Err> denotes the type exception_ptr.

        • Mandates: err != exception_ptr() is true.

      2. Otherwise, make_exception_ptr(system_error(err)) if decay_t<Err> denotes the type error_code.

      3. Otherwise, make_exception_ptr(err).

34.2. Queries and queryables [exec.queryable]

34.2.1. General [exec.queryable.general]

  1. A queryable object is a read-only collection of key/value pairs where each key is a customization point object known as a query object. A query is an invocation of a query object with a queryable object as its first argument and a (possibly empty) set of additional arguments. A query imposes syntactic and semantic requirements on its invocations.

  2. Let q be a query object, let args be a (possibly empty) pack of subexpressions, let env be a subexpression that refers to a queryable object o of type O, and let cenv be a subexpression referring to o such that decltype((cenv)) is const O&. The expression q(env, args...) is equal to ([concepts.equality]) the expression q(cenv, args...).

  3. The type of a query expression can not be void.

  4. The expression q(env, args...) is equality-preserving ([concepts.equality]) and does not modify the query object or the arguments.

  5. If the expression env.query(q, args...) is well-formed, then it is expression-equivalent to q(env, args...).

  6. Unless otherwise specified, the result of a query is valid as long as the queryable object is valid.

34.2.2. queryable concept [exec.queryable.concept]

namespace std {
  template<class T>
    concept queryable = destructible<T>; // exposition only
}
  1. The exposition-only queryable concept specifies the constraints on the types of queryable objects.

  2. Let env be an object of type Env. The type Env models queryable if for each callable object q and a pack of subexpressions args, if requires { q(env, args...) } is true then q(env, args...) meets any semantic requirements imposed by q.

34.3. Asynchronous operations [async.ops]

  1. An execution resource is a program entity that manages a (possibly dynamic) set of execution agents ([thread.req.lockable.general]), which it uses to execute parallel work on behalf of callers. [Example 1: The currently active thread, a system-provided thread pool, and uses of an API associated with an external hardware accelerator are all examples of execution resources. -- end example] Execution resources execute asynchronous operations. An execution resource is either valid or invalid.

  2. An asynchronous operation is a distinct unit of program execution that:

    1. ... is explicitly created.

    2. ... can be explicitly started once at most.

    3. ... once started, eventually completes exactly once with a (possibly empty) set of result datums and in exactly one of three dispositions: success, failure, or cancellation.

      • A successful completion, also known as a value completion, can have an arbitrary number of result datums.

      • A failure completion, also known as an error completion, has a single result datum.

      • A cancellation completion, also known as a stopped completion, has no result datum.

      An asynchronous operation’s async result is its disposition and its (possibly empty) set of result datums.

    4. ... can complete on a different execution resource than the execution resource on which it started.

    5. ... can create and start other asynchronous operations called child operations. A child operation is an asynchronous operation that is created by the parent operation and, if started, completes before the parent operation completes. A parent operation is the asynchronous operation that created a particular child operation.

    An asynchronous operation can in fact execute synchronously; that is, it can complete during the execution of its start operation on the thread of execution that started it.

  3. An asynchronous operation has associated state known as its operation state.

  4. An asynchronous operation has an associated environment. An environment is a queryable object ([exec.queryable]) representing the execution-time properties of the operation’s caller. The caller of an asynchronous operation is its parent operation or the function that created it. An asynchronous operation’s operation state owns the operation’s environment.

  5. An asynchronous operation has an associated receiver. A receiver is an aggregation of three handlers for the three asynchronous completion dispositions: a value completion handler for a value completion, an error completion handler for an error completion, and a stopped completion handler for a stopped completion. A receiver has an associated environment. An asynchronous operation’s operation state owns the operation’s receiver. The environment of an asynchronous operation is equal to its receiver’s environment.

  6. For each completion disposition, there is a completion function. A completion function is a customization point object ([customization.point.object]) that accepts an asynchronous operation’s receiver as the first argument and the result datums of the asynchronous operation as additional arguments. The value completion function invokes the receiver’s value completion handler with the value result datums; likewise for the error completion function and the stopped completion function. A completion function has an associated type known as its completion tag that is the unqualified type of the completion function. A valid invocation of a completion function is called a completion operation.

  7. The lifetime of an asynchronous operation, also known as the operation’s async lifetime, begins when its start operation begins executing and ends when its completion operation begins executing. If the lifetime of an asynchronous operation’s associated operation state ends before the lifetime of the asynchronous operation, the behavior is undefined. After an asynchronous operation executes a completion operation, its associated operation state is invalid. Accessing any part of an invalid operation state is undefined behavior.

  8. An asynchronous operation shall not execute a completion operation before its start operation has begun executing. After its start operation has begun executing, exactly one completion operation shall execute. The lifetime of an asynchronous operation’s operation state can end during the execution of the completion operation.

  9. A sender is a factory for one or more asynchronous operations. Connecting a sender and a receiver creates an asynchronous operation. The asynchronous operation’s associated receiver is equal to the receiver used to create it, and its associated environment is equal to the environment associated with the receiver used to create it. The lifetime of an asynchronous operation’s associated operation state does not depend on the lifetimes of either the sender or the receiver from which it was created. A sender is started when it is connected to a receiver and the resulting asynchronous operation is started. A sender’s async result is the async result of the asynchronous operation created by connecting it to a receiver. A sender sends its results by way of the asynchronous operation(s) it produces, and a receiver receives those results. A sender is either valid or invalid; it becomes invalid when its parent sender (see below) becomes invalid.

  10. A scheduler is an abstraction of an execution resource with a uniform, generic interface for scheduling work onto that resource. It is a factory for senders whose asynchronous operations execute value completion operations on an execution agent belonging to the scheduler’s associated execution resource. A schedule-expression obtains such a sender from a scheduler. A schedule sender is the result of a schedule expression. On success, an asynchronous operation produced by a schedule sender executes a value completion operation with an empty set of result datums. Multiple schedulers can refer to the same execution resource. A scheduler can be valid or invalid. A scheduler becomes invalid when the execution resource to which it refers becomes invalid, as do any schedule senders obtained from the scheduler, and any operation states obtained from those senders.

  11. An asynchronous operation has one or more associated completion schedulers for each of its possible dispositions. A completion scheduler is a scheduler whose associated execution resource is used to execute a completion operation for an asynchronous operation. A value completion scheduler is a scheduler on which an asynchronous operation’s value completion operation can execute. Likewise for error completion schedulers and stopped completion schedulers.

  12. A sender has an associated queryable object ([exec.queryable]) known as its attributes that describes various characteristics of the sender and of the asynchronous operation(s) it produces. For each disposition, there is a query object for reading the associated completion scheduler from a sender’s attributes; i.e., a value completion scheduler query object for reading a sender’s value completion scheduler, etc. If a completion scheduler query is well-formed, the returned completion scheduler is unique for that disposition for any asynchronous operation the sender creates. A schedule sender is required to have a value completion scheduler attribute whose value is equal to the scheduler that produced the schedule sender.

  13. A completion signature is a function type that describes a completion operation. An asynchronous operation has a finite set of possible completion signatures corresponding to the completion operations that the asynchronous operation potentially evaluates ([basic.def.odr]). For a completion function set, receiver rcvr, and pack of arguments args, let c be the completion operation set(rcvr, args...), and let F be the function type decltype(auto(set))(decltype((args))...). A completion signature Sig is associated with c if and only if MATCHING-SIG(Sig, F) is true ([exec.general]). Together, a sender type and an environment type Env determine the set of completion signatures of an asynchronous operation that results from connecting the sender with a receiver that has an environment of type Env. The type of the receiver does not affect an asynchronous operation’s completion signatures, only the type of the receiver’s environment.

  14. A sender algorithm is a function that takes and/or returns a sender. There are three categories of sender algorithms:

    • A sender factory is a function that takes non-senders as arguments and that returns a sender.

    • A sender adaptor is a function that constructs and returns a parent sender from a set of one or more child senders and a (possibly empty) set of additional arguments. An asynchronous operation created by a parent sender is a parent operation to the child operations created by the child senders.

    • A sender consumer is a function that takes one or more senders and a (possibly empty) set of additional arguments, and whose return type is not the type of a sender.

34.4. Header <execution> synopsis [exec.syn]

namespace std {
  // [exec.general], helper concepts
  template<class T>
    concept movable-value = see below; // exposition only

  template<class From, class To>
    concept decays-to = same_as<decay_t<From>, To>; // exposition only

  template<class T>
    concept class-type = decays-to<T, T> && is_class_v<T>;  // exposition only

  // [exec.queryable], queryable objects
  template<class T>
    concept queryable = see above; // exposition only

  // [exec.queries], queries
  struct forwarding_query_t { see below };
  struct get_allocator_t { see below };
  struct get_stop_token_t { see below };

  inline constexpr forwarding_query_t forwarding_query{};
  inline constexpr get_allocator_t get_allocator{};
  inline constexpr get_stop_token_t get_stop_token{};

  template<class T>
    using stop_token_of_t =
      remove_cvref_t<decltype(get_stop_token(declval<T>()))>;

  template<class T>
    concept forwarding-query = // exposition only
      forwarding_query(T{});
}

namespace std::execution {
  // [exec.queries], queries
  enum class forward_progress_guarantee {
    concurrent,
    parallel,
    weakly_parallel
  };
  struct get_domain_t { see below };
  struct get_scheduler_t { see below };
  struct get_delegation_scheduler_t { see below };
  struct get_forward_progress_guarantee_t { see below };
  template<class CPO>
    struct get_completion_scheduler_t { see below };

  inline constexpr get_domain_t get_domain{};
  inline constexpr get_scheduler_t get_scheduler{};
  inline constexpr get_delegation_scheduler_t get_delegation_scheduler{};
  inline constexpr get_forward_progress_guarantee_t get_forward_progress_guarantee{};
  template<class CPO>
    inline constexpr get_completion_scheduler_t<CPO> get_completion_scheduler{};

  struct empty_env {};
  struct get_env_t { see below };
  inline constexpr get_env_t get_env{};

  template<class T>
    using env_of_t = decltype(get_env(declval<T>()));

  // [exec.domain.default], execution domains
  struct default_domain;

  // [exec.sched], schedulers
  struct scheduler_t {};

  template<class Sch>
    concept scheduler = see below;

  // [exec.recv], receivers
  struct receiver_t {};

  template<class Rcvr>
    concept receiver = see below;

  template<class Rcvr, class Completions>
    concept receiver_of = see below;

  struct set_value_t { see below };
  struct set_error_t { see below };
  struct set_stopped_t { see below };

  inline constexpr set_value_t set_value{};
  inline constexpr set_error_t set_error{};
  inline constexpr set_stopped_t set_stopped{};

  // [exec.opstate], operation states
  struct operation_state_t {};

  template<class O>
    concept operation_state = see below;

  struct start_t { see below };
  inline constexpr start_t start{};

  // [exec.snd], senders
  struct sender_t {};

  template<class Sndr>
    concept sender = see below;

  template<class Sndr, class Env = empty_env>
    concept sender_in = see below;

  template<class Sndr, class Rcvr>
    concept sender_to = see below;

  template<class... Ts>
    struct type-list; // exposition only

  // [exec.getcomplsigs], completion signatures
  struct get_completion_signatures_t { see below };
  inline constexpr get_completion_signatures_t get_completion_signatures {};

  template<class Sndr, class Env = empty_env>
      requires sender_in<Sndr, Env>
    using completion_signatures_of_t = call-result-t<get_completion_signatures_t, Sndr, Env>;

  template<class... Ts>
    using decayed-tuple = tuple<decay_t<Ts>...>; // exposition only

  template<class... Ts>
    using variant-or-empty = see below; // exposition only

  template<class Sndr,
           class Env = empty_env,
           template<class...> class Tuple = decayed-tuple,
           template<class...> class Variant = variant-or-empty>
      requires sender_in<Sndr, Env>
    using value_types_of_t = see below;

  template<class Sndr,
           class Env = empty_env,
           template<class...> class Variant = variant-or-empty>
      requires sender_in<Sndr, Env>
    using error_types_of_t = see below;

  template<class Sndr, class Env = empty_env>
      requires sender_in<Sndr, Env>
    inline constexpr bool sends_stopped = see below;

  template<class Sndr, class Env>
    using single-sender-value-type = see below; // exposition only

  template<class Sndr, class Env>
    concept single-sender = see below; // exposition only

  template<sender Sndr>
    using tag_of_t = see below;

  // [exec.snd.transform], sender transformations
  template<class Domain, sender Sndr, queryable... Env>
      requires (sizeof...(Env) <= 1)
    constexpr sender decltype(auto) transform_sender(
      Domain dom, Sndr&& sndr, const Env&... env) noexcept(see below);

  // [exec.snd.transform.env], environment transformations
  template<class Domain, sender Sndr, queryable Env>
    constexpr queryable decltype(auto) transform_env(
      Domain dom, Sndr&& sndr, Env&& env) noexcept;

  // [exec.snd.apply], sender algorithm application
  template<class Domain, class Tag, sender Sndr, class... Args>
    constexpr decltype(auto) apply_sender(
      Domain dom, Tag, Sndr&& sndr, Args&&... args) noexcept(see below);

  // [exec.connect], the connect sender algorithm
  struct connect_t { see below };
  inline constexpr connect_t connect{};

  template<class Sndr, class Rcvr>
    using connect_result_t =
      decltype(connect(declval<Sndr>(), declval<Rcvr>()));

  // [exec.factories], sender factories
  struct just_t { see below };
  struct just_error_t { see below };
  struct just_stopped_t { see below };
  struct schedule_t { see below };

  inline constexpr just_t just{};
  inline constexpr just_error_t just_error{};
  inline constexpr just_stopped_t just_stopped{};
  inline constexpr schedule_t schedule{};
  inline constexpr unspecified read{};

  template<scheduler Sndr>
    using schedule_result_t = decltype(schedule(declval<Sndr>()));

  // [exec.adapt], sender adaptors
  template<class-type D>
    struct sender_adaptor_closure { };

  struct starts_on_t { see below };
  struct continues_on_t { see below };
  struct on_t { see below };
  struct schedule_from_t { see below };
  struct then_t { see below };
  struct upon_error_t { see below };
  struct upon_stopped_t { see below };
  struct let_value_t { see below };
  struct let_error_t { see below };
  struct let_stopped_t { see below };
  struct bulk_t { see below };
  struct split_t { see below };
  struct when_all_t { see below };
  struct when_all_with_variant_t { see below };
  struct into_variant_t { see below };
  struct stopped_as_optional_t { see below };
  struct stopped_as_error_t { see below };

  inline constexpr starts_on_t starts_on{};
  inline constexpr continues_on_t continues_on{};
  inline constexpr on_t on{};
  inline constexpr schedule_from_t schedule_from{};
  inline constexpr then_t then{};
  inline constexpr upon_error_t upon_error{};
  inline constexpr upon_stopped_t upon_stopped{};
  inline constexpr let_value_t let_value{};
  inline constexpr let_error_t let_error{};
  inline constexpr let_stopped_t let_stopped{};
  inline constexpr bulk_t bulk{};
  inline constexpr split_t split{};
  inline constexpr when_all_t when_all{};
  inline constexpr when_all_with_variant_t when_all_with_variant{};
  inline constexpr into_variant_t into_variant{};
  inline constexpr stopped_as_optional_t stopped_as_optional{};
  inline constexpr stopped_as_error_t stopped_as_error{};

  // [exec.utils], sender and receiver utilities
  // [exec.utils.cmplsigs]
  template<class Fn>
    concept completion-signature = // exposition only
      see below;

  template<completion-signature... Fns>
    struct completion_signatures {};

  template<class Sigs> // exposition only
    concept valid-completion-signatures = see below;

  // [exec.utils.tfxcmplsigs]
  template<
    valid-completion-signatures InputSignatures,
    valid-completion-signatures AdditionalSignatures = completion_signatures<>,
    template<class...> class SetValue = see below,
    template<class> class SetError = see below,
    valid-completion-signatures SetStopped = completion_signatures<set_stopped_t()>>
  using transform_completion_signatures = completion_signatures<see below>;

  template<
    sender Sndr,
    class Env = empty_env,
    valid-completion-signatures AdditionalSignatures = completion_signatures<>,
    template<class...> class SetValue = see below,
    template<class> class SetError = see below,
    valid-completion-signatures SetStopped = completion_signatures<set_stopped_t()>>
      requires sender_in<Sndr, Env>
  using transform_completion_signatures_of =
    transform_completion_signatures<
      completion_signatures_of_t<Sndr, Env>,
      AdditionalSignatures, SetValue, SetError, SetStopped>;

  // [exec.ctx], execution resources
  // [exec.run.loop], run_loop
  class run_loop;
}

namespace std::this_thread {
  // [exec.consumers], consumers
  struct sync_wait_t { see below };
  struct sync_wait_with_variant_t { see below };

  inline constexpr sync_wait_t sync_wait{};
  inline constexpr sync_wait_with_variant_t sync_wait_with_variant{};
}

namespace std::execution {
  // [exec.as.awaitable]
  struct as_awaitable_t { see below };
  inline constexpr as_awaitable_t as_awaitable{};

  // [exec.with.awaitable.senders]
  template<class-type Promise>
    struct with_awaitable_senders;
}
  1. The exposition-only type variant-or-empty<Ts...> is defined as follows:

    1. If sizeof...(Ts) is greater than zero, variant-or-empty<Ts...> denotes variant<Us...> where Us... is the pack decay_t<Ts>... with duplicate types removed.

    2. Otherwise, variant-or-empty<Ts...> denotes the exposition-only class type:

      namespace std::execution {
        struct empty-variant { // exposition only
          empty-variant() = delete;
        };
      }
      
  2. For types Sndr and Env, single-sender-value-type<Sndr, Env> is an alias for:

    1. value_types_of_t<Sndr, Env, decay_t, type_identity_t> if that type is well-formed,

    2. Otherwise, void if value_types_of_t<Sndr, Env, tuple, variant> is variant<tuple<>> or variant<>,

    3. Otherwise, value_types_of_t<Sndr, Env, decayed-tuple, type_identity_t> if that type is well-formed,

    4. Otherwise, single-sender-value-type<Sndr, Env> is ill-formed.

  3. The exposition-only concept single-sender is defined as follows:

    namespace std::execution {
      template<class Sndr, class Env>
        concept single-sender =
          sender_in<Sndr, Env> &&
          requires {
            typename single-sender-value-type<Sndr, Env>;
          };
    }
    

34.5. Queries [exec.queries]

34.5.1. forwarding_query [exec.fwd.env]

  1. forwarding_query asks a query object whether it should be forwarded through queryable adaptors.

  2. The name forwarding_query denotes a query object. For some query object q of type Q, forwarding_query(q) is expression-equivalent to:

    1. MANDATE-NOTHROW(q.query(forwarding_query)) if that expression is well-formed.

      • Mandates: The expression above has type bool and is a core constant expression if q is a core constant expression.

    2. Otherwise, true if derived_from<Q, forwarding_query_t> is true.

    3. Otherwise, false.

34.5.2. get_allocator [exec.get.allocator]

  1. get_allocator asks a queryable object for its associated allocator.

  2. The name get_allocator denotes a query object. For a subexpression env, get_allocator(env) is expression-equivalent to MANDATE-NOTHROW(as_const(env).query(get_allocator)).

    • Mandates: If the expression above is well-formed, its type satisfies simple-allocator ([allocator.requirements.general]).

  3. forwarding_query(get_allocator) is a core constant expression and has value true.

34.5.3. get_stop_token [exec.get.stop.token]

  1. get_stop_token asks a queryable object for an associated stop token.

  2. The name get_stop_token denotes a query object. For a subexpression env, get_stop_token(env) is expression-equivalent to:

    1. MANDATE-NOTHROW(as_const(env).query(get_stop_token)) if that expression is well-formed.

      • Mandates: The type of the expression above satisfies stoppable_token.

    2. Otherwise, never_stop_token{}.

  3. forwarding_query(get_stop_token) is a core constant expression and has value true.

34.5.4. execution::get_env [exec.get.env]

  1. execution::get_env is a customization point object. For a subexpression o, execution::get_env(o) is expression-equivalent to:

    1. MANDATE-NOTHROW(as_const(o).get_env()) if that expression is well-formed.

      • Mandates: The type of the expression above satisfies queryable ([exec.queryable]).

    2. Otherwise, empty_env{}.

  2. The value of get_env(o) shall be valid while o is valid.

  3. When passed a sender object, get_env returns the sender’s associated attributes. When passed a receiver, get_env returns the receiver’s associated execution environment.

34.5.5. execution::get_domain [exec.get.domain]

  1. get_domain asks a queryable object for its associated execution domain tag.

  2. The name get_domain denotes a query object. For a subexpression env, get_domain(env) is expression-equivalent to MANDATE-NOTHROW(as_const(env).query(get_domain)).

  3. forwarding_query(execution::get_domain) is a core constant expression and has value true.

34.5.6. execution::get_scheduler [exec.get.scheduler]

  1. get_scheduler asks a queryable object for its associated scheduler.

  2. The name get_scheduler denotes a query object. For a subexpression env, get_scheduler(env) is expression-equivalent to MANDATE-NOTHROW(as_const(env).query(get_scheduler)).

    • Mandates: If the expression above is well-formed, its type satisfies scheduler.

  3. forwarding_query(execution::get_scheduler) is a core constant expression and has value true.

34.5.7. execution::get_delegation_scheduler [exec.get.delegation.scheduler]

  1. get_delegation_scheduler asks a queryable object for a scheduler that can be used to delegate work to for the purpose of forward progress delegation ([intro.progress]).

  2. The name get_delegation_scheduler denotes a query object. For a subexpression env, get_delegation_scheduler(env) is expression-equivalent to MANDATE-NOTHROW(as_const(env).query(get_delegation_scheduler)).

    • Mandates: If the expression above is well-formed, its type satisfies scheduler.

  3. forwarding_query(execution::get_delegation_scheduler) is a core constant expression and has value true.

34.5.8. execution::get_forward_progress_guarantee [exec.get.forward.progress.guarantee]

namespace std::execution {
  enum class forward_progress_guarantee {
    concurrent,
    parallel,
    weakly_parallel
  };
}
  1. get_forward_progress_guarantee asks a scheduler about the forward progress guarantee of execution agents created by that scheduler’s associated execution resource ([intro.progress]).

  2. The name get_forward_progress_guarantee denotes a query object. For a subexpression sch, let Sch be decltype((sch)). If Sch does not satisfy scheduler, get_forward_progress_guarantee is ill-formed. Otherwise, get_forward_progress_guarantee(sch) is expression-equivalent to:

    1. MANDATE-NOTHROW(as_const(sch).query(get_forward_progress_guarantee)), if that expression is well-formed.

      • Mandates: The type of the expression above is forward_progress_guarantee.

    2. Otherwise, forward_progress_guarantee::weakly_parallel.

  3. If get_forward_progress_guarantee(sch) for some scheduler sch returns forward_progress_guarantee::concurrent, all execution agents created by that scheduler’s associated execution resource shall provide the concurrent forward progress guarantee. If it returns forward_progress_guarantee::parallel, all such execution agents shall provide at least the parallel forward progress guarantee.

34.5.9. execution::get_completion_scheduler [exec.completion.scheduler]

  1. get_completion_scheduler<completion-tag> obtains the completion scheduler associated with a completion tag from a sender’s attributes.

  2. The name get_completion_scheduler denotes a query object template. For a subexpression q, the expression get_completion_scheduler<completion-tag>(q) is ill-formed if completion-tag is not one of set_value_t, set_error_t, or set_stopped_t. Otherwise, get_completion_scheduler<completion-tag>(q) is expression-equivalent to MANDATE-NOTHROW(as_const(q).query(get_completion_scheduler<completion-tag>)).

    • Mandates: If the expression above is well-formed, its type satisfies scheduler.

  3. Let completion-fn be a completion function ([async.ops]); let completion-tag be the associated completion tag of completion-fn; let args be a pack of subexpressions; and let sndr be a subexpression such that sender<decltype((sndr))> is true and get_completion_scheduler<completion-tag>(get_env(sndr)) is well-formed and denotes a scheduler sch. If an asynchronous operation created by connecting sndr with a receiver rcvr causes the evaluation of completion-fn(rcvr, args...), the behavior is undefined unless the evaluation happens on an execution agent that belongs to sch’s associated execution resource.

  4. The expression forwarding_query(get_completion_scheduler<completion-tag>) is a core constant expression and has value true.

34.6. Schedulers [exec.sched]

  1. The scheduler concept defines the requirements of a scheduler type ([async.ops]). schedule is a customization point object that accepts a scheduler. A valid invocation of schedule is a schedule-expression.

    namespace std::execution {
      template<class Sch>
        concept scheduler =
          derived_from<typename remove_cvref_t<Sch>::scheduler_concept, scheduler_t> &&
          queryable<Sch> &&
          requires(Sch&& sch) {
            { schedule(std::forward<Sch>(sch)) } -> sender;
            { auto(get_completion_scheduler<set_value_t>(
                get_env(schedule(std::forward<Sch>(sch))))) }
                  -> same_as<remove_cvref_t<Sch>>;
          } &&
          equality_comparable<remove_cvref_t<Sch>> &&
          copy_constructible<remove_cvref_t<Sch>>;
    }
    
  2. Let Sch be the type of a scheduler and let Env be the type of an execution environment for which sender_in<schedule_result_t<Sch>, Env> is satisfied. Then sender-in-of<schedule_result_t<Sch>, Env> shall be modeled.

  3. None of a scheduler’s copy constructor, destructor, equality comparison, or swap member functions shall exit via an exception. None of these member functions, nor a scheduler type’s schedule function, shall introduce data races as a result of potentially concurrent ([intro.races]) invocations of those functions from different threads.

  4. For any two values sch1 and sch2 of some scheduler type Sch, sch1 == sch2 shall return true only if both sch1 and sch2 share the same associated execution resource.

  5. For a given scheduler expression sch, the expression get_completion_scheduler<set_value_t>(get_env(schedule(sch))) shall compare equal to sch.

  6. For a given scheduler expression sch, if the expression get_domain(sch) is well-formed, then the expression get_domain(get_env(schedule(sch))) is also well-formed and has the same type.

  7. A scheduler type’s destructor shall not block pending completion of any receivers connected to the sender objects returned from schedule. The ability to wait for completion of submitted function objects can be provided by the associated execution resource of the scheduler.

34.7. Receivers [exec.recv]

34.7.1. Receiver concepts [exec.recv.concepts]

  1. A receiver represents the continuation of an asynchronous operation. The receiver concept defines the requirements for a receiver type ([async.ops]). The receiver_of concept defines the requirements for a receiver type that is usable as the first argument of a set of completion operations corresponding to a set of completion signatures. The get_env customization point object is used to access a receiver’s associated environment.

    namespace std::execution {
      template<class Rcvr>
        concept receiver =
          derived_from<typename remove_cvref_t<Rcvr>::receiver_concept, receiver_t> &&
          requires(const remove_cvref_t<Rcvr>& rcvr) {
            { get_env(rcvr) } -> queryable;
          } &&
          move_constructible<remove_cvref_t<Rcvr>> &&  // rvalues are movable, and
          constructible_from<remove_cvref_t<Rcvr>, Rcvr>; // lvalues are copyable
    
      template<class Signature, class Rcvr>
        concept valid-completion-for = // exposition only
          requires (Signature* sig) {
            []<class Tag, class... Args>(Tag(*)(Args...))
                requires callable<Tag, remove_cvref_t<Rcvr>, Args...>
            {}(sig);
          };
    
      template<class Rcvr, class Completions>
        concept has-completions = // exposition only
          requires (Completions* completions) {
            []<valid-completion-for<Rcvr>...Sigs>(completion_signatures<Sigs...>*)
            {}(completions);
          };
    
      template<class Rcvr, class Completions>
        concept receiver_of =
          receiver<Rcvr> && has-completions<Rcvr, Completions>;
    }
    
  2. Class types that are marked final do not model the receiver concept.

  3. Let rcvr be a receiver and let op_state be an operation state associated with an asynchronous operation created by connecting rcvr with a sender. Let token be a stop token equal to get_stop_token(get_env(rcvr)). token shall remain valid for the duration of the asynchronous operation’s lifetime ([async.ops]). This means that, unless it knows about further guarantees provided by the type of rcvr, the implementation of op_state can not use token after it executes a completion operation. This also implies that any stop callbacks registered on token must be destroyed before the invocation of the completion operation.

34.7.2. execution::set_value [exec.set.value]

  1. set_value is a value completion function ([async.ops]). Its associated completion tag is set_value_t. The expression set_value(rcvr, vs...) for a subexpression rcvr and pack of subexpressions vs is ill-formed if rcvr is an lvalue or an rvalue of const type. Otherwise, it is expression-equivalent to MANDATE-NOTHROW(rcvr.set_value(vs...)).

34.7.3. execution::set_error [exec.set.error]

  1. set_error is an error completion function ([async.ops]). Its associated completion tag is set_error_t. The expression set_error(rcvr, err) for some subexpressions rcvr and err is ill-formed if rcvr is an lvalue or an rvalue of const type. Otherwise, it is expression-equivalent to MANDATE-NOTHROW(rcvr.set_error(err)).

34.7.4. execution::set_stopped [exec.set.stopped]

  1. set_stopped is a stopped completion function ([async.ops]). Its associated completion tag is set_stopped_t. The expression set_stopped(rcvr) for a subexpression rcvr is ill-formed if rcvr is an lvalue or an rvalue of const type. Otherwise, it is expression-equivalent to MANDATE-NOTHROW(rcvr.set_stopped()).

34.8. Operation states [exec.opstate]

  1. The operation_state concept defines the requirements of an operation state type ([async.ops]).

    namespace std::execution {
      template<class O>
        concept operation_state =
          derived_from<typename O::operation_state_concept, operation_state_t> &&
          is_object_v<O> &&
          requires (O& o) {
            { start(o) } noexcept;
          };
    }
    
  2. If an operation_state object is destroyed during the lifetime of its asynchronous operation ([async.ops]), the behavior is undefined. The operation_state concept does not impose requirements on any operations other than destruction and start, including copy and move operations. Invoking any such operation on an object whose type models operation_state can lead to undefined behavior.

  3. The program is ill-formed if it performs a copy or move construction or assigment operation on an operation state object created by connecting a library-provided sender.

34.8.1. execution::start [exec.opstate.start]

  1. The name start denotes a customization point object that starts ([async.ops]) the asynchronous operation associated with the operation state object. For a subexpression op, the expression start(op) is ill-formed if op is an rvalue. Otherwise, it is expression-equivalent to MANDATE-NOTHROW(op.start()).

  2. If op.start() does not start ([async.ops]) the asynchronous operation associated with the operation state op, the behavior of calling start(op) is undefined.

34.9. Senders [exec.snd]

34.9.1. General [exec.snd.general]

  1. For the purposes of this subclause, a sender is an object whose type satisfies the sender concept ([async.ops]).

  2. Subclauses [exec.factories] and [exec.adapt] define customizable algorithms that return senders. Each algorithm has a default implementation. Let sndr be the result of an invocation of such an algorithm or an object equal to the result ([concepts.equality]), and let Sndr be decltype((sndr)). Let rcvr be a receiver of type Rcvr with associated environment env of type Env such that sender_to<Sndr, Rcvr> is true. For the default implementation of the algorithm that produced sndr, connecting sndr to rcvr and starting the resulting operation state ([async.ops]) necessarily results in the potential evaluation ([basic.def.odr]) of a set of completion operations whose first argument is a subexpression equal to rcvr. Let Sigs be a pack of completion signatures corresponding to this set of completion operations. Then the type of the expression get_completion_signatures(sndr, env) is a specialization of the class template completion_signatures ([exec.utils.cmplsigs]), the set of whose template arguments is Sigs. If a user-provided implementation of the algorithm that produced sndr is selected instead of the default, any completion signature that is in the set of types denoted by completion_signatures_of_t<Sndr, Env> and that is not part of Sigs shall correspond to error or stopped completion operations, unless otherwise specified.

  3. This subclause makes use of the following exposition-only entities.

    1. For a queryable object env, FWD-ENV(env) is an expression whose type satisfies queryable such that for a query object q and a pack of subexpressions as, the expression FWD-ENV(env).query(q, as...) is ill-formed if forwarding_query(q) is false; otherwise, it is expression-equivalent to env.query(q, as...).

    2. For a query object q and a subexpression v, MAKE-ENV(q, v) is an expression env whose type satisfies queryable such that the result of env.query(q) has a value equal to v ([concepts.equality]). Unless otherwise stated, the object to which env.query(q) refers remains valid while env remains valid.

    3. For two queryable objects env1 and env2, a query object q and a pack of subexpressions as, JOIN-ENV(env1, env2) is an expression env3 whose type satisfies queryable such that env3.query(q, as...) is expression-equivalent to:

      • env1.query(q, as...) if that expression is well-formed,

      • otherwise, env2.query(q, as...) if that expression is well-formed,

      • otherwise, env3.query(q, as...) is ill-formed.

    4. The results of FWD-ENV, MAKE-ENV, and JOIN-ENV can be context-dependent; i.e., they can evaluate to expressions with different types and value categories in different contexts for the same arguments.

    5. For a scheduler sch, SCHED-ATTRS(sch) is an expression o1 whose type satisfies queryable such that o1.query(get_completion_scheduler<Tag>) is a expression with the same type and value as sch where Tag is one of set_value_t or set_stopped_t, and such that o1.query(get_domain) is expression-equivalent to sch.query(get_domain). SCHED-ENV(sch) is an expression o2 whose type satisfies queryable such that o1.query(get_scheduler) is a prvalue with the same type and value as sch, and such that o2.query(get_domain) is expression-equivalent to sch.query(get_domain).

    6. For two subexpressions rcvr and expr, SET-VALUE(rcvr, expr) is expression-equivalent to (expr, set_value(std::move(rcvr))) if the type of expr is void; otherwise, set_value(std::move(rcvr), expr). TRY-EVAL(rcvr, expr) is equivalent to:

      try {
        expr;
      } catch(...) {
        set_error(std::move(rcvr), current_exception());
      }
      

      if expr is potentially-throwing; otherwise, expr. TRY-SET-VALUE(rcvr, expr) is TRY-EVAL(rcvr, SET-VALUE(rcvr, expr)) except that rcvr is evaluated only once.

    7. template<class Default = default_domain, class Sndr>
        constexpr auto completion-domain(const Sndr& sndr) noexcept;
      
      1. COMPL-DOMAIN(T) is the type of the expression get_domain(get_completion_scheduler<T>(get_env(sndr))).

      2. Effects: If all of the types COMPL-DOMAIN(set_value_t), COMPL-DOMAIN(set_error_t), and COMPL-DOMAIN(set_stopped_t) are ill-formed, completion-domain<Default>(sndr) is a default-constructed prvalue of type Default. Otherwise, if they all share a common type ([meta.trans.other]) (ignoring those types that are ill-formed), then completion-domain<Default>(sndr) is a default-constructed prvalue of that type. Otherwise, completion-domain<Default>(sndr) is ill-formed.

    8. template<class Tag, class Env, class Default>
        constexpr decltype(auto) query-with-default(
          Tag, const Env& env, Default&& value) noexcept(see below);
      
      1. Let e be the expression Tag()(env) if that expression is well-formed; otherwise, it is static_cast<Default>(std::forward<Default>(value)).

      2. Returns: e.

      3. Remarks: The expression in the noexcept clause is noexcept(e).

    9. template<class Sndr>
        constexpr auto get-domain-early(const Sndr& sndr) noexcept;
      
      1. Effects: Equivalent to: return Domain(); where Domain is the decayed type of the first of the following expressions that is well-formed:

        • get_domain(get_env(sndr))

        • completion-domain(sndr)

        • default_domain()

    10. template<class Sndr, class Env>
        constexpr auto get-domain-late(const Sndr& sndr, const Env& env) noexcept;
      
      1. Effects: Equivalent to:

        • If sender-for<Sndr, continues_on_t> is true, then return Domain(); where Domain is the type of the following expression:

          [] {
            auto [_, sch, _] = sndr;
            return query-or-default(get_domain, sch, default_domain());
          }();
          

          The continues_on algorithm works in tandem with schedule_from ([exec.schedule.from])) to give scheduler authors a way to customize both how to transition onto (continues_on) and off of (schedule_from) a given execution context. Thus, continues_on ignores the domain of the predecessor and uses the domain of the destination scheduler to select a customization, a property that is unique to continues_on. That is why it is given special treatment here.

        • Otherwise, return Domain(); where Domain is the first of the following expressions that is well-formed and whose type is not void:

          • get_domain(get_env(sndr))

          • completion-domain<void>(sndr)

          • get_domain(env)

          • get_domain(get_scheduler(env))

          • default_domain().

    11. template<callable Fun>
        requires is_nothrow_move_constructible_v<Fun>
      struct emplace-from { // exposition only
        Fun fun; // exposition only
        using type = call-result-t<Fun>;
      
        constexpr operator type() && noexcept(nothrow-callable<Fun>) {
          return std::move(fun)();
        }
      
        constexpr type operator()() && noexcept(nothrow-callable<Fun>) {
          return std::move(fun)();
        }
      };
      
      1. emplace-from is used to emplace non-movable types into tuple, optional, variant, and similar types.

    12. struct on-stop-request { // exposition only
        inplace_stop_source& stop-src; // exposition only
        void operator()() noexcept { stop-src.request_stop(); }
      };
      
    13. template<class T0, class T1, ... class Tn>
      struct product-type {  // exposition only
        T0 t0;      // exposition only
        T1 t1;      // exposition only
          ...
        Tn tn;      // exposition only
      
        template<size_t I, class Self>
        constexpr decltype(auto) get(this Self&& self) noexcept; // exposition only
      
        template<class Self, class Fn>
        constexpr decltype(auto) apply(this Self&& self, Fn&& fn) // exposition only
          noexcept(see below);
      };
      
      1. product-type is presented here in pseudo-code form for the sake of exposition. It can be approximated in standard C++ with a tuple-like implementation that takes care to keep the type an aggregate that can be used as the initializer of a structured binding declaration.

      2. An expression of type product-type is usable as the initializer of a structured binding declaration [dcl.struct.bind].

      3. template<size_t I, class Self>
        constexpr decltype(auto) get(this Self&& self) noexcept;
        
        1. Effects: Equivalent to:

          auto& [...ts] = self;
          return std::forward_like<Self>(ts...[I]);
          
      4. template<class Self, class Fn>
        constexpr decltype(auto) apply(this Self&& self, Fn&& fn) noexcept(see below);
        
        1. Effects: Equivalent to:

          auto& [...ts] = self;
          return std::forward<Fn>(fn)(std::forward_like<Self>(ts)...);
          
        2. Requires: The expression in the return statement above is well-formed.

        3. Remarks: The expression in the noexcept clause is true if the return statement above is not potentially throwing; otherwise, false.

    14. template<class Tag, class Data = see below, class... Child>
        constexpr auto make-sender(Tag tag, Data&& data, Child&&... child);
      
      1. Mandates: The following expressions are true:

        • semiregular<Tag>

        • movable-value<Data>

        • (sender<Child> &&...)

      2. Returns: A prvalue of type basic-sender<Tag, decay_t<Data>, decay_t<Child>...> that has been direct-list-initialized with the forwarded arguments, where basic-sender is the following exposition-only class template except as noted below:

        namespace std::execution {
          template<class Tag>
          concept completion-tag = // exposition only
            same_as<Tag, set_value_t> || same_as<Tag, set_error_t> || same_as<Tag, set_stopped_t>;
        
          template<template<class...> class T, class... Args>
          concept valid-specialization = requires { typename T<Args...>; }; // exposition only
        
          struct default-impls {  // exposition only
            static constexpr auto get-attrs = see below;
            static constexpr auto get-env = see below;
            static constexpr auto get-state = see below;
            static constexpr auto start = see below;
            static constexpr auto complete = see below;
          };
        
          template<class Tag>
          struct impls-for : default-impls {}; // exposition only
        
          template<class Sndr, class Rcvr> // exposition only
          using state-type = decay_t<call-result-t<
            decltype(impls-for<tag_of_t<Sndr>>::get-state), Sndr, Rcvr&>>;
        
          template<class Index, class Sndr, class Rcvr> // exposition only
          using env-type = call-result-t<
            decltype(impls-for<tag_of_t<Sndr>>::get-env), Index,
            state-type<Sndr, Rcvr>&, const Rcvr&>;
        
          template<class Sndr, size_t I = 0>
          using child-type = decltype(declval<Sndr>().template get<I+2>()); // exposition only
        
          template<class Sndr>
          using indices-for = remove_reference_t<Sndr>::indices-for; // exposition only
        
          template<class Sndr, class Rcvr>
          struct basic-state { // exposition only
            basic-state(Sndr&& sndr, Rcvr&& rcvr) noexcept(see below)
              : rcvr(std::move(rcvr))
              , state(impls-for<tag_of_t<Sndr>>::get-state(std::forward<Sndr>(sndr), rcvr)) { }
        
            Rcvr rcvr; // exposition only
            state-type<Sndr, Rcvr> state; // exposition only
          };
        
          template<class Sndr, class Rcvr, class Index>
            requires valid-specialization<env-type, Index, Sndr, Rcvr>
          struct basic-receiver {  // exposition only
            using receiver_concept = receiver_t;
        
            using tag-t = tag_of_t<Sndr>; // exposition only
            using state-t = state-type<Sndr, Rcvr>; // exposition only
            static constexpr const auto& complete = impls-for<tag-t>::complete; // exposition only
        
            template<class... Args>
              requires callable<decltype(complete), Index, state-t&, Rcvr&, set_value_t, Args...>
            void set_value(Args&&... args) && noexcept {
              complete(Index(), op->state, op->rcvr, set_value_t(), std::forward<Args>(args)...);
            }
        
            template<class Error>
              requires callable<decltype(complete), Index, state-t&, Rcvr&, set_error_t, Error>
            void set_error(Error&& err) && noexcept {
              complete(Index(), op->state, op->rcvr, set_error_t(), std::forward<Error>(err));
            }
        
            void set_stopped() && noexcept
              requires callable<decltype(complete), Index, state-t&, Rcvr&, set_stopped_t> {
              complete(Index(), op->state, op->rcvr, set_stopped_t());
            }
        
            auto get_env() const noexcept -> env-type<Index, Sndr, Rcvr> {
              return impls-for<tag-t>::get-env(Index(), op->state, op->rcvr);
            }
        
            basic-state<Sndr, Rcvr>* op; // exposition only
          };
        
          constexpr auto connect-all = see below; // exposition only
        
          template<class Sndr, class Rcvr>
          using connect-all-result = call-result-t<  // exposition only
            decltype(connect-all), basic-state<Sndr, Rcvr>*, Sndr, indices-for<Sndr>>;
        
          template<class Sndr, class Rcvr>
            requires valid-specialization<state-type, Sndr, Rcvr> &&
                     valid-specialization<connect-all-result, Sndr, Rcvr>
          struct basic-operation : basic-state<Sndr, Rcvr> {  // exposition only
            using operation_state_concept = operation_state_t;
            using tag-t = tag_of_t<Sndr>; // exposition only
        
            connect-all-result<Sndr, Rcvr> inner-ops; // exposition only
        
            basic-operation(Sndr&& sndr, Rcvr&& rcvr) noexcept(see below)  // exposition only
              : basic-state<Sndr, Rcvr>(std::forward<Sndr>(sndr), std::move(rcvr))
              , inner-ops(connect-all(this, std::forward<Sndr>(sndr), indices-for<Sndr>()))
            {}
        
            void start() & noexcept {
              auto& [...ops] = inner-ops;
              impls-for<tag-t>::start(this->state, this->rcvr, ops...);
            }
          };
        
          template<class Sndr, class Env>
          using completion-signatures-for = see below; // exposition only
        
          template<class Tag, class Data, class... Child>
          struct basic-sender : product-type<Tag, Data, Child...> {  // exposition only
            using sender_concept = sender_t;
            using indices-for = index_sequence_for<Child...>; // exposition only
        
            decltype(auto) get_env() const noexcept {
              auto& [_, data, ...child] = *this;
              return impls-for<Tag>::get-attrs(data, child...);
            }
        
            template<decays-to<basic-sender> Self, receiver Rcvr>
            auto connect(this Self&& self, Rcvr rcvr) noexcept(see below)
              -> basic-operation<Self, Rcvr> {
              return {std::forward<Self>(self), std::move(rcvr)};
            }
        
            template<decays-to<basic-sender> Self, class Env>
            auto get_completion_signatures(this Self&& self, Env&& env) noexcept
              -> completion-signatures-for<Self, Env> {
              return {};
            }
          };
        }
        
      3. Remarks: The default template argument for the Data template parameter denotes an unspecified empty trivially copyable class type that models semiregular.

      4. It is unspecified whether a specialization of basic-sender is an aggregate.

      5. An expression of type basic-sender is usable as the initializer of a structured binding declaration [dcl.struct.bind].

      6. The expression in the noexcept clause of the constructor of basic-state is:

        is_nothrow_move_constructible_v<Rcvr> &&
        nothrow-callable<decltype(impls-for<tag_of_t<Sndr>>::get-state), Sndr, Rcvr&>
        
      7. The object connect-all is initialized with a callable object equivalent to the following lambda:

        []<class Sndr, class Rcvr, size_t... Is>(
          basic-state<Sndr, Rcvr>* op, Sndr&& sndr, index_sequence<Is...>) noexcept(see below)
            -> decltype(auto) {
            auto& [_, data, ...child] = sndr;
            return product-type{connect(
              std::forward_like<Sndr>(child),
              basic-receiver<Sndr, Rcvr, integral_constant<size_t, Is>>{op})...};
          }
        
        1. Requires: The expression in the return statement is well-formed.

        2. Remarks: The expression in the noexcept clause is true if the return statement is not potentially throwing; otherwise, false.

      8. The expression in the noexcept clause of the constructor of basic-operation is:

        is_nothrow_constructible_v<basic-state<Self, Rcvr>, Self, Rcvr> &&
        noexcept(connect-all(this, std::forward<Sndr>(sndr), indices-for<Sndr>()))
        
      9. The expression in the noexcept clause of the connect member function of basic-sender is:

        is_nothrow_constructible_v<basic-operation<Self, Rcvr>, Self, Rcvr>
        
      10. The member default-impls::get-attrs is initialized with a callable object equivalent to the following lambda:

        [](const auto&, const auto&... child) noexcept -> decltype(auto) {
          if constexpr (sizeof...(child) == 1)
            return (FWD-ENV(get_env(child)), ...);
          else
            return empty_env();
        }
        
      11. The member default-impls::get-env is initialized with a callable object equivalent to the following lambda:

        [](auto, auto&, const auto& rcvr) noexcept -> decltype(auto) {
          return FWD-ENV(get_env(rcvr));
        }
        
      12. The member default-impls::get-state is initialized with a callable object equivalent to the following lambda:

        []<class Sndr, class Rcvr>(Sndr&& sndr, Rcvr& rcvr) noexcept -> decltype(auto) {
          auto& [_, data, ...child] = sndr;
          return std::forward_like<Sndr>(data);
        }
        
      13. The member default-impls::start is initialized with a callable object equivalent to the following lambda:

        [](auto&, auto&, auto&... ops) noexcept -> void {
          (execution::start(ops), ...);
        }
        
      14. The member default-impls::complete is initialized with a callable object equivalent to the following lambda:

        []<class Index, class Rcvr, class Tag, class... Args>(
          Index, auto& state, Rcvr& rcvr, Tag, Args&&... args) noexcept
            -> void requires callable<Tag, Rcvr, Args...> {
          // Mandates: Index::value == 0
          Tag()(std::move(rcvr), std::forward<Args>(args)...);
        }
        
      15. For a subexpression sndr let Sndr be decltype((sndr)). Let rcvr be a receiver with an associated environment of type Env such that sender_in<Sndr, Env> is true. completion-signatures-for<Sndr, Env> denotes a specialization of completion_signatures, the set of whose template arguments correspond to the set of completion operations that are potentially evaluated as a result of starting ([async.ops]) the operation state that results from connecting sndr and rcvr. When sender_in<Sndr, Env> is false, the type denoted by completion-signatures-for<Sndr, Env>, if any, is not a specialization of completion_signatures.

        Recommended practice: When sender_in<Sndr, Env> is false, implementations are encouraged to use the type denoted by completion-signatures-for<Sndr, Env> to communicate to users why.

    15. template<sender Sndr, queryable Env>
        constexpr auto write-env(Sndr&& sndr, Env&& env); // exposition only
      
      1. write-env is an exposition-only sender adaptor that, when connected with a receiver rcvr, connects the adapted sender with a receiver whose execution environment is the result of joining the queryable argument env to the result of get_env(rcvr).

      2. Let write-env-t be an exposition-only empty class type.

      3. Returns: make-sender(write-env-t(), std::forward<Env>(env), std::forward<Sndr>(sndr)).

      4. Remarks: The exposition-only class template impls-for ([exec.snd.general]) is specialized for write-env-t as follows:

        template<>
        struct impls-for<write-env-t> : default-impls {
          static constexpr auto get-env =
            [](auto, const auto& state, const auto& rcvr) noexcept {
              return JOIN-ENV(state, get_env(rcvr));
            };
        };
        

34.9.2. Sender concepts [exec.snd.concepts]

  1. The sender concept defines the requirements for a sender type ([async.ops]). The sender_in concept defines the requirements for a sender type that can create asynchronous operations given an associated environment type. The sender_to concept defines the requirements for a sender type that can connect with a specific receiver type. The get_env customization point object is used to access a sender’s associated attributes. The connect customization point object is used to connect ([async.ops]) a sender and a receiver to produce an operation state.

    namespace std::execution {
      template<class Sigs>
        concept valid-completion-signatures = see below; // exposition only
    
      template<class Sndr>
        concept is-sender = // exposition only
          derived_from<typename Sndr::sender_concept, sender_t>;
    
      template<class Sndr>
        concept enable-sender = // exposition only
          is-sender<Sndr> ||
          is-awaitable<Sndr, env-promise<empty_env>>;  // [exec.awaitables]
    
      template<class Sndr>
        concept sender =
          bool(enable-sender<remove_cvref_t<Sndr>>) && // atomic constraint ([temp.constr.atomic])
          requires (const remove_cvref_t<Sndr>& sndr) {
            { get_env(sndr) } -> queryable;
          } &&
          move_constructible<remove_cvref_t<Sndr>> &&  // senders are movable and
          constructible_from<remove_cvref_t<Sndr>, Sndr>; // decay copyable
    
      template<class Sndr, class Env = empty_env>
        concept sender_in =
          sender<Sndr> &&
          queryable<Env> &&
          requires (Sndr&& sndr, Env&& env) {
            { get_completion_signatures(std::forward<Sndr>(sndr), std::forward<Env>(env)) }
              -> valid-completion-signatures;
          };
    
      template<class Sndr, class Rcvr>
        concept sender_to =
          sender_in<Sndr, env_of_t<Rcvr>> &&
          receiver_of<Rcvr, completion_signatures_of_t<Sndr, env_of_t<Rcvr>>> &&
          requires (Sndr&& sndr, Rcvr&& rcvr) {
            connect(std::forward<Sndr>(sndr), std::forward<Rcvr>(rcvr));
          };
    }
    
  2. Given a subexpression sndr, let Sndr be decltype((sndr)) and let rcvr be a receiver with an associated environment whose type is Env. A completion operation is a permissible completion for Sndr and Env if its completion signature appears in the argument list of the specialization of completion_signatures denoted by completion_signatures_of_t<Sndr, Env>. Sndr and Env model sender_in<Sndr, Env> if all the completion operations that are potentially evaluated by connecting sndr to rcvr and starting the resulting operation state are permissible completions for Sndr and Env.

  3. A type models the exposition-only concept valid-completion-signatures if it denotes a specialization of the completion_signatures class template.

  4. The exposition-only concepts sender-of and sender-in-of define the requirements for a sender type that completes with a given unique set of value result types.

    namespace std::execution {
      template<class... As>
        using value-signature = set_value_t(As...); // exposition only
    
      template<class Sndr, class Env, class... Values>
        concept sender-in-of =
          sender_in<Sndr, Env> &&
          MATCHING-SIG( // see [exec.general]
            set_value_t(Values...),
            value_types_of_t<Sndr, Env, value-signature, type_identity_t>);
    
      template<class Sndr, class... Values>
        concept sender-of = sender-in-of<Sndr, empty_env, Values...>;
    }
    
  5. Let sndr be an expression such that decltype((sndr)) is Sndr. The type tag_of_t<Sndr> is as follows:

    • If the declaration auto&& [tag, data, ...children] = sndr; would be well-formed, tag_of_t<Sndr> is an alias for decltype(auto(tag)).

    • Otherwise, tag_of_t<Sndr> is ill-formed.

  6. Let sender-for be an exposition-only concept defined as follows:

    namespace std::execution {
      template<class Sndr, class Tag>
      concept sender-for =
        sender<Sndr> &&
        same_as<tag_of_t<Sndr>, Tag>;
    }
    
  7. For a type T, SET-VALUE-SIG(T) denotes the type set_value_t() if T is cv void; otherwise, it denotes the type set_value_t(T).

  8. Library-provided sender types:

    • Always expose an overload of a member connect that accepts an rvalue sender.

    • Only expose an overload of a member connect that accepts an lvalue sender if they model copy_constructible.

34.9.3. Awaitable helpers [exec.awaitables]

  1. The sender concepts recognize awaitables as senders. For [exec], an awaitable is an expression that would be well-formed as the operand of a co_await expression within a given context.

  2. For a subexpression c, let GET-AWAITER(c, p) be expression-equivalent to the series of transformations and conversions applied to c as the operand of an await-expression in a coroutine, resulting in lvalue e as described by [expr.await], where p is an lvalue referring to the coroutine’s promise, which has type Promise. This includes the invocation of the promise type’s await_transform member if any, the invocation of the operator co_await picked by overload resolution if any, and any necessary implicit conversions and materializations.

    I have opened cwg#250 to give these transformations a term-of-art so we can more easily refer to it here.

  3. Let is-awaitable be the following exposition-only concept:

    namespace std {
      template<class T>
      concept await-suspend-result = see below; // exposition only
    
      template<class A, class Promise>
      concept is-awaiter = // exposition only
        requires (A& a, coroutine_handle<Promise> h) {
          a.await_ready() ? 1 : 0;
          { a.await_suspend(h) } -> await-suspend-result;
          a.await_resume();
        };
    
      template<class C, class Promise>
      concept is-awaitable =
        requires (C (*fc)() noexcept, Promise& p) {
          { GET-AWAITER(fc(), p) } -> is-awaiter<Promise>;
        };
    }
    

    await-suspend-result<T> is true if and only if one of the following is true:

    • T is void, or

    • T is bool, or

    • T is a specialization of coroutine_handle.

  4. For a subexpression c such that decltype((c)) is type C, and an lvalue p of type Promise, await-result-type<C, Promise> denotes the type decltype(GET-AWAITER(c, p).await_resume()).

  5. Let with-await-transform be the exposition-only class template:

    namespace std::execution {
      template<class T, class Promise>
        concept has-as-awaitable = // exposition only
          requires (T&& t, Promise& p) {
            { std::forward<T>(t).as_awaitable(p) } -> is-awaitable<Promise&>;
          };
    
      template<class Derived>
        struct with-await-transform {
          template<class T>
            T&& await_transform(T&& value) noexcept {
              return std::forward<T>(value);
            }
    
          template<has-as-awaitable<Derived> T>
            decltype(auto) await_transform(T&& value)
              noexcept(noexcept(std::forward<T>(value).as_awaitable(declval<Derived&>()))) {
              return std::forward<T>(value).as_awaitable(static_cast<Derived&>(*this));
            }
        };
    }
    
  6. Let env-promise be the exposition-only class template:

    namespace std::execution {
      template<class Env>
      struct env-promise : with-await-transform<env-promise<Env>> {
        unspecified get_return_object() noexcept;
        unspecified initial_suspend() noexcept;
        unspecified final_suspend() noexcept;
        void unhandled_exception() noexcept;
        void return_void() noexcept;
        coroutine_handle<> unhandled_stopped() noexcept;
    
        const Env& get_env() const noexcept;
      };
    }
    

    Specializations of env-promise are only used for the purpose of type computation; its members need not be defined.

34.9.4. execution::default_domain [exec.domain.default]

namespace std::execution {
  struct default_domain {
    template<sender Sndr, queryable... Env>
        requires (sizeof...(Env) <= 1)
      static constexpr sender decltype(auto) transform_sender(Sndr&& sndr, const Env&... env)
        noexcept(see below);

    template<sender Sndr, queryable Env>
      static constexpr queryable decltype(auto) transform_env(Sndr&& sndr, Env&& env) noexcept;

    template<class Tag, sender Sndr, class... Args>
      static constexpr decltype(auto) apply_sender(Tag, Sndr&& sndr, Args&&... args)
        noexcept(see below);
  };
}
34.9.4.1. Static members [exec.domain.default.statics]
template<sender Sndr, queryable... Env>
    requires (sizeof...(Env) <= 1)
  constexpr sender decltype(auto) transform_sender(Sndr&& sndr, const Env&... env)
    noexcept(see below);
  1. Let e be the expression tag_of_t<Sndr>().transform_sender(std::forward<Sndr>(sndr), env...) if that expression is well-formed; otherwise, std::forward<Sndr>(sndr).

  2. Returns: e.

  3. Remarks: The exception specification is equivalent to noexcept(e).

template<sender Sndr, queryable Env>
  constexpr queryable decltype(auto) transform_env(Sndr&& sndr, Env&& env) noexcept;
  1. Let e be the expression tag_of_t<Sndr>().transform_env(std::forward<Sndr>(sndr), std::forward<Env>(env)) if that expression is well-formed; otherwise, static_cast<Env>(std::forward<Env>(env)).

  2. Mandates: noexcept(e) is true.

  3. Returns: e.

template<class Tag, sender Sndr, class... Args>
  constexpr decltype(auto) apply_sender(Tag, Sndr&& sndr, Args&&... args)
    noexcept(see below);
  1. Let e be the expression Tag().apply_sender(std::forward<Sndr>(sndr), std::forward<Args>(args)...).

  2. Constraints: e is a well-formed expression.

  3. Returns: e.

  4. Remarks: The exception specification is equivalent to noexcept(e).

34.9.5. execution::transform_sender [exec.snd.transform]

namespace std::execution {
  template<class Domain, sender Sndr, queryable... Env>
      requires (sizeof...(Env) <= 1)
    constexpr sender decltype(auto) transform_sender(Domain dom, Sndr&& sndr, const Env&... env)
      noexcept(see below);
}
  1. Let transformed-sndr be the expression dom.transform_sender(std::forward<Sndr>(sndr), env...) if that expression is well-formed; otherwise, default_domain().transform_sender(std::forward<Sndr>(sndr), env...). Let final-sndr be the expression transformed-sndr if transformed-sndr and sndr have the same type ignoring cv qualifiers; otherwise, it is the expression transform_sender(dom, transformed-sndr, env...).

  2. Returns: final-sndr.

  3. Remarks: The exception specification is equivalent to noexcept(final-sndr).

34.9.6. execution::transform_env [exec.snd.transform.env]

namespace std::execution {
  template<class Domain, sender Sndr, queryable Env>
    constexpr queryable decltype(auto) transform_env(Domain dom, Sndr&& sndr, Env&& env) noexcept;
}
  1. Let e be the expression dom.transform_env(std::forward<Sndr>(sndr), std::forward<Env>(env)) if that expression is well-formed; otherwise, default_domain().transform_env(std::forward<Sndr>(sndr), std::forward<Env>(env)).

  2. Mandates: noexcept(e) is true.

  3. Returns: e.

34.9.7. execution::apply_sender [exec.snd.apply]

namespace std::execution {
  template<class Domain, class Tag, sender Sndr, class... Args>
    constexpr decltype(auto) apply_sender(Domain dom, Tag, Sndr&& sndr, Args&&... args)
      noexcept(see below);
}
  1. Let e be the expression dom.apply_sender(Tag(), std::forward<Sndr>(sndr), std::forward<Args>(args)...) if that expression is well-formed; otherwise, default_domain().apply_sender(Tag(), std::forward<Sndr>(sndr), std::forward<Args>(args)...).

  2. Constraints: The expression e is well-formed.

  3. Returns: e.

  4. Remarks: The exception specification is equivalent to noexcept(e).

34.9.8. execution::get_completion_signatures [exec.getcomplsigs]

  1. get_completion_signatures is a customization point object. Let sndr be an expression such that decltype((sndr)) is Sndr, and let env be an expression such that decltype((env)) is Env. Let new_sndr be the expression transform_sender(decltype(get-domain-late(sndr, env)){}, sndr, env), and let NewSndr be decltype((new_sndr)). Then get_completion_signatures(sndr, env) is expression-equivalent to (void(sndr), void(env), CS()) except that void(sndr) and void(env) are indeterminately sequenced, where CS is:

    1. decltype(new_sndr.get_completion_signatures(env)) if that type is well-formed,

    2. Otherwise, remove_cvref_t<NewSndr>::completion_signatures if that type is well-formed,

    3. Otherwise, if is-awaitable<NewSndr, env-promise<Env>> is true, then:

      completion_signatures<
        SET-VALUE-SIG(await-result-type<NewSndr,
                      env-promise<Env>>), // see [exec.snd.concepts]
        set_error_t(exception_ptr),
        set_stopped_t()>
      
    4. Otherwise, CS is ill-formed.

  2. Let rcvr be an rvalue whose type Rcvr models receiver, and let Sndr be the type of a sender such that sender_in<Sndr, env_of_t<Rcvr>> is true. Let Sigs... be the template arguments of the completion_signatures specialization named by completion_signatures_of_t<Sndr, env_of_t<Rcvr>>. Let CSO be a completion function. If sender Sndr or its operation state cause the expression CSO(rcvr, args...) to be potentially evaluated ([basic.def.odr]) then there shall be a signature Sig in Sigs... such that MATCHING-SIG(decayed-typeof<CSO>(decltype(args)...), Sig) is true ([exec.general]).

34.9.9. execution::connect [exec.connect]

  1. connect connects ([async.ops]) a sender with a receiver.

  2. The name connect denotes a customization point object. For subexpressions sndr and rcvr, let Sndr be decltype((sndr)) and Rcvr be decltype((rcvr)), let new_sndr be the expression transform_sender(decltype(get-domain-late(sndr, get_env(rcvr))){}, sndr, get_env(rcvr)), and let DS and DR be decay_t<decltype((new_sndr))> and decay_t<Rcvr>, respectively.

  3. Let connect-awaitable-promise be the following exposition-only class:

    namespace std::execution {
      struct connect-awaitable-promise
        : with-await-transform<connect-awaitable-promise> {
    
        connect-awaitable-promise(DS&, DR& rcvr) noexcept : rcvr(rcvr) {}
    
        suspend_always initial_suspend() noexcept { return {}; }
        [[noreturn]] suspend_always final_suspend() noexcept { terminate(); }
        [[noreturn]] void unhandled_exception() noexcept { terminate(); }
        [[noreturn]] void return_void() noexcept { terminate(); }
    
        coroutine_handle<> unhandled_stopped() noexcept {
          set_stopped(std::move(rcvr));
          return noop_coroutine();
        }
    
        operation-state-task get_return_object() noexcept {
          return operation-state-task{
            coroutine_handle<connect-awaitable-promise>::from_promise(*this)};
        }
    
        env_of_t<DR> get_env() const noexcept {
          return execution::get_env(rcvr);
        }
    
      private:
        DR& rcvr; // exposition only
      };
    }
    
  4. Let operation-state-task be the following exposition-only class:

    namespace std::execution {
      struct operation-state-task {
        using operation_state_concept = operation_state_t;
        using promise_type = connect-awaitable-promise;
    
        explicit operation-state-task(coroutine_handle<> h) noexcept : coro(h) {}
        operation-state-task(operation-state-task&& o) noexcept
          : coro(exchange(o.coro, {})) {}
        ~operation-state-task() { if (coro) coro.destroy(); }
    
        void start() & noexcept {
          coro.resume();
        }
    
      private:
        coroutine_handle<> coro; // exposition only
      };
    }
    
  5. Let V name the type await-result-type<DS, connect-awaitable-promise>, let Sigs name the type:

    completion_signatures<
      SET-VALUE-SIG(V), // see [exec.snd.concepts]
      set_error_t(exception_ptr),
      set_stopped_t()>
    

    and let connect-awaitable be an exposition-only coroutine defined as follows:

    namespace std::execution {
      template<class Fun, class... Ts>
      auto suspend-complete(Fun fun, Ts&&... as) noexcept { // exposition only
        auto fn = [&, fun]() noexcept { fun(std::forward<Ts>(as)...); };
    
        struct awaiter {
          decltype(fn) fn;
    
          static constexpr bool await_ready() noexcept { return false; }
          void await_suspend(coroutine_handle<>) noexcept { fn(); }
          [[noreturn]] void await_resume() noexcept { unreachable(); }
        };
        return awaiter{fn};
      }
    
      operation-state-task connect-awaitable(DS sndr, DR rcvr) requires receiver_of<DR, Sigs> {
        exception_ptr ep;
        try {
          if constexpr (same_as<V, void>) {
            co_await std::move(sndr);
            co_await suspend-complete(set_value, std::move(rcvr));
          } else {
            co_await suspend-complete(set_value, std::move(rcvr), co_await std::move(sndr));
          }
        } catch(...) {
          ep = current_exception();
        }
        co_await suspend-complete(set_error, std::move(rcvr), std::move(ep));
      }
    }
    
  6. The expression connect(sndr, rcvr) is expression-equivalent to:

    1. new_sndr.connect(rcvr) if that expression is well-formed.

      • Mandates: The type of the expression above satisfies operation_state.

    2. Otherwise, connect-awaitable(new_sndr, rcvr).

    3. Mandates: sender<Sndr> && receiver<Rcvr> is true.

34.9.10. Sender factories [exec.factories]

34.9.10.1. execution::schedule [exec.schedule]
  1. schedule obtains a schedule sender ([async.ops]) from a scheduler.

  2. The name schedule denotes a customization point object. For a subexpression sch, the expression schedule(sch) is expression-equivalent to sch.schedule().

    1. If the expression get_completion_scheduler<set_value_t>( get_env(sch.schedule())) == sch is ill-formed or evaluates to false, the behavior of calling schedule(sch) is undefined.

    2. Mandates: The type of sch.schedule() satisfies sender.

34.9.10.2. execution::just, execution::just_error, execution::just_stopped [exec.just]
  1. just, just_error, and just_stopped are sender factories whose asynchronous operations complete synchronously in their start operation with a value completion operation, an error completion operation, or a stopped completion operation respectively.

  2. The names just, just_error, and just_stopped denote customization point objects. Let just-cpo be one of just, just_error, or just_stopped. For a pack of subexpressions ts, let Ts be the pack of types decltype((ts)). The expression just-cpo(ts...) is ill-formed if:

    • (movable-value<Ts> &&...) is false, or

    • just-cpo is just_error and sizeof...(ts) == 1 is false, or

    • just-cpo is just_stopped and sizeof...(ts) == 0 is false;

    Otherwise, it is expression-equivalent to make-sender(just-cpo, product-type{ts...}).

  3. For just, just_error, and just_stopped, let set-cpo be set_value, set_error, and set_stopped respectively. The exposition-only class template impls-for ([exec.snd.general]) is specialized for just-cpo as follows:

    namespace std::execution {
      template<>
      struct impls-for<decayed-typeof<just-cpo>> : default-impls {
        static constexpr auto start =
          [](auto& state, auto& rcvr) noexcept -> void {
            auto& [...ts] = state;
            set-cpo(std::move(rcvr), std::move(ts)...);
          };
      };
    }
    
34.9.10.3. execution::read_env [exec.read.env]
  1. read_env is a sender factory for a sender whose asynchronous operation completes synchronously in its start operation with a value completion result equal to a value read from the receiver’s associated environment.

  2. read_env is a customization point object. For some query object q, the expression read_env(q) is expression-equivalent to make-sender(read_env, q).

  3. The exposition-only class template impls-for ([exec.snd.general]) is specialized for read_env as follows:

    namespace std::execution {
      template<>
      struct impls-for<decayed-typeof<read_env>> : default-impls {
        static constexpr auto start =
          [](auto query, auto& rcvr) noexcept -> void {
            TRY-SET-VALUE(rcvr, query(get_env(rcvr)));
          };
      };
    }
    

34.9.11. Sender adaptors [exec.adapt]

34.9.11.1. General [exec.adapt.general]
  1. [exec.adapt] specifies a set of sender adaptors.

  2. The bitwise inclusive OR operator is overloaded for the purpose of creating sender chains. The adaptors also support function call syntax with equivalent semantics.

  3. Unless otherwise specified:

    1. A sender adaptor is prohibited from causing observable effects, apart from moving and copying its arguments, before the returned sender is connected with a receiver using connect, and start is called on the resulting operation state.

    2. A parent sender ([async.ops]) with a single child sender sndr has an associated attribute object equal to FWD-ENV(get_env(sndr)) ([exec.fwd.env]).

    3. A parent sender with more than one child sender has an associated attributes object equal to empty_env{}.

    4. When a parent sender is connected to a receiver rcvr, any receiver used to connect a child sender has an associated environment equal to FWD-ENV(get_env(rcvr)).

    These requirements apply to any function that is selected by the implementation of the sender adaptor.

  4. If a sender returned from a sender adaptor specified in [exec.adapt] is specified to include set_error_t(Err) among its set of completion signatures where decay_t<Err> denotes the type exception_ptr, but the implementation does not potentially evaluate an error completion operation with an exception_ptr argument, the implementation is allowed to omit the exception_ptr error completion signature from the set.

34.9.11.2. Sender adaptor closure objects [exec.adapt.objects]
  1. A pipeable sender adaptor closure object is a function object that accepts one or more sender arguments and returns a sender. For a pipeable sender adaptor closure object c and an expression sndr such that decltype((sndr)) models sender, the following expressions are equivalent and yield a sender:

    c(sndr)
    sndr | c
    

    Given an additional pipeable sender adaptor closure object d, the expression c | d produces another pipeable sender adaptor closure object e:

    e is a perfect forwarding call wrapper ([func.require]) with the following properties:

    • Its target object is an object d2 of type decltype(auto(d)) direct-non-list-initialized with d.

    • It has one bound argument entity, an object c2 of type decltype(auto(c)) direct-non-list-initialized with c.

    • Its call pattern is d2(c2(arg)), where arg is the argument used in a function call expression of e.

The expression c | d is well-formed if and only if the initializations of the state entities ([func.def]) of e are all well-formed.

  1. An object t of type T is a pipeable sender adaptor closure object if T models derived_from<sender_adaptor_closure<T>>, T has no other base classes of type sender_adaptor_closure<U> for any other type U, and T does not satisfy sender.

  2. The template parameter D for sender_adaptor_closure can be an incomplete type. Before any expression of type cv D appears as an operand to the | operator, D shall be complete and model derived_from<sender_adaptor_closure<D>>. The behavior of an expression involving an object of type cv D as an operand to the | operator is undefined if overload resolution selects a program-defined operator| function.

  3. A pipeable sender adaptor object is a customization point object that accepts a sender as its first argument and returns a sender.

  4. If a pipeable sender adaptor object accepts only one argument, then it is a pipeable sender adaptor closure object.

  5. If a pipeable sender adaptor object adaptor accepts more than one argument, then let sndr be an expression such that decltype((sndr)) models sender, let args... be arguments such that adaptor(sndr, args...) is a well-formed expression as specified below, and let BoundArgs be a pack that denotes decltype(auto(args)).... The expression adaptor(args...) produces a pipeable sender adaptor closure object f that is a perfect forwarding call wrapper with the following properties:

    • Its target object is a copy of adaptor.

    • Its bound argument entities bound_args consist of objects of types BoundArgs... direct-non-list-initialized with std::forward<decltype((args))>(args)..., respectively.

    • Its call pattern is adaptor(rcvr, bound_args...), where rcvr is the argument used in a function call expression of f.

    The expression adaptor(args...) is well-formed if and only if the initializations of the bound argument entities of the result, as specified above, are all well-formed.

34.9.11.3. execution::starts_on [exec.starts.on]
  1. starts_on adapts an input sender into a sender that will start on an execution agent belonging to a particular scheduler’s associated execution resource.

  2. The name starts_on denotes a customization point object. For subexpressions sch and sndr, if decltype((sch)) does not satisfy scheduler, or decltype((sndr)) does not satisfy sender, starts_on(sch, sndr) is ill-formed.

  3. Otherwise, the expression starts_on(sch, sndr) is expression-equivalent to:

    transform_sender(
      query-or-default(get_domain, sch, default_domain()),
      make-sender(starts_on, sch, sndr))
    

    except that sch is evaluated only once.

  4. Let out_sndr and env be subexpressions such that OutSndr is decltype((out_sndr)). If sender-for<OutSndr, starts_on_t> is false, then the expressions starts_on.transform_env(out_sndr, env) and starts_on.transform_sender(out_sndr, env) are ill-formed; otherwise:

    • starts_on.transform_env(out_sndr, env) is equivalent to:

      auto&& [_, sch, _] = out_sndr;
      return JOIN-ENV(SCHED-ENV(sch), FWD-ENV(env));
      
    • starts_on.transform_sender(out_sndr, env) is equivalent to:

      auto&& [_, sch, sndr] = out_sndr;
      return let_value(
        schedule(sch),
        [sndr = std::forward_like<OutSndr>(sndr)]() mutable
          noexcept(is_nothrow_move_constructible_v<decay_t>) {
          return std::move(sndr);
        });
      
  5. Let out_sndr be a subexpression denoting a sender returned from starts_on(sch, sndr) or one equal to such, and let OutSndr be the type decltype((out_sndr)). Let out_rcvr be a subexpression denoting a receiver that has an environment of type Env such that sender_in<OutSndr, Env> is true. Let op be an lvalue referring to the operation state that results from connecting out_sndr with out_rcvr. Calling start(op) shall start sndr on an execution agent of the associated execution resource of sch. If scheduling onto sch fails, an error completion on out_rcvr shall be executed on an unspecified execution agent.

34.9.11.4. execution::continues_on [exec.continues.on]
  1. continues_on adapts a sender into one that completes on the specified scheduler.

  2. The name continues_on denotes a pipeable sender adaptor object. For subexpressions sch and sndr, if decltype((sch)) does not satisfy scheduler, or decltype((sndr)) does not satisfy sender, continues_on(sndr, sch) is ill-formed.

  3. Otherwise, the expression continues_on(sndr, sch) is expression-equivalent to:

    transform_sender(
      get-domain-early(sndr),
      make-sender(continues_on, sch, sndr))
    

    except that sndr is evaluated only once.

  4. The exposition-only class template impls-for is specialized for continues_on_t as follows:

    namespace std::execution {
      template<>
      struct impls-for<continues_on_t> : default-impls {
        static constexpr auto get_attrs =
          [](const auto& data, const auto& child) noexcept -> decltype(auto) {
            return JOIN-ENV(SCHED-ATTRS(data), FWD-ENV(get_env(child)));
          };
      };
    }
    
  5. Let sndr and env be subexpressions such that Sndr is decltype((sndr)). If sender-for<Sndr, continues_on_t> is false, then the expression continues_on.transform_sender(sndr, env) is ill-formed; otherwise, it is equal to:

    auto [_, data, child] = sndr;
    return schedule_from(std::move(data), std::move(child));
    

    This causes the continues_on(sndr, sch) sender to become schedule_from(sch, sndr) when it is connected with a receiver whose execution domain does not customize continues_on.

  6. Let out_sndr be a subexpression denoting a sender returned from continues_on(sndr, sch) or one equal to such, and let OutSndr be the type decltype((out_sndr)). Let out_rcvr be a subexpression denoting a receiver that has an environment of type Env such that sender_in<OutSndr, Env> is true. Let op be an lvalue referring to the operation state that results from connecting out_sndr with out_rcvr. Calling start(op) shall start sndr on the current execution agent and execute completion operations on out_rcvr on an execution agent of the execution resource associated with sch. If scheduling onto sch fails, an error completion on out_rcvr shall be executed on an unspecified execution agent.

34.9.11.5. execution::schedule_from [exec.schedule.from]
  1. schedule_from schedules work dependent on the completion of a sender onto a scheduler’s associated execution resource. schedule_from is not meant to be used in user code; it is used in the implementation of continues_on.

  2. The name schedule_from denotes a customization point object. For some subexpressions sch and sndr, let Sch be decltype((sch)) and Sndr be decltype((sndr)). If Sch does not satisfy scheduler, or Sndr does not satisfy sender, schedule_from(sch, sndr) is ill-formed.

  3. Otherwise, the expression schedule_from(sch, sndr) is expression-equivalent to:

    transform_sender(
      query-or-default(get_domain, sch, default_domain()),
      make-sender(schedule_from, sch, sndr))
    

    except that sch is evaluated only once.

  4. The exposition-only class template impls-for ([exec.snd.general]) is specialized for schedule_from_t as follows:

    namespace std::execution {
      template<>
      struct impls-for<schedule_from_t> : default-impls {
        static constexpr auto get-attrs = see below;
        static constexpr auto get-state = see below;
        static constexpr auto complete = see below;
      };
    }
    
    1. The member impls-for<schedule_from_t>::get-attrs is initialized with a callable object equivalent to the following lambda:

      [](const auto& data, const auto& child) noexcept -> decltype(auto) {
        return JOIN-ENV(SCHED-ATTRS(data), FWD-ENV(get_env(child)));
      }
      
    2. The member impls-for<schedule_from_t>::get-state is initialized with a callable object equivalent to the following lambda:

      []<class Sndr, class Rcvr>(Sndr&& sndr, Rcvr& rcvr) noexcept(see below)
          requires sender_in<child-type<Sndr>, env_of_t<Rcvr>> {
      
        auto& [_, sch, child] = sndr;
      
        using sched_t = decltype(auto(sch));
        using variant_t = see below;
        using receiver_t = see below;
        using operation_t = connect_result_t<schedule_result_t<sched_t>, receiver_t>;
        constexpr bool nothrow = noexcept(connect(schedule(sch), receiver_t{nullptr}));
      
        struct state-type {
          Rcvr& rcvr;          // exposition only
          variant_t async-result; // exposition only
          operation_t op-state;   // exposition only
      
          explicit state-type(sched_t sch, Rcvr& rcvr) noexcept(nothrow)
            : rcvr(rcvr), op-state(connect(schedule(sch), receiver_t{this})) {}
        };
      
        return state-type{sch, rcvr};
      }
      
      1. Objects of the local class state-type can be used to initialize a structured binding.

      2. Let Sigs be a pack of the arguments to the completion_signatures specialization named by completion_signatures_of_t<child-type<Sndr>, env_of_t<Rcvr>>. Let as-tuple be an alias template that transforms a completion signature Tag(Args...) into the tuple specialization decayed-tuple<Tag, Args...>. Then variant_t denotes the type variant<monostate, as-tuple<Sigs>...>, except with duplicate types removed.

      3. receiver_t is an alias for the following exposition-only class:

        namespace std::execution {
          struct receiver-type {
            using receiver_concept = receiver_t;
            state-type* state; // exposition only
        
            void set_value() && noexcept {
              visit(
                [this]<class Tuple>(Tuple& result) noexcept -> void {
                  if constexpr (!same_as<monostate, Tuple>) {
                    auto& [tag, ...args] = result;
                    tag(std::move(state->rcvr), std::move(args)...);
                  }
                },
                state->async-result);
            }
        
            template<class Error>
            void set_error(Error&& err) && noexcept {
              execution::set_error(std::move(state->rcvr), std::forward<Error>(err));
            }
        
            void set_stopped() && noexcept {
              execution::set_stopped(std::move(state->rcvr));
            }
        
            decltype(auto) get_env() const noexcept {
              return FWD-ENV(execution::get_env(state->rcvr));
            }
          };
        }
        
      4. The expression in the noexcept clause of the lambda is true if the construction of the returned state-type object is not potentially throwing; otherwise, false.

    3. The member impls-for<schedule_from_t>::complete is initialized with a callable object equivalent to the following lambda:

      []<class Tag, class... Args>(auto, auto& state, auto& rcvr, Tag, Args&&... args) noexcept -> void {
        using result_t = decayed-tuple<Tag, Args...>;
        constexpr bool nothrow = is_nothrow_constructible_v<result_t, Tag, Args...>;
      
        TRY-EVAL(rcvr, [&]() noexcept(nothrow) {
          state.async-result.template emplace<result_t>(Tag(), std::forward<Args>(args)...);
        }());
      
        if (state.async-result.valueless_by_exception())
          return;
        if (state.async-result.index() == 0)
          return;
      
        start(state.op-state);
      };
      
  5. Let out_sndr be a subexpression denoting a sender returned from schedule_from(sch, sndr) or one equal to such, and let OutSndr be the type decltype((out_sndr)). Let out_rcvr be a subexpression denoting a receiver that has an environment of type Env such that sender_in<OutSndr, Env> is true. Let op be an lvalue referring to the operation state that results from connecting out_sndr with out_rcvr. Calling start(op) shall start sndr on the current execution agent and execute completion operations on out_rcvr on an execution agent of the execution resource associated with sch. If scheduling onto sch fails, an error completion on out_rcvr shall be executed on an unspecified execution agent.

34.9.11.6. execution::on [exec.on]
  1. The on sender adaptor has two forms:

    • on(sch, sndr), which starts a sender sndr on an execution agent belonging to a scheduler sch’s associated execution resource and that, upon sndr’s completion, transfers execution back to the execution resource on which the on sender was started.

    • on(sndr, sch, closure), which upon completion of a sender sndr, transfers execution to an execution agent belonging to a scheduler sch’s associated execution resource, then executes a sender adaptor closure closure with the async results of the sender, and that then transfers execution back to the execution resource on which sndr completed.

  2. The name on denotes a pipeable sender adaptor object. For subexpressions sch and sndr, on(sch, sndr) is ill-formed if any of the following is true:

    • decltype((sch)) does not satisfy scheduler, or

    • decltype((sndr)) does not satisfy sender and sndr is not a pipeable sender adaptor closure object ([exec.adapt.objects]), or

    • decltype((sndr)) satisfies sender and sndr is also a pipeable sender adaptor closure object.

  3. Otherwise, if decltype((sndr)) satisfies sender, the expression on(sch, sndr) is expression-equivalent to:

    transform_sender(
      query-or-default(get_domain, sch, default_domain()),
      make-sender(on, sch, sndr))
    

    except that sch is evaluated only once.

  4. For subexpressions sndr, sch, and closure, if decltype((sch)) does not satisfy scheduler, or decltype((sndr)) does not satisfy sender, or closure is not a pipeable sender adaptor closure object ([exec.adapt.objects]), the expression on(sndr, sch, closure) is ill-formed; otherwise, it is expression-equivalent to:

    transform_sender(
      get-domain-early(sndr),
      make-sender(on, product-type{sch, closure}, sndr))
    

    except that sndr is evaluated only once.

  5. Let out_sndr and env be subexpressions, let OutSndr be decltype((out_sndr)), and let Env be decltype((env)). If sender-for<OutSndr, on_t> is false, then the expressions on.transform_env(out_sndr, env) and on.transform_sender(out_sndr, env) are ill-formed; otherwise:

    1. Let not-a-scheduler be an unspecified empty class type, and let not-a-sender be the exposition-only type:

      struct not-a-sender {
        using sender_concept = sender_t;
      
        auto get_completion_signatures(auto&&) const {
          return see below;
        }
      };
      

      where the member function get_completion_signatures returns an object of a type that is not a specialization of the completion_signatures class template.

    2. The expression on.transform_env(out_sndr, env) has effects equivalent to:

      auto&& [_, data, _] = out_sndr;
      if constexpr (scheduler<decltype(data)>) {
        return JOIN-ENV(SCHED-ENV(std::forward_like<OutSndr>(data)), FWD-ENV(std::forward<Env>(env)));
      } else {
        return std::forward<Env>(env);
      }
      
    3. The expression on.transform_sender(out_sndr, env) has effects equivalent to:

      auto&& [_, data, child] = out_sndr;
      if constexpr (scheduler<decltype(data)>) {
        auto orig_sch =
          query-with-default(get_scheduler, env, not-a-scheduler());
      
        if constexpr (same_as<decltype(orig_sch), not-a-scheduler>) {
          return not-a-sender{};
        } else {
          return continues_on(
            starts_on(std::forward_like<OutSndr>(data), std::forward_like<OutSndr>(child)),
            std::move(orig_sch));
        }
      } else {
        auto& [sch, closure] = data;
        auto orig_sch = query-with-default(
          get_completion_scheduler<set_value_t>,
          get_env(child),
          query-with-default(get_scheduler, env, not-a-scheduler()));
      
        if constexpr (same_as<decltype(orig_sch), not-a-scheduler>) {
          return not-a-sender{};
        } else {
          return write-env(
            continues_on(
              std::forward_like<OutSndr>(closure)(
                continues_on(
                  write-env(std::forward_like<OutSndr>(child), SCHED-ENV(orig_sch)),
                  sch)),
              orig_sch),
            SCHED-ENV(sch));
        }
      }
      
    4. Recommended practice: Implementations should use the return type of not-a-sender::get_completion_signatures to inform users that their usage of on is incorrect because there is no available scheduler onto which to restore execution.

  6. Let out_sndr be a subexpression denoting a sender returned from on(sch, sndr) or one equal to such, and let OutSndr be the type decltype((out_sndr)). Let out_rcvr be a subexpression denoting a receiver that has an environment of type Env such that sender_in<OutSndr, Env> is true. Let op be an lvalue referring to the operation state that results from connecting out_sndr with out_rcvr. Calling start(op) shall:

    1. Remember the current scheduler, get_scheduler(get_env(rcvr)).

    2. Start sndr on an execution agent belonging to sch’s associated execution resource.

    3. Upon sndr’s completion, transfer execution back to the execution resource associated with the scheduler remembered in step 1.

    4. Forward sndr’s async result to out_rcvr.

    If any scheduling operation fails, an error completion on out_rcvr shall be executed on an unspecified execution agent.

  7. Let out_sndr be a subexpression denoting a sender returned from on(sndr, sch, closure) or one equal to such, and let OutSndr be the type decltype((out_sndr)). Let out_rcvr be a subexpression denoting a receiver that has an environment of type Env such that sender_in<OutSndr, Env> is true. Let op be an lvalue referring to the operation state that results from connecting out_sndr with out_rcvr. Calling start(op) shall:

    1. Remember the current scheduler, which is the first of the following expressions that is well-formed:

      • get_completion_scheduler<set_value_t>(get_env(sndr))

      • get_scheduler(get_env(rcvr))

    2. Start sndr on the current execution agent.

    3. Upon sndr’s completion, transfer execution to an agent owned by sch’s associated execution resource.

    4. Forward sndr’s async result as if by connecting and starting a sender closure(S), where S is a sender that completes synchronously with sndr’s async result.

    5. Upon completion of the operation started in step 4, transfer execution back to the execution resource associated with the scheduler remembered in step 1 and forward the operation’s async result to out_rcvr.

    If any scheduling operation fails, an error completion on out_rcvr shall be executed on an unspecified execution agent.

34.9.11.7. execution::then, execution::upon_error, execution::upon_stopped [exec.then]
  1. then attaches an invocable as a continuation for an input sender’s value completion operation. upon_error and upon_stopped do the same for the error and stopped completion operations respectively, sending the result of the invocable as a value completion.

  2. The names then, upon_error, and upon_stopped denote pipeable sender adaptor objects. Let the expression then-cpo be one of then, upon_error, or upon_stopped. For subexpressions sndr and f, if decltype((sndr)) does not satisfy sender, or decltype((f)) does not satisfy movable-value, then-cpo(sndr, f) is ill-formed.

  3. Otherwise, the expression then-cpo(sndr, f) is expression-equivalent to:

    transform_sender(
      get-domain-early(sndr),
      make-sender(then-cpo, f, sndr))
    

    except that sndr is evaluated only once.

  4. For then, upon_error, and upon_stopped, let set-cpo be set_value, set_error, and set_stopped respectively. The exposition-only class template impls-for ([exec.snd.general]) is specialized for then-cpo as follows:

    namespace std::execution {
      template<>
      struct impls-for<decayed-typeof<then-cpo>> : default-impls {
        static constexpr auto complete =
          []<class Tag, class... Args>
            (auto, auto& fn, auto& rcvr, Tag, Args&&... args) noexcept -> void {
              if constexpr (same_as<Tag, decayed-typeof<set-cpo>>) {
                TRY-SET-VALUE(rcvr,
                              invoke(std::move(fn), std::forward<Args>(args)...));
              } else {
                Tag()(std::move(rcvr), std::forward<Args>(args)...);
              }
            };
      };
    }
    
  5. The expression then-cpo(sndr, f) has undefined behavior unless it returns a sender out_sndr that:

    1. Invokes f or a copy of such with the value, error, or stopped result datums of sndr for then, upon_error, and upon_stopped respectively, using the result value of f as out_sndr’s value completion, and

    2. Forwards all other completion operations unchanged.

34.9.11.8. execution::let_value, execution::let_error, execution::let_stopped, [exec.let]
  1. let_value, let_error, and let_stopped transform a sender’s value, error, and stopped completions respectively into a new child asynchronous operation by passing the sender’s result datums to a user-specified callable, which returns a new sender that is connected and started.

  2. For let_value, let_error, and let_stopped, let set-cpo be set_value, set_error, and set_stopped respectively. Let the expression let-cpo be one of let_value, let_error, or let_stopped. For a subexpression sndr, let let-env(sndr) be expression-equivalent to the first well-formed expression below:

    • SCHED-ENV(get_completion_scheduler<decayed-typeof<set-cpo>>(get_env(sndr)))

    • MAKE-ENV(get_domain, get_domain(get_env(sndr)))

    • (void(sndr), empty_env{})

  3. The names let_value, let_error, and let_stopped denote pipeable sender adaptor objects. For subexpressions sndr and f, let F be the decayed type of f. If decltype((sndr)) does not satisfy sender or if decltype((f)) does not satisfy movable-value, the expression let-cpo(sndr, f) is ill-formed. If F does not satisfy invocable, the expression let_stopped(sndr, f) is ill-formed.

  4. Otherwise, the expression let-cpo(sndr, f) is expression-equivalent to:

    transform_sender(
      get-domain-early(sndr),
      make-sender(let-cpo, f, sndr))
    

    except that sndr is evaluated only once.

  5. The exposition-only class template impls-for ([exec.snd.general]) is specialized for let-cpo as follows:

    namespace std::execution {
      template<class State, class Rcvr, class... Args>
      void let-bind(State& state, Rcvr& rcvr, Args&&... args); // exposition only
    
      template<>
      struct impls-for<decayed-typeof<let-cpo>> : default-impls {
        static constexpr auto get-state = see below;
        static constexpr auto complete = see below;
      };
    }
    
    1. Let receiver2 denote the following exposition-only class template:

      namespace std::execution {
        template<class Rcvr, class Env>
        struct receiver2 {
          using receiver_concept = receiver_t;
      
          template<class... Args>
          void set_value(Args&&... args) && noexcept {
            execution::set_value(std::move(rcvr), std::forward<Args>(args)...);
          }
      
          template<class Error>
          void set_error(Error&& err) && noexcept {
            execution::set_error(std::move(rcvr), std::forward<Error>(err));
          }
      
          void set_stopped() && noexcept {
            execution::set_stopped(std::move(rcvr));
          }
      
          decltype(auto) get_env() const noexcept {
            return JOIN-ENV(env, FWD-ENV(execution::get_env(rcvr)));
          }
      
          Rcvr& rcvr; // exposition only
          Env env; // exposition only
        };
      }
      
    2. impls-for<decayed-typeof<let-cpo>>::get-state is initialized with a callable object equivalent to the following:

      []<class Sndr, class Rcvr>(Sndr&& sndr, Rcvr& rcvr) requires see below {
        auto& [_, fn, child] = sndr;
        using fn_t = decay_t<decltype(fn)>;
        using env_t = decltype(let-env(child));
        using args_variant_t = see below;
        using ops2_variant_t = see below;
      
        struct state-type {
          fn_t fn;    // exposition only
          env_t env;    // exposition only
          args_variant_t args;    // exposition only
          ops2_variant_t ops2;    // exposition only
        };
        return state-type{std::forward_like<Sndr>(fn), let-env(child), {}, {}};
      }
      
      1. Let Sigs be a pack of the arguments to the completion_signatures specialization named by completion_signatures_of_t<child-type<Sndr>, env_of_t<Rcvr>>. Let LetSigs be a pack of those types in Sigs with a return type of decayed-typeof<set-cpo>. Let as-tuple be an alias template such that as-tuple<Tag(Args...)> denotes the type decayed-tuple<Args...>. Then args_variant_t denotes the type variant<monostate, as-tuple<LetSigs>...> except with duplicate types removed.

      2. Given a type Tag and a pack Args, let as-sndr2 be an alias template such that as-sndr2<Tag(Args...)> denotes the type call-result-t<Fn, decay_t<Args>&...>. Then ops2_variant_t denotes the type variant<monostate, connect_result_t<as-sndr2<LetSigs>, receiver2<Rcvr, Env>>...> except with duplicate types removed.

      3. The requires-clause constraining the above lambda is satisfied if and only if the types args_variant_t and ops2_variant_t are well-formed.

    3. The exposition-only function template let-bind has effects equivalent to:

      using args_t = decayed-tuple<Args...>;
      auto mkop2 = [&] {
        return connect(
          apply(std::move(state.fn),
                state.args.template emplace<args_t>(std::forward<Args>(args)...)),
          receiver2{rcvr, std::move(state.env)});
      };
      start(state.ops2.template emplace<decltype(mkop2())>(emplace-from{mkop2}));
      
    4. impls-for<decayed-typeof<let-cpo>>::complete is initialized with a callable object equivalent to the following:

      []<class Tag, class... Args>
        (auto, auto& state, auto& rcvr, Tag, Args&&... args) noexcept -> void {
          if constexpr (same_as<Tag, decayed-typeof<set-cpo>>) {
            TRY-EVAL(rcvr, let-bind(state, rcvr, std::forward<Args>(args)...));
          } else {
            Tag()(std::move(rcvr), std::forward<Args>(args)...);
          }
        }
      
  6. Let sndr and env be subexpressions, and let Sndr be decltype((sndr)). If sender-for<Sndr, decayed-typeof<let-cpo>> is false, then the expression let-cpo.transform_env(sndr, env) is ill-formed. Otherwise, it is equal to JOIN-ENV(let-env(sndr), FWD-ENV(env)).

  7. Let the subexpression out_sndr denote the result of the invocation let-cpo(sndr, f) or an object equal to such, and let the subexpression rcvr denote a receiver such that the expression connect(out_sndr, rcvr) is well-formed. The expression connect(out_sndr, rcvr) has undefined behavior unless it creates an asynchronous operation ([async.ops]) that, when started:

    • invokes f when set-cpo is called with sndr’s result datums,

    • makes its completion dependent on the completion of a sender returned by f, and

    • propagates the other completion operations sent by sndr.

34.9.11.9. execution::bulk [exec.bulk]
  1. bulk runs a task repeatedly for every index in an index space.

  2. The name bulk denotes a pipeable sender adaptor object. For subexpressions sndr, shape, and f, let Shape be decltype(auto(shape)). If decltype((sndr)) does not satisfy sender, or if Shape does not satisfy integral, or if decltype((f)) does not satisfy movable-value, bulk(sndr, shape, f) is ill-formed.

  3. Otherwise, the expression bulk(sndr, shape, f) is expression-equivalent to:

    transform_sender(
      get-domain-early(sndr),
      make-sender(bulk, product-type{shape, f}, sndr))
    

    except that sndr is evaluated only once.

  4. The exposition-only class template impls-for ([exec.snd.general]) is specialized for bulk_t as follows:

    namespace std::execution {
      template<>
      struct impls-for<bulk_t> : default-impls {
        static constexpr auto complete = see below;
      };
    }
    
    1. The member impls-for<bulk_t>::complete is initialized with a callable object equivalent to the following lambda:

      []<class Index, class State, class Rcvr, class Tag, class... Args>
        (Index, State& state, Rcvr& rcvr, Tag, Args&&... args) noexcept -> void requires see below {
          if constexpr (same_as<Tag, set_value_t>) {
            auto& [shape, f] = state;
            constexpr bool nothrow = noexcept(f(auto(shape), args...));
            TRY-EVAL(rcvr, [&]() noexcept(nothrow) {
              for (decltype(auto(shape)) i = 0; i < shape; ++i) {
                f(auto(i), args...);
              }
              Tag()(std::move(rcvr), std::forward<Args>(args)...);
            }());
          } else {
            Tag()(std::move(rcvr), std::forward<Args>(args)...);
          }
        }
      
      1. The expression in the requires-clause of the lambda above is true if and only if Tag denotes a type other than set_value_t or if the expression f(auto(shape), args...) is well-formed.

  5. Let the subexpression out_sndr denote the result of the invocation bulk(sndr, shape, f) or an object equal to such, and let the subexpression rcvr denote a receiver such that the expression connect(out_sndr, rcvr) is well-formed. The expression connect(out_sndr, rcvr) has undefined behavior unless it creates an asynchronous operation ([async.ops]) that, when started:

    • on a value completion operation, invokes f(i, args...) for every i of type Shape from 0 to shape, where args is a pack of lvalue subexpressions referring to the value completion result datums of the input sender, and

    • propagates all completion operations sent by sndr.

34.9.11.10. execution::split [exec.split]
  1. split adapts an arbitrary sender into a sender that can be connected multiple times.

  2. Let split-env be the type of an environment such that, given an instance env, the expression get_stop_token(env) is well-formed and has type inplace_stop_token.

  3. The name split denotes a pipeable sender adaptor object. For a subexpression sndr, let Sndr be decltype((sndr)). If sender_in<Sndr, split-env> is false, split(sndr) is ill-formed.

  4. Otherwise, the expression split(sndr) is expression-equivalent to:

    transform_sender(
      get-domain-early(sndr),
      make-sender(split, {}, sndr))
    

    except that sndr is evaluated only once.

    • The default implementation of transform_sender will have the effect of connecting the sender to a receiver. It will return a sender with a different tag type.

  5. Let local-state denote the following exposition-only class template:

    namespace std::execution {
      struct local-state-base {          // exposition only
        virtual ~local-state-base() = default;
        virtual void notify() noexcept = 0; // exposition only
      };
    
      template<class Sndr, class Rcvr>
      struct local-state : local-state-base { // exposition only
        using on-stop-callback =     // exposition only
          stop_callback_of_t<stop_token_of_t<env_of_t<Rcvr>>, on-stop-request>;
    
        local-state(Sndr&& sndr, Rcvr& rcvr) noexcept;
        ~local-state();
    
        void notify() noexcept override;
    
      private:
        optional<on-stop-callback> on_stop; // exposition only
        shared-state<Sndr>* sh_state; // exposition only
        Rcvr* rcvr; // exposition only
      };
    }
    
    1. local-state(Sndr&& sndr, Rcvr& rcvr) noexcept;
      1. Effects: Equivalent to:

        auto& [_, data, _] = sndr;
        this->sh_state = data.sh_state.get();
        this->sh_state->inc-ref();
        this->rcvr = addressof(rcvr);
        
    2. ~local-state();
      1. Effects: Equivalent to:

        sh_state->dec-ref();
        
    3. void notify() noexcept override;
      1. Effects: Equivalent to:

        on_stop.reset();
        visit(
          [this](const auto& tupl) noexcept -> void {
            apply(
              [this](auto tag, const auto&... args) noexcept -> void {
                tag(std::move(*rcvr), args...);
              },
              tupl);
          },
          sh_state->result);
        
  6. Let split-receiver denote the following exposition-only class template:

    namespace std::execution {
      template<class Sndr>
      struct split-receiver {
        using receiver_concept = receiver_t;
    
        template<class Tag, class... Args>
        void complete(Tag, Args&&... args) noexcept { // exposition only
          using tuple_t = decayed-tuple<Tag, Args...>;
          try {
            sh_state->result.template emplace<tuple_t>(Tag(), std::forward<Args>(args)...);
          } catch (...) {
            using tuple_t = tuple<set_error_t, exception_ptr>;
            sh_state->result.template emplace<tuple_t>(set_error, current_exception());
          }
          sh_state->notify();
        }
    
        template<class... Args>
        void set_value(Args&&... args) && noexcept {
          complete(execution::set_value, std::forward<Args>(args)...);
        }
    
        template<class Error>
        void set_error(Error&& err) && noexcept {
          complete(execution::set_error, std::forward<Error>(err));
        }
    
        void set_stopped() && noexcept {
          complete(execution::set_stopped);
        }
    
        struct env { // exposition only
          shared-state<Sndr>* sh-state; // exposition only
    
          inplace_stop_token query(get_stop_token_t) const noexcept {
            return sh-state->stop_src.get_token();
          }
        };
    
        env get_env() const noexcept {
          return env{sh_state};
        }
    
        shared-state<Sndr>* sh_state; // exposition only
      };
    }
    
  7. Let shared-state denote the following exposition-only class template:

    namespace std::execution {
      template<class Sndr>
      struct shared-state {
        using variant-type = see below;   // exposition only
        using state-list-type = see below;    // exposition only
    
        explicit shared-state(Sndr&& sndr);
    
        void start-op() noexcept;  // exposition only
        void notify() noexcept;  // exposition only
        void inc-ref() noexcept; // exposition only
        void dec-ref() noexcept; // exposition only
    
        inplace_stop_source stop_src{};   // exposition only
        variant-type result{};   // exposition only
        state-list-type waiting_states;    // exposition only
        atomic<bool> completed{false};   // exposition only
        atomic<size_t> ref_count{1};   // exposition only
        connect_result_t<Sndr, split-receiver<Sndr>> op_state;    // exposition only
      };
    }
    
    1. Let Sigs be a pack of the arguments to the completion_signatures specialization named by completion_signatures_of_t<Sndr>. For type Tag and pack Args, let as-tuple be an alias template such that as-tuple<Tag(Args...)> denotes the type decayed-tuple<Tag, Args...>. Then variant-type denotes the type variant<tuple<set_stopped_t>, tuple<set_error_t, exception_ptr>, as-tuple<Sigs>...>, but with duplicate types removed.

    2. Let state-list-type be a type that stores a list of pointers to local-state-base objects and that permits atomic insertion.

    3.   explicit shared-state(Sndr&& sndr);
      1. Effects: Initializes op_state with the result of connect(std::forward<Sndr>(sndr), split-receiver{this}).

      2. Postcondition: waiting_states is empty, and completed is false.

    4.   void start-op() noexcept;
      1. Effects: Calls inc-ref(). If stop_src.stop_requested() is true, calls notify(); otherwise, calls start(op_state).

    5.   void notify() noexcept;
      1. Effects: Atomically does the following:

        • Sets completed to true, and

        • Exchanges waiting_states with an empty list, storing the old value in a local prior_states.

        Then, for each pointer p in prior_states, calls p->notify(). Finally, calls dec-ref().

    6.   void inc-ref() noexcept;
      1. Effects: Increments ref_count.

    7.   void dec-ref() noexcept;
      1. Effects: Decrements ref_count. If the new value of ref_count is 0, calls delete this.

      2. Synchronization: If dec-ref() does not decrement the ref_count to 0 then synchronizes with the call to dec-ref() that decrements ref_count to 0.

  8. Let split-impl-tag be an empty exposition-only class type. Given an expression sndr, the expression split.transform_sender(sndr) is equivalent to:

    auto&& [tag, _, child] = sndr;
    auto* sh_state = new shared-state{std::forward_like<decltype((sndr))>(child)};
    return make-sender(split-impl-tag(), shared-wrapper{sh_state, tag});
    

    where shared-wrapper is an exposition-only class that manages the reference count of the shared-state object pointed to by sh_state. shared-wrapper models copyable with move operations nulling out the moved-from object, copy operations incrementing the reference count by calling sh_state->inc-ref(), and assignment operations performing a copy-and-swap operation. The destructor has no effect if sh_state is null; otherwise, it decrements the reference count by calling sh_state->dec-ref().

  9. The exposition-only class template impls-for ([exec.snd.general]) is specialized for split-impl-tag as follows:

    namespace std::execution {
      template<>
      struct impls-for<split-impl-tag> : default-impls {
        static constexpr auto get-state = see below;
        static constexpr auto start = see below;
      };
    }
    
    1. The member impls-for<split-impl-tag>::get-state is initialized with a callable object equivalent to the following lambda expression:

      []<class Sndr>(Sndr&& sndr, auto& rcvr) noexcept {
        return local-state{std::forward<Sndr>(sndr), rcvr};
      }
      
    2. The member impls-for<split-impl-tag>::start is initialized with a callable object that has a function call operator equivalent to the following:

      template<class Sndr, class Rcvr>
      void operator()(local-state<Sndr, Rcvr>& state, Rcvr& rcvr) const noexcept;
      
      1. Effects: If state.sh_state->completed is true, calls state.notify() and returns. Otherwise, does the following in order:

        1. Calls:

          state.on_stop.emplace(
            get_stop_token(get_env(rcvr)),
            on-stop-request{state.sh_state->stop_src});
          
        2. Then atomically does the following:

          • Reads the value c of state.sh_state->completed, and

          • Inserts addressof(state) into state.sh_state->waiting_states if c is false.

        3. If c is true, calls state.notify() and returns.

        4. Otherwise, if addressof(state) is the first item added to state.sh_state->waiting_states, calls state.sh_state->start-op().

34.9.11.11. execution::when_all [exec.when.all]
  1. when_all and when_all_with_variant both adapt multiple input senders into a sender that completes when all input senders have completed. when_all only accepts senders with a single value completion signature and on success concatenates all the input senders' value result datums into its own value completion operation. when_all_with_variant(sndrs...) is semantically equivalent to when_all(into_variant(sndrs)...), where sndrs is a pack of subexpressions whose types model sender.

  2. The names when_all and when_all_with_variant denote customization point objects. Let sndrs be a pack of subexpressions, let Sndrs be a pack of the types decltype((sndrs))..., and let CD be the type common_type_t<decltype(get-domain-early(sndrs))...>. The expressions when_all(sndrs...) and when_all_with_variant(sndrs...) are ill-formed if any of the following is true:

    • sizeof...(sndrs) is 0, or

    • (sender<Sndrs> && ...) is false, or

    • CD is ill-formed.

  3. The expression when_all(sndrs...) is expression-equivalent to:

    transform_sender(
      CD(),
      make-sender(when_all, {}, sndrs...))
    
  4. The exposition-only class template impls-for ([exec.snd.general]) is specialized for when_all_t as follows:

    namespace std::execution {
      template<>
      struct impls-for<when_all_t> : default-impls {
        static constexpr auto get-attrs = see below;
        static constexpr auto get-env = see below;
        static constexpr auto get-state = see below;
        static constexpr auto start = see below;
        static constexpr auto complete = see below;
      };
    }
    
    1. The member impls-for<when_all_t>::get-attrs is initialized with a callable object equivalent to the following lambda expression:

      [](auto&&, auto&&... child) noexcept {
        if constexpr (same_as<CD, default_domain>) {
          return empty_env();
        } else {
          return MAKE-ENV(get_domain, CD());
        }
      }
      
    2. The member impls-for<when_all_t>::get-env is initialized with a callable object equivalent to the following lambda expression:

      []<class State, class Rcvr>(auto&&, State& state, const Receiver& rcvr) noexcept {
        return JOIN-ENV(
          MAKE-ENV(get_stop_token, state.stop_src.get_token()), get_env(rcvr));
      }
      
    3. The member impls-for<when_all_t>::get-state is initialized with a callable object equivalent to the following lambda expression:

      []<class Sndr, class Rcvr>(Sndr&& sndr, Rcvr& rcvr) noexcept(e) -> decltype(e) {
        return e;
      }
      

      where e is the expression:

      std::forward<Sndr>(sndr).apply(make-state<Rcvr>())
      

      and where make-state is the following exposition-only class template:

      template<class Sndr, class Env>
      concept max-1-sender-in = sender_in<Sndr, Env> && // exposition only
        (tuple_size_v<value_types_of_t<Sndr, Env, tuple, tuple>> <= 1);
      
      enum class disposition { started, error, stopped }; // exposition only
      
      template<class Rcvr>
      struct make-state {
        template<max-1-sender-in<env_of_t<Rcvr>>... Sndrs>
        auto operator()(auto, auto, Sndrs&&... sndrs) const {
          using values_tuple = see below;
          using errors_variant = see below;
          using stop_callback = stop_callback_of_t<stop_token_of_t<env_of_t<Rcvr>>, on-stop-request>;
      
          struct state-type {
            void arrive(Rcvr& rcvr) noexcept {
              if (0 == --count) {
                complete(rcvr);
              }
            }
      
            void complete(Rcvr& rcvr) noexcept; // see below
      
            atomic<size_t> count{sizeof...(sndrs)};   // exposition only
            inplace_stop_source stop_src{};   // exposition only
            atomic<disposition> disp{disposition::started};   // exposition only
            errors_variant errors{};   // exposition only
            values_tuple values{};   // exposition only
            optional<stop_callback> on_stop{nullopt};   // exposition only
          };
      
          return state-type{};
        }
      };
      
      1. Let copy-fail be exception_ptr if decay-copying any of the child senders' result datums can potentially throw; otherwise, none-such, where none-such is an unspecified empty class type.

      2. The alias values_tuple denotes the type tuple<value_types_of_t<Sndrs, env_of_t<Rcvr>, decayed-tuple, optional>...> if that type is well-formed; otherwise, tuple<>.

      3. The alias errors_variant denotes the type variant<none-such, copy-fail, Es...> with duplicate types removed, where Es is the pack of the decayed types of all the child senders' possible error result datums.

      4. The member void state::complete(Rcvr& rcvr) noexcept behaves as follows:

        1. If disp is equal to disposition::started, evaluates:

          auto tie = []<class... T>(tuple<T...>& t) noexcept { return tuple<T&...>(t); };
          auto set = [&](auto&... t) noexcept { set_value(std::move(rcvr), std::move(t)...); };
          
          on_stop.reset();
          apply(
            [&](auto&... opts) noexcept {
              apply(set, tuple_cat(tie(*opts)...));
            },
            values);
          
        2. Otherwise, if disp is equal to disposition::error, evaluates:

          on_stop.reset();
          visit(
            [&]<class Error>(Error& error) noexcept {
              if constexpr (!same_as<Error, none-such>) {
                set_error(std::move(rcvr), std::move(error));
              }
            },
            errors);
          
        3. Otherwise, evaluates:

          on_stop.reset();
          set_stopped(std::move(rcvr));
          
    4. The member impls-for<when_all_t>::start is initialized with a callable object equivalent to the following lambda expression:

      []<class State, class Rcvr, class... Ops>(
          State& state, Rcvr& rcvr, Ops&... ops) noexcept -> void {
        state.on_stop.emplace(
          get_stop_token(get_env(rcvr)),
          on-stop-request{state.stop_src});
        if (state.stop_src.stop_requested()) {
          state.on_stop.reset();
          set_stopped(std::move(rcvr));
        } else {
          (start(ops), ...);
        }
      }
      
    5. The member impls-for<when_all_t>::complete is initialized with a callable object equivalent to the following lambda expression:

      []<class Index, class State, class Rcvr, class Set, class... Args>(
          this auto& complete, Index, State& state, Rcvr& rcvr, Set, Args&&... args) noexcept -> void {
        if constexpr (same_as<Set, set_error_t>) {
          if (disposition::error != state.disp.exchange(disposition::error)) {
            state.stop_src.request_stop();
            TRY-EMPLACE-ERROR(state.errors, std::forward<Args>(args)...);
          }
        } else if constexpr (same_as<Set, set_stopped_t>) {
          auto expected = disposition::started;
          if (state.disp.compare_exchange_strong(expected, disposition::stopped)) {
            state.stop_src.request_stop();
          }
        } else if constexpr (!same_as<decltype(State::values), tuple<>>) {
          if (state.disp == disposition::started) {
            auto& opt = get<Index::value>(state.values);
            TRY-EMPLACE-VALUE(complete, opt, std::forward<Args>(args)...);
          }
        }
      
        state.arrive(rcvr);
      }
      

      where TRY-EMPLACE-ERROR(v, e), for subexpressions v and e, is equivalent to:

      try {
        v.template emplace<decltype(auto(e))>(e);
      } catch (...) {
        v.template emplace<exception_ptr>(current_exception());
      }
      

      if the expression decltype(auto(e))(e) is potentially throwing; otherwise, v.template emplace<decltype(auto(e))>(e); and where TRY-EMPLACE-VALUE(c, o, as...), for subexpressions c, o, and pack of subexpressions as, is equivalent to:

      try {
        o.emplace(as...);
      } catch (...) {
        c(Index(), state, rcvr, set_error, current_exception());
        return;
      }
      

      if the expression decayed-tuple<decltype(as)...>{as...} is potentially throwing; otherwise, o.emplace(as...).

  5. The expression when_all_with_variant(sndrs...) is expression-equivalent to:

    transform_sender(
      CD(),
      make-sender(when_all_with_variant, {}, sndrs...));
    
  6. Given subexpressions sndr and env, if sender-for<decltype((sndr)), when_all_with_variant_t> is false, then the expression when_all_with_variant.transform_sender(sndr, env) is ill-formed; otherwise, it is equivalent to:

    auto&& [_, _, ...child] = sndr;
    return when_all(into_variant(std::forward_like<decltype((sndr))>(child))...);
    

    This causes the when_all_with_variant(sndrs...) sender to become when_all(into_variant(sndrs)...) when it is connected with a receiver whose execution domain does not customize when_all_with_variant.

34.9.11.12. execution::into_variant [exec.into.variant]
  1. into_variant adapts a sender with multiple value completion signatures into a sender with just one value completion signature consisting of a variant of tuples.

  2. The name into_variant denotes a pipeable sender adaptor object. For a subexpression sndr, let Sndr be decltype((sndr)). If Sndr does not satisfy sender, into_variant(sndr) is ill-formed.

  3. Otherwise, the expression into_variant(sndr) is expression-equivalent to:

    transform_sender(
      get-domain-early(sndr),
      make-sender(into_variant, {}, sndr))
    

    except that sndr is only evaluated once.

  4. The exposition-only class template impls-for ([exec.snd.general]) is specialized for into_variant as follows:

    namespace std::execution {
      template<>
      struct impls-for<into_variant_t> : default-impls {
        static constexpr auto get-state = see below;
        static constexpr auto complete = see below;
      };
    }
    
    1. The member impls-for<into_variant_t>::get-state is initialized with a callable object equivalent to the following lambda:

      []<class Sndr, class Rcvr>(Sndr&& sndr, Rcvr& rcvr) noexcept
        -> type_identity<value_types_of_t<child-type<Sndr>, env_of_t<Rcvr>>> {
        return {};
      }
      
    2. The member impls-for<into_variant_t>::complete is initialized with a callable object equivalent to the following lambda:

      []<class State, class Rcvr, class Tag, class... Args>(
          auto, State, Rcvr& rcvr, Tag, Args&&... args) noexcept -> void {
        if constexpr (same_as<Tag, set_value_t>) {
          using variant_type = typename State::type;
          TRY-SET-VALUE(rcvr, variant_type(decayed-tuple<Args...>{std::forward<Args>(args)...}));
        } else {
          Tag()(std::move(rcvr), std::forward<Args>(args)...);
        }
      }
      
34.9.11.13. execution::stopped_as_optional [exec.stopped.as.optional]
  1. stopped_as_optional maps a sender’s stopped completion operation into a value completion operation as an disengaged optional. The sender’s value completion operation is also converted into an optional. The result is a sender that never completes with stopped, reporting cancellation by completing with an disengaged optional.

  2. The name stopped_as_optional denotes a pipeable sender adaptor object. For a subexpression sndr, let Sndr be decltype((sndr)). The expression stopped_as_optional(sndr) is expression-equivalent to:

    transform_sender(
      get-domain-early(sndr),
      make-sender(stopped_as_optional, {}, sndr))
    

    except that sndr is only evaluated once.

  3. Let sndr and env be subexpressions such that Sndr is decltype((sndr)) and Env is decltype((env)). If sender-for<Sndr, stopped_as_optional_t> is false, or if the type single-sender-value-type<Sndr, Env> is ill-formed or void, then the expression stopped_as_optional.transform_sender(sndr, env) is ill-formed; otherwise, it is equivalent to:

    auto&& [_, _, child] = sndr;
    using V = single-sender-value-type<Sndr, Env>;
    return let_stopped(
        then(std::forward_like<Sndr>(child),
             []<class... Ts>(Ts&&... ts) noexcept(is_nothrow_constructible_v<V, Ts...>) {
               return optional<V>(in_place, std::forward<Ts>(ts)...);
             }),
        []() noexcept { return just(optional<V>()); });
    
34.9.11.14. execution::stopped_as_error [exec.stopped.as.error]
  1. stopped_as_error maps an input sender’s stopped completion operation into an error completion operation as a custom error type. The result is a sender that never completes with stopped, reporting cancellation by completing with an error.

  2. The name stopped_as_error denotes a pipeable sender adaptor object. For some subexpressions sndr and err, let Sndr be decltype((sndr)) and let Err be decltype((err)). If the type Sndr does not satisfy sender or if the type Err doesn’t satisfy movable-value, stopped_as_error(sndr, err) is ill-formed. Otherwise, the expression stopped_as_error(sndr, err) is expression-equivalent to:

    transform_sender(
      get-domain-early(sndr),
      make-sender(stopped_as_error, err, sndr))
    

    except that sndr is only evaluated once.

  3. Let sndr and env be subexpressions such that Sndr is decltype((sndr)) and Env is decltype((env)). If sender-for<Sndr, stopped_as_error_t> is false, then the expression stopped_as_error.transform_sender(sndr, env) is ill-formed; otherwise, it is equivalent to:

    auto&& [_, err, child] = sndr;
    using E = decltype(auto(err));
    return let_stopped(
        std::forward_like<Sndr>(child),
        [err = std::forward_like<Sndr>(err)]() mutable noexcept(is_nothrow_move_constructible_v<E>) {
          return just_error(std::move(err));
        });
    

34.9.12. Sender consumers [exec.consumers]

34.9.12.1. this_thread::sync_wait [exec.sync.wait]
  1. this_thread::sync_wait and this_thread::sync_wait_with_variant are used to block the current thread of execution until the specified sender completes and to return its async result. sync_wait mandates that the input sender has exactly one value completion signature.

  2. Let sync-wait-env be the following exposition-only class type:

    namespace std::this_thread {
      struct sync-wait-env {
        execution::run_loop* loop; // exposition only
    
        auto query(execution::get_scheduler_t) const noexcept {
          return loop->get_scheduler();
        }
    
        auto query(execution::get_delegation_scheduler_t) const noexcept {
          return loop->get_scheduler();
        }
      };
    }
    
  3. Let sync-wait-result-type and sync-wait-with-variant-result-type be exposition-only alias templates defined as follows:

    namespace std::this_thread {
      template<execution::sender_in<sync-wait-env> Sndr>
        using sync-wait-result-type =
          optional<execution::value_types_of_t<Sndr, sync-wait-env, decayed-tuple, type_identity_t>>;
    
      template<execution::sender_in<sync-wait-env> Sndr>
        using sync-wait-with-variant-result-type =
          optional<execution::value_types_of_t<Sndr, sync-wait-env>>;
    }
    
  4. The name this_thread::sync_wait denotes a customization point object. For a subexpression sndr, let Sndr be decltype((sndr)). If sender_in<Sndr, sync-wait-env> is false, the expression this_thread::sync_wait(sndr) is ill-formed. Otherwise, it is expression-equivalent to the following, except that sndr is evaluated only once:

    apply_sender(get-domain-early(sndr), sync_wait, sndr)
    

    Mandates:

    • The type sync-wait-result-type<Sndr> is well-formed.

    • same_as<decltype(e), sync-wait-result-type<Sndr>> is true, where e is the apply_sender expression above.

  5. Let sync-wait-state and sync-wait-receiver be the following exposition-only class templates:

    namespace std::this_thread {
      template<class Sndr>
      struct sync-wait-state { // exposition only
        execution::run_loop loop;  // exposition only
        exception_ptr error; // exposition only
        sync-wait-result-type<Sndr> result;  // exposition only
      };
    
      template<class Sndr>
      struct sync-wait-receiver { // exposition only
        using receiver_concept = execution::receiver_t;
        sync-wait-state<Sndr>* state; // exposition only
    
        template<class... Args>
        void set_value(Args&&... args) && noexcept;
    
        template<class Error>
        void set_error(Error&& err) && noexcept;
    
        void set_stopped() && noexcept;
    
        sync-wait-env get_env() const noexcept { return {&state->loop}; }
      };
    }
    
    1. template<class... Args>
      void set_value(Args&&... args) && noexcept;
      
      1. Effects: Equivalent to:

        try {
          state->result.emplace(std::forward<Args>(args)...);
        } catch (...) {
          state->error = current_exception();
        }
        state->loop.finish();
        
    2. template<class Error>
      void set_error(Error&& err) && noexcept;
      
      1. Effects: Equivalent to:

        state->error = AS-EXCEPT-PTR(std::forward<Error>(err)); // see [exec.general]
        state->loop.finish();
        
    3. void set_stopped() && noexcept;
      
      1. Effects: Equivalent to state->loop.finish().

  6. For a subexpression sndr, let Sndr be decltype((sndr)). If sender_to<Sndr, sync-wait-receiver<Sndr>> is false, the expression sync_wait.apply_sender(sndr) is ill-formed; otherwise, it is equivalent to:

    sync-wait-state<Sndr> state;
    auto op = connect(sndr, sync-wait-receiver<Sndr>{&state});
    start(op);
    
    state.loop.run();
    if (state.error) {
      rethrow_exception(std::move(state.error));
    }
    return std::move(state.result);
    
  7. The behavior of this_thread::sync_wait(sndr) is undefined unless:

    1. It blocks the current thread of execution ([defns.block]) with forward progress guarantee delegation ([intro.progress]) until the specified sender completes. The default implementation of sync_wait achieves forward progress guarantee delegation by providing a run_loop scheduler via the get_delegation_scheduler query on the sync-wait-receiver’s environment. The run_loop is driven by the current thread of execution.

    2. It returns the specified sender’s async results as follows:

      1. For a value completion, the result datums are returned in a tuple in an engaged optional object.

      2. For an error completion, an exception is thrown.

      3. For a stopped completion, a disengaged optional object is returned.

  8. The name this_thread::sync_wait_with_variant denotes a customization point object. For a subexpression sndr, let Sndr be decltype(into_variant(sndr)). If sender_in<Sndr, sync-wait-env> is false, this_thread::sync_wait_with_variant(sndr) is ill-formed. Otherwise, it is expression-equivalent to the following, except sndr is evaluated only once:

    apply_sender(get-domain-early(sndr), sync_wait_with_variant, sndr)
    

    Mandates:

    • The type sync-wait-with-variant-result-type<Sndr> is well-formed.

    • same_as<decltype(e), sync-wait-with-variant-result-type<Sndr>> is true, where e is the apply_sender expression above.

  9. If callable<sync_wait_t, Sndr> is false, the expression sync_wait_with_variant.apply_sender(sndr) is ill-formed. Otherwise, it is equivalent to:

    using result_type = sync-wait-with-variant-result-type<Sndr>;
    if (auto opt_value = sync_wait(into_variant(sndr))) {
      return result_type(std::move(get<0>(*opt_value)));
    }
    return result_type(nullopt);
    
  10. The behavior of this_thread::sync_wait_with_variant(sndr) is undefined unless:

    1. It blocks the current thread of execution ([defns.block]) with forward progress guarantee delegation ([intro.progress]) until the specified sender completes. The default implementation of sync_wait_with_variant achieves forward progress guarantee delegation by relying on the forward progress guarantee delegation provided by sync_wait.

    2. It returns the specified sender’s async results as follows:

      1. For a value completion, the result datums are returned in an engaged optional object that contains a variant of tuples.

      2. For an error completion, an exception is thrown.

      3. For a stopped completion, a disengaged optional object is returned.

34.10. Sender/receiver utilities [exec.utils]

34.10.1. execution::completion_signatures [exec.utils.cmplsigs]

  1. completion_signatures is a type that encodes a set of completion signatures ([async.ops]).

  2. [Example:

    struct my_sender {
      using sender_concept = sender_t;
      using completion_signatures =
        execution::completion_signatures<
          set_value_t(),
          set_value_t(int, float),
          set_error_t(exception_ptr),
          set_error_t(error_code),
          set_stopped_t()>;
    };
    
    // Declares my_sender to be a sender that can complete by calling
    // one of the following for a receiver expression rcvr:
    //    set_value(rcvr)
    //    set_value(rcvr, int{...}, float{...})
    //    set_error(rcvr, exception_ptr{...})
    //    set_error(rcvr, error_code{...})
    //    set_stopped(rcvr)
    

    -- end example]

  3. [exec.utils.cmplsigs] makes use of the following exposition-only entities:

    template<class Fn>
      concept completion-signature = see below;
    
    template<bool>
      struct indirect-meta-apply {
        template<template<class...> class T, class... As>
          using meta-apply = T<As...>; // exposition only
      };
    
    template<class...>
      concept always-true = true; // exposition only
    
    1. A type Fn satisfies completion-signature if and only if it is a function type with one of the following forms:

      • set_value_t(Vs...), where Vs is a pack of object or reference types.

      • set_error_t(Err), where Err is an object or reference type.

      • set_stopped_t()

    template<class Tag,
              valid-completion-signatures Completions,
              template<class...> class Tuple,
              template<class...> class Variant>
      using gather-signatures = see below;
    
    1. Let Fns be a pack of the arguments of the completion_signatures specialization named by Completions, let TagFns be a pack of the function types in Fns whose return types are Tag, and let Tsn be a pack of the function argument types in the n-th type in TagFns. Then, given two variadic templates Tuple and Variant, the type gather-signatures<Tag, Completions, Tuple, Variant> names the type META-APPLY(Variant, META-APPLY(Tuple, Ts0...), META-APPLY(Tuple, Ts1...), ... META-APPLY(Tuple, Tsm-1...)), where m is the size of the pack TagFns and META-APPLY(T, As...) is equivalent to:

      typename indirect-meta-apply<always-true<As...>>::template meta-apply<T, As...>;
      
    2. The purpose of META-APPLY is to make it valid to use non-variadic templates as Variant and Tuple arguments to gather-signatures.

  4. namespace std::execution {
      template<completion-signature... Fns>
        struct completion_signatures {};
    
      template<class Sndr,
                class Env = empty_env,
                template<class...> class Tuple = decayed-tuple,
                template<class...> class Variant = variant-or-empty>
          requires sender_in<Sndr, Env>
        using value_types_of_t =
            gather-signatures<set_value_t, completion_signatures_of_t<Sndr, Env>, Tuple, Variant>;
    
      template<class Sndr,
                class Env = empty_env,
                template<class...> class Variant = variant-or-empty>
          requires sender_in<Sndr, Env>
        using error_types_of_t =
            gather-signatures<set_error_t, completion_signatures_of_t<Sndr, Env>, type_identity_t, Variant>;
    
      template<class Sndr, class Env = empty_env>
          requires sender_in<Sndr, Env>
        inline constexpr bool sends_stopped =
            !same_as<
              type-list<>,
              gather-signatures<set_stopped_t, completion_signatures_of_t<Sndr, Env>, type-list, type-list>>;
    }
    

34.10.2. execution::transform_completion_signatures [exec.utils.tfxcmplsigs]

  1. transform_completion_signatures is an alias template used to transform one set of completion signatures into another. It takes a set of completion signatures and several other template arguments that apply modifications to each completion signature in the set to generate a new specialization of completion_signatures.

  2. [Example:

    // Given a sender Sndr and an environment Env, adapt the completion
    // signatures of Sndr by lvalue-ref qualifying the values, adding an additional
    // exception_ptr error completion if its not already there, and leaving the
    // other completion signatures alone.
    template<class... Args>
      using my_set_value_t =
        completion_signatures<
          set_value_t(add_lvalue_reference_t<Args>...)>;
    
    using my_completion_signatures =
      transform_completion_signatures<
        completion_signatures_of_t<Sndr, Env>,
        completion_signatures<set_error_t(exception_ptr)>,
        my_set_value_t>;
    

    -- end example]

  3. [exec.utils.tfxcmplsigs] makes use of the following exposition-only entities:

    template<class... As>
      using default-set-value =
        completion_signatures<set_value_t(As...)>;
    
    template<class Err>
      using default-set-error =
        completion_signatures<set_error_t(Err)>;
    
  4. namespace std::execution {
      template<valid-completion-signatures InputSignatures,
              valid-completion-signatures AdditionalSignatures =
                  completion_signatures<>,
              template<class...> class SetValue = default-set-value,
              template<class> class SetError = default-set-error,
              valid-completion-signatures SetStopped =
                  completion_signatures<set_stopped_t()>>
      using transform_completion_signatures =
        completion_signatures<see below>;
    }
    
    1. SetValue shall name an alias template such that for any pack of types As, the type SetValue<As...> is either ill-formed or else valid-completion-signatures<SetValue<As...>> is satisfied.

    2. SetError shall name an alias template such that for any type Err, SetError<Err> is either ill-formed or else valid-completion-signatures<SetError<Err>> is satisfied.

    Then:

    1. Let Vs... be a pack of the types in the type-list named by gather-signatures<set_value_t, InputSignatures, SetValue, type-list>.

    2. Let Es... be a pack of the types in the type-list named by gather-signatures<set_error_t, InputSignatures, type_identity_t, error-list>, where error-list is an alias template such that error-list<Ts...> is type-list<SetError<Ts>...>.

    3. Let Ss name the type completion_signatures<> if gather-signatures<set_stopped_t, InputSignatures, type-list, type-list> is an alias for the type type-list<>; otherwise, SetStopped.

    Then:

    1. If any of the above types are ill-formed, then transform_completion_signatures<InputSignatures, AdditionalSignatures, SetValue, SetError, SetStopped> is ill-formed.

    2. Otherwise, transform_completion_signatures<InputSignatures, AdditionalSignatures, SetValue, SetError, SetStopped> is the type completion_signatures<Sigs...> where Sigs... is the unique set of types in all the template arguments of all the completion_signatures specializations in the set AdditionalSignatures, Vs..., Es..., Ss.

34.11. Execution contexts [exec.ctx]

34.11.1. execution::run_loop [exec.run.loop]

  1. A run_loop is an execution resource on which work can be scheduled. It maintains a thread-safe first-in-first-out queue of work. Its run() member function removes elements from the queue and executes them in a loop on the thread of execution that calls run().

  2. A run_loop instance has an associated count that corresponds to the number of work items that are in its queue. Additionally, a run_loop instance has an associated state that can be one of starting, running, or finishing.

  3. Concurrent invocations of the member functions of run_loop other than run and its destructor do not introduce data races. The member functions pop_front, push_back, and finish execute atomically.

  4. Recommended practice: Implementations are encouraged to use an intrusive queue of operation states to hold the work units to make scheduling allocation-free.

    namespace std::execution {
      class run_loop {
        // [exec.run.loop.types] Associated types
        class run-loop-scheduler; // exposition only
        class run-loop-sender; // exposition only
        struct run-loop-opstate-base { // exposition only
          virtual void execute() = 0;  // exposition only
          run_loop* loop;  // exposition only
          run-loop-opstate-base* next;  // exposition only
        };
        template<class Rcvr>
          using run-loop-opstate = unspecified; // exposition only
    
        // [exec.run.loop.members] Member functions:
        run-loop-opstate-base* pop-front(); // exposition only
        void push-back(run-loop-opstate-base*); // exposition only
    
      public:
        // [exec.run.loop.ctor] construct/copy/destroy
        run_loop() noexcept;
        run_loop(run_loop&&) = delete;
        ~run_loop();
    
        // [exec.run.loop.members] Member functions:
        run-loop-scheduler get_scheduler();
        void run();
        void finish();
      };
    }
    
34.11.1.1. Associated types [exec.run.loop.types]
class run-loop-scheduler;
  1. run-loop-scheduler is an unspecified type that models scheduler.

  2. Instances of run-loop-scheduler remain valid until the end of the lifetime of the run_loop instance from which they were obtained.

  3. Two instances of run-loop-scheduler compare equal if and only if they were obtained from the same run_loop instance.

  4. Let sch be an expression of type run-loop-scheduler. The expression schedule(sch) has type run-loop-sender and is not potentially-throwing if sch is not potentially-throwing.

class run-loop-sender;
  1. run-loop-sender is an exposition-only type that satisfies sender. For any type Env, completion_signatures_of_t<run-loop-sender, Env> is:

    completion_signatures<set_value_t(), set_error_t(exception_ptr), set_stopped_t()>
    
  2. An instance of run-loop-sender remains valid until the end of the lifetime of its associated run_loop instance.

  3. Let sndr be an expression of type run-loop-sender, let rcvr be an expression such that receiver_of<decltype((rcvr)), CS> is true where CS is the completion_signatures specialization above. Let C be either set_value_t or set_stopped_t. Then:

    • The expression connect(sndr, rcvr) has type run-loop-opstate<decay_t<decltype((rcvr))>> and is potentially-throwing if and only if (void(sndr), auto(rcvr)) is potentially-throwing.

    • The expression get_completion_scheduler<C>(get_env(sndr)) is potentially-throwing if and only if sndr is potentially-throwing, has type run-loop-scheduler, and compares equal to the run-loop-scheduler instance from which sndr was obtained.

template<class Rcvr>
  struct run-loop-opstate;
  1. run-loop-opstate<Rcvr> inherits privately and unambiguously from run-loop-opstate-base.

  2. Let o be a non-const lvalue of type run-loop-opstate<Rcvr>, and let REC(o) be a non-const lvalue reference to an instance of type Rcvr that was initialized with the expression rcvr passed to the invocation of connect that returned o. Then:

    • The object to which REC(o) refers remains valid for the lifetime of the object to which o refers.

    • The type run-loop-opstate<Rcvr> overrides run-loop-opstate-base::execute() such that o.execute() is equivalent to:

      if (get_stop_token(REC(o)).stop_requested()) {
        set_stopped(std::move(REC(o)));
      } else {
        set_value(std::move(REC(o)));
      }
      
    • The expression start(o) is equivalent to:

      try {
        o.loop->push-back(addressof(o));
      } catch(...) {
        set_error(std::move(REC(o)), current_exception());
      }
      
34.11.1.2. Constructor and destructor [exec.run.loop.ctor]
run_loop() noexcept;
  1. Postconditions: count is 0 and state is starting.

~run_loop();
  1. Effects: If count is not 0 or if state is running, invokes terminate(). Otherwise, has no effects.

34.11.1.3. Member functions [exec.run.loop.members]
run-loop-opstate-base* pop-front();
  1. Effects: Blocks ([defns.block]) until one of the following conditions is true:

    • count is 0 and state is finishing, in which case pop-front returns nullptr; or

    • count is greater than 0, in which case an item is removed from the front of the queue, count is decremented by 1, and the removed item is returned.

void push-back(run-loop-opstate-base* item);
  1. Effects: Adds item to the back of the queue and increments count by 1.

  2. Synchronization: This operation synchronizes with the pop-front operation that obtains item.

run-loop-scheduler get_scheduler();
  1. Returns: An instance of run-loop-scheduler that can be used to schedule work onto this run_loop instance.

void run();
  1. Precondition: state is starting.

  2. Effects: Sets the state to running. Then, equivalent to:

    while (auto* op = pop-front()) {
      op->execute();
    }
    
  3. Remarks: When state changes, it does so without introducing data races.

void finish();
  1. Effects: Changes state to finishing.

  2. Synchronization: finish synchronizes with the pop-front operation that returns nullptr.

34.12. Coroutine utilities [exec.coro.utils]

34.12.1. execution::as_awaitable [exec.as.awaitable]

  1. as_awaitable transforms an object into one that is awaitable within a particular coroutine. [exec.coro.utils] makes use of the following exposition-only entities:

    namespace std::execution {
      template<class Sndr, class Promise>
        concept awaitable-sender =
          single-sender<Sndr, env_of_t<Promise>> &&
          sender_to<Sndr, awaitable-receiver> && // see below
          requires (Promise& p) {
            { p.unhandled_stopped() } -> convertible_to<coroutine_handle<>>;
          };
    
      template<class Sndr, class Promise>
        class sender-awaitable;
    }
    
    1. The type sender-awaitable<Sndr, Promise> is equivalent to:

      namespace std::execution {
        template<class Sndr, class Promise>
        class sender-awaitable {
          struct unit {};                                          // exposition only
          using value-type =                                       // exposition only
            single-sender-value-type<Sndr, env_of_t<Promise>>;
          using result-type =                                      // exposition only
            conditional_t<is_void_v<value-type>, unit, value-type>;
          struct awaitable-receiver;                               // exposition only
      
          variant<monostate, result-type, exception_ptr> result{}; // exposition only
          connect_result_t<Sndr, awaitable-receiver> state;        // exposition only
      
        public:
          sender-awaitable(Sndr&& sndr, Promise& p);
          static constexpr bool await_ready() noexcept { return false; }
          void await_suspend(coroutine_handle<Promise>) noexcept { start(state); }
          value-type await_resume();
        };
      }
      
      1. awaitable-receiver is equivalent to:

        struct awaitable-receiver {
          using receiver_concept = receiver_t;
          variant<monostate, result-type, exception_ptr>* result-ptr; // exposition only
          coroutine_handle<Promise> continuation;                     // exposition only
          // ... see below
        };
        

        Let rcvr be an rvalue expression of type awaitable-receiver, let crcvr be a const lvalue that refers to rcvr, let vs be a pack of subexpressions, and let err be an expression of type Err. Then:

        1. If constructible_from<result-type, decltype((vs))...> is satisfied, the expression set_value(rcvr, vs...) is equivalent to:

          try {
            rcvr.result-ptr->template emplace<1>(vs...);
          } catch(...) {
            rcvr.result-ptr->template emplace<2>(current_exception());
          }
          rcvr.continuation.resume();
          

          Otherwise, set_value(rcvr, vs...) is ill-formed.

        2. The expression set_error(rcvr, err) is equivalent to:

          rcvr.result-ptr->template emplace<2>(AS-EXCEPT-PTR(err)); // see [exec.general]
          rcvr.continuation.resume();
          
        3. The expression set_stopped(rcvr) is equivalent to:

          static_cast<coroutine_handle<>>(rcvr.continuation.promise().unhandled_stopped()).resume();
          
        4. For any expression tag whose type satisfies forwarding-query and for any pack of subexpressions as, get_env(crcvr).query(tag, as...) is expression-equivalent to tag(get_env(as_const(crcvr.continuation.promise())), as...).

      2. sender-awaitable(Sndr&& sndr, Promise& p);

        1. Effects: Initializes state with connect(std::forward<Sndr>(sndr), awaitable-receiver{addressof(result), coroutine_handle<Promise>::from_promise(p)}).

      3. value-type await_resume();

        1. Effects: Equivalent to:

          if (result.index() == 2)
            rethrow_exception(get<2>(result));
          if constexpr (!is_void_v<value-type>)
            return std::forward<value-type>(get<1>(result));
          
  2. as_awaitable is a customization point object. For subexpressions expr and p where p is an lvalue, Expr names the type decltype((expr)) and Promise names the type decay_t<decltype((p))>, as_awaitable(expr, p) is expression-equivalent to:

    1. expr.as_awaitable(p) if that expression is well-formed.

      • Mandates: is-awaitable<A, Promise> is true, where A is the type of the expression above.

    2. Otherwise, (void(p), expr) if is-awaitable<Expr, U> is true, where U is an unspecified class type that is not Promise and that lacks a member named await_transform.

      • Preconditions: is-awaitable<Expr, Promise> is true and the expression co_await expr in a coroutine with promise type U is expression-equivalent to the same expression in a coroutine with promise type Promise.

    3. Otherwise, sender-awaitable{expr, p} if awaitable-sender<Expr, Promise> is true.

    4. Otherwise, (void(p), expr).

    except that the evaluations of expr and p are indeterminately sequenced.

34.12.2. execution::with_awaitable_senders [exec.with.awaitable.senders]

  1. with_awaitable_senders, when used as the base class of a coroutine promise type, makes senders awaitable in that coroutine type.

    In addition, it provides a default implementation of unhandled_stopped() such that if a sender completes by calling set_stopped, it is treated as if an uncatchable "stopped" exception were thrown from the await-expression. The coroutine is never resumed, and the unhandled_stopped of the coroutine caller’s promise type is called.

    namespace std::execution {
      template<class-type Promise>
        struct with_awaitable_senders {
          template<class OtherPromise>
            requires (!same_as<OtherPromise, void>)
          void set_continuation(coroutine_handle<OtherPromise> h) noexcept;
    
          coroutine_handle<> continuation() const noexcept { return continuation; }
    
          coroutine_handle<> unhandled_stopped() noexcept {
            return stopped-handler(continuation.address());
          }
    
          template<class Value>
          see below await_transform(Value&& value);
    
        private:
          // exposition only
          [[noreturn]] static coroutine_handle<> default-unhandled-stopped(void*) noexcept {
            terminate();
          }
          coroutine_handle<> continuation{}; // exposition only
          // exposition only
          coroutine_handle<> (*stopped-handler)(void*) noexcept = &default-unhandled-stopped;
        };
    }
    
  2. template<class OtherPromise>
      requires (!same_as<OtherPromise, void>)
    void set_continuation(coroutine_handle<OtherPromise> h) noexcept;
    
    1. Effects: Equivalent to:

    continuation = h;
    if constexpr ( requires(OtherPromise& other) { other.unhandled_stopped(); } ) {
      stopped-handler = [](void* p) noexcept -> coroutine_handle<> {
        return coroutine_handle<OtherPromise>::from_address(p)
          .promise().unhandled_stopped();
      };
    } else {
      stopped-handler = &default-unhandled-stopped;
    }
    
  3. template<class Value>
    call-result-t<as_awaitable_t, Value, Promise&> await_transform(Value&& value);
    
    1. Effects: Equivalent to:

    return as_awaitable(std::forward<Value>(value), static_cast<Promise&>(*this));
    

Index

Terms defined by this specification

References

Informative References

[HPX]
Hartmut Kaiser; et al. HPX - The C++ Standard Library for Parallelism and Concurrency. URL: https://doi.org/10.21105/joss.02352
[N4885]
Thomas Köppe. Working Draft, Standard for Programming Language C++. 17 March 2021. URL: https://wg21.link/n4885
[P0443R14]
Jared Hoberock, Michael Garland, Chris Kohlhoff, Chris Mysen, H. Carter Edwards, Gordon Brown, D. S. Hollman. A Unified Executors Proposal for C++. 15 September 2020. URL: https://wg21.link/p0443r14
[P0981R0]
Richard Smith, Gor Nishanov. Halo: coroutine Heap Allocation eLision Optimization: the joint response. 18 March 2018. URL: https://wg21.link/p0981r0
[P1056R1]
Lewis Baker, Gor Nishanov. Add lazy coroutine (coroutine task) type. 7 October 2018. URL: https://wg21.link/p1056r1
[P1895R0]
Lewis Baker, Eric Niebler, Kirk Shoop. tag_invoke: A general pattern for supporting customisable functions. 8 October 2019. URL: https://wg21.link/p1895r0
[P1897R3]
Lee Howes. Towards C++23 executors: A proposal for an initial set of algorithms. 16 May 2020. URL: https://wg21.link/p1897r3
[P2175R0]
Lewis Baker. Composable cancellation for sender-based async operations. 15 December 2020. URL: https://wg21.link/p2175r0
[P2855R1]
Ville Voutilainen. Member customization points for Senders and Receivers. 22 February 2024. URL: https://wg21.link/p2855r1
[P2999R3]
Eric Niebler. Sender Algorithm Customization. 13 December 2023. URL: https://wg21.link/p2999r3
[P3149R3]
Ian Petersen, Ján Ondrušek; Jessica Wong; Kirk Shoop; Lee Howes; Lucian Radu Teodorescu;. async_scope — Creating scopes for non-sequential concurrency. 22 May 2024. URL: https://wg21.link/p3149r3
[P3175R3]
Eric Niebler. Reconsidering the std::execution::on algorithm. 2024-06-24. URL: https://wg21.link/P3175R3
[P3187R1]
Kirk Shoop, Lewis Baker. remove ensure_started and start_detached from P2300. 21 March 2024. URL: https://wg21.link/p3187r1
[P3303R1]
Eric Niebler. Fixing Lazy Sender Algorithm Customization. 2024-06-24. URL: https://wg21.link/P3303R1