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4356. connect() should use get_allocator(get_env(rcvr)) to allocate the coroutine-state for a connect-awaitable coroutine

Section: 33.9.10 [exec.connect] Status: New Submitter: Lewis Baker Opened: 2025-08-27 Last modified: 2025-10-23

Priority: 2

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Discussion:

The wording for connect() (33.9.10 [exec.connect]) handles passing awaitable types as the sender argument by calling the connect-awaitable() coroutine and having it execute a co_await expression.

The connect-awaitable() coroutine will typically need to dynamically allocate storage for the coroutine state and, as specified, this currently just always uses the global default allocator. This is because the connect-awaitable-promise type does not define any member operator new/delete overloads.

It seems desirable for this facility to use the allocator obtained from the receiver, by calling get_allocator(get_env(rcvr)), in order to allocate storage for the coroutine-state instead of always using global operator new. This would give the user at least some level of control over how this allocation is performed.

[2025-10-23; Reflector poll.]

Set priority to 2 after reflector poll.

"The get_allocator query doesn't have a default and I think that isn't covered in the proposed resolution."

Proposed resolution:

This wording is relative to N5014.

1. Modify 33.9.10 [exec.connect] as indicated:

   [Drafting note: This should cover the design intent, although we may want to spell this out more explicitly in terms of the exact semantics in a similar way to 25.8.5 [coro.generator.promise] p17, which lists overloads of operator new() and describes the rebound allocator type which allocates storage in chunks of size __STDCPP_DEFAULT_NEW_ALIGNMENT__.]

   -5- Let V name the type await-result-type<DS, connect-awaitable-promise>, let Sigs name the type

   […]
   

   and let connect-awaitable be an exposition-only coroutine defined as follows:

   namespace std::execution {
     […]
   }  
   

   Any dynamically allocated storage required for the coroutine state allocated by an invocation of the form connect-awaitable(sndr, rcvr) is allocated using the allocator obtained from get_allocator(get_env(rcvr)).

   -6- […]