2095. promise and packaged_task missing constructors needed for uses-allocator construction

Section: 33.6.6 [futures.promise], 33.6.10 [futures.task] Status: LEWG Submitter: Jonathan Wakely Opened: 2011-11-01 Last modified: 2016-08-06

Priority: 4

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Discussion:

This example is ill-formed according to C++11 because uses_allocator<promise<R>, A>::value is true, but is_constructible<promise<R>, A, promise<R>&&>::value is false. Similarly for packaged_task.

#include <future>
#include <memory>
#include <tuple>

using namespace std;

typedef packaged_task<void()> task;
typedef promise<void> prom;
allocator<task> a;

tuple<task, prom> t1{ allocator_arg, a };
tuple<task, prom> t2{ allocator_arg, a, task{}, prom{} };

[2012, Portland]

This is an allocator issue, and should be dealt with directly by LWG.

[2013-03-06]

Jonathan suggests to make the new constructors non-explicit and makes some representational improvements.

[2013-09 Chicago]

Move to deferred.

This issue has much in common with similar problems with std::function that are being addressed by the polymorphic allocators proposal currently under evaluation in LEWG. Defer further discussion on this topic until the final outcome of that paper and its proposed resolution is known.

[2014-02-20 Re-open Deferred issues as Priority 4]

[2016-08 Chicago]

Fri PM: Send to LEWG - and this also applies to function in LFTS.

Proposed resolution:

[This wording is relative to the FDIS.]

  1. Add to 33.6.6 [futures.promise], class template promise synopsis, as indicated:

    namespace std {
      template <class R>
      class promise {
      public:
        promise();
        template <class Allocator>
        promise(allocator_arg_t, const Allocator& a);
        template <class Allocator>
        promise(allocator_arg_t, const Allocator& a, promise&& rhs) noexcept;
        promise(promise&& rhs) noexcept;
        promise(const promise& rhs) = delete;
        ~promise();	
        […]
      };
      […]
    }
    
  2. Change 33.6.6 [futures.promise] as indicated:

    promise(promise&& rhs) noexcept;
    template <class Allocator>
    promise(allocator_arg_t, const Allocator& a, promise&& rhs) noexcept;
    

    -5- Effects: constructs a new promise object and transfers ownership of the shared state of rhs (if any) to the newly-constructed object.

    -6- Postcondition: rhs has no shared state.

    -?- [Note: a is not used — end note]

  3. Add to 33.6.10 [futures.task], class template packaged_task synopsis, as indicated:

    namespace std {
      template<class> class packaged_task; // undefined
    
      template<class R, class... ArgTypes>
      class packaged_task<R(ArgTypes...)> {
      public:
        // construction and destruction
        packaged_task() noexcept;
        template <class Allocator>
          packaged_task(allocator_arg_t, const Allocator& a) noexcept;
        template <class F>
          explicit packaged_task(F&& f);
        template <class F, class Allocator>
          explicit packaged_task(allocator_arg_t, const Allocator& a, F&& f);
        ~packaged_task();
    	
        // no copy
        packaged_task(const packaged_task&) = delete;
        template<class Allocator>
          packaged_task(allocator_arg_t, const Allocator& a, const packaged_task&) = delete;
        packaged_task& operator=(const packaged_task&) = delete;
        
        // move support
        packaged_task(packaged_task&& rhs) noexcept;
        template <class Allocator>
          packaged_task(allocator_arg_t, const Allocator& a, packaged_task&& rhs) noexcept;
        packaged_task& operator=(packaged_task&& rhs) noexcept;
        void swap(packaged_task& other) noexcept;
        […]
      };
      […]
    }
    
  4. Change 33.6.10.1 [futures.task.members] as indicated:

    packaged_task() noexcept;
    template <class Allocator>
      packaged_task(allocator_arg_t, const Allocator& a) noexcept;
    

    -1- Effects: constructs a packaged_task object with no shared state and no stored task.

    -?- [Note: a is not used — end note]

    […]

    packaged_task(packaged_task&& rhs) noexcept;
    template <class Allocator>
      packaged_task(allocator_arg_t, const Allocator& a, packaged_task&& rhs) noexcept;
    

    -5- Effects: constructs a new packaged_task object and transfers ownership of rhs's shared state to *this, leaving rhs with no shared state. Moves the stored task from rhs to *this.

    -6- Postcondition: rhs has no shared state.

    -?- [Note: a is not used — end note]