3003. <future> still has type-erased allocators in promise

Section: 33.6.6 [futures.promise] Status: New Submitter: Billy O'Neal III Opened: 2017-07-16 Last modified: 2017-11-29

Priority: 2

View other active issues in [futures.promise].

View all other issues in [futures.promise].

View all issues with New status.

Discussion:

In Toronto Saturday afternoon LWG discussed LWG 2976 which finishes the job of removing allocator support from packaged_task. LWG confirmed that, despite the removal of packaged_task allocators "because it looks like std::function" was incorrect, they wanted to keep the allocator removals anyway, in large part due to this resolution being a response to an NB comment.

If we don't want the type erased allocator situation at all, then we should remove them from the remaining place they exist in <future>, namely, in promise.

This change also resolves potential implementation divergence on whether allocator::construct is intended to be used on elements constructed in the shared state, and allows the emplace-construction-in-future paper, P0319, to be implemented without potential problems there.

[28-Nov-2017 Mailing list discussion - set priority to P2]

Lots of people on the ML feel strongly about this; the suggestion was made that a paper would be welcomed laying out the rationale for removing allocator support here (and in other places).

Proposed resolution:

This resolution is relative to N4659.

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

    template<class R> 
    class promise {
    public:
      promise();
      template <class Allocator>
        promise(allocator_arg_t, const Allocator& a);
      […]
    };
    template <class R>
      void swap(promise<R>& x, promise<R>& y) noexcept;
    template <class R, class Alloc>
      struct uses_allocator<promise<R>, Alloc>;
    
    […]
    template <class R, class Alloc>
      struct uses_allocator<promise<R>, Alloc>
        : true_type { };
    

    -3- Requires: Alloc shall be an Allocator (20.5.3.5 [allocator.requirements]).

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

    -4- Effects: constructs a promise object and a shared state. The second constructor uses the allocator a to allocate memory for the shared state.