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Showing content from https://cplusplus.github.io/LWG/issue2921 below:

Issue 2921: packaged_task and type-erased allocators

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Section: 32.10.10 [futures.task] Status: C++17 Submitter: United States Opened: 2017-02-03 Last modified: 2025-06-23

Priority: Not Prioritized

View all other issues in [futures.task].

View all issues with C++17 status.

Discussion:

The constructor that type-erases an allocator has all of the problems of the similar function constructor that was removed for this CD. This constructor from packaged_task should similarly be removed as well. If we prefer to keep this constructor, the current wording is underspecified, as the Allocator argument is not required to be type satisfying the Allocator requirements, nor is allocator_traits used.

Proposed change:

Strike

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

from the class definition in p2, and from 30.6.9.1 [futures.task.members] p2. Strike the last sentence of 30.6.9.1p4. In p3, revise "These constructors" to "This constructor"

[Kona 2017-03-02]

Accepted as Immediate to resolve NB comment.

[Sofia 2025-06-21; this was reverted by P3503R3]

Proposed resolution:

This wording is relative to N4618.

1. Modify 32.10.10 [futures.task] as follows:

   Strike

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

   from the class definition in p2, and from [futures.task.members] p2.

2. Modify 32.10.10.2 [futures.task.members]/3:

   Remarks: These constructors This constructor shall not participate in overload resolution if decay_t<F> is the same type as packaged_task<R(ArgTypes...)>.

3. Strike the last sentence of 32.10.10.2 [futures.task.members]/4:

   The constructors that take an Allocator argument use it to allocate memory needed to store the internal data structures.

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