2752. "Throws:" clauses of async and packaged_task are unimplementable

Section: 32.10.9 [futures.async], 32.10.10.2 [futures.task.members] Status: C++17 Submitter: Billy Robert O'Neal III Opened: 2016-07-07 Last modified: 2017-07-30

Priority: 3

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Discussion:

std::async is a request from the user for type erasure; as any given function gets passed to async which returns only future<ReturnType>. Therefore, it needs to be able to allocate memory, as other issues (e.g. LWG 2202) indicate. However, async's Throws clause doesn't allow an implementation to do this, as it permits only future_error.

std::packaged_task's constructor allocates memory using a user supplied allocator. An implementation needs to call the user's allocate to allocate such memory. The user's allocate function is not constrained to throwing only bad_alloc; it can raise whatever it wants, but packaged_task's constructor prohibits this.

[2016-07 Chicago]

Alisdair thinks the third bullet is not quite right.

Previous resolution [SUPERSEDED]:

1. Change 32.10.9 [futures.async] p6 to:

   Throws: system_error if policy == launch::async and the implementation is unable to start a new thread, or std::bad_alloc if memory for the internal data structures could not be allocated.

2. Change 32.10.10.2 [futures.task.members] p5 to:

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

   Throws:

   1. (?) — Aany exceptions thrown by the copy or move constructor of f., or
   2. (?) — For the first version, std::bad_alloc if memory for the internal data structures could not be allocated.
   3. (?) — For the second version, any exceptions thrown by std::allocator_traits<Allocator>::template rebind<unspecified>::allocate.

[2016-08 Chicago]

Wed PM: Move to Tentatively Ready

Proposed resolution:

This wording is relative to N4606.

1. Change 32.10.9 [futures.async] p6 to:

   Throws: system_error if policy == launch::async and the implementation is unable to start a new thread, or std::bad_alloc if memory for the internal data structures could not be allocated.

2. Change 32.10.10.2 [futures.task.members] p5 to:

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

   Throws:

   1. (?) — Aany exceptions thrown by the copy or move constructor of f., or
   2. (?) — For the first version, std::bad_alloc if memory for the internal data structures could not be allocated.
   3. (?) — For the second version, any exceptions thrown by std::allocator_traits<Allocator>::template rebind_traits<unspecified>::allocate.