Currently, the internal state class _Packaged_state used by packaged_task stores a function.
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template <class _Ret, class... _ArgTypes>
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class _Packaged_state<_Ret(_ArgTypes...)> : public _Associated_state<typename _P_arg_type<_Ret>::type> {
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public:
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using _Mybase = _Associated_state<typename _P_arg_type<_Ret>::type>;
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using _Mydel = typename _Mybase::_Mydel;
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using _Function_type = function<_Ret(_ArgTypes...)>; // TRANSITION, ABI, should not use std::function
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private:
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_Function_type _Fn;
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However, function requires the stored target object to be copy constructible while packaged_task doesn't, which rendered our implementation strategy non-conforming (reject-valid) for a long while, see #321.
#4946 partially fixed the non-conformance. However, the fix was imperfect:
- it introduced handling for move-only functors which
function isn't supposed to support,
- it was broken when one specializes
function for program-defined types, although I don't think anyone should do this, and
- when the functor's copy constructor is eligible but ill-formed, there's still hard error from it, which isn't supposed to happen for
packaged_task.
In vNext where ABI breakage is available, we need to change the internal storage type. It seems that the implementation details of that type can be shared with move_only_function, but it doesn't seem possible to directly use move_only_function because move_only_function is only available since C++23.
vNext note: Resolving this issue will require breaking binary compatibility. We won't be able to accept pull requests for this issue until the vNext branch is available. See #169 for more information.
Currently, the internal state class
_Packaged_stateused bypackaged_taskstores afunction.STL/stl/inc/future
Lines 481 to 486 in 37120ed
STL/stl/inc/future
Lines 550 to 551 in 37120ed
However,
functionrequires the stored target object to be copy constructible whilepackaged_taskdoesn't, which rendered our implementation strategy non-conforming (reject-valid) for a long while, see #321.#4946 partially fixed the non-conformance. However, the fix was imperfect:
functionisn't supposed to support,functionfor program-defined types, although I don't think anyone should do this, andpackaged_task.In vNext where ABI breakage is available, we need to change the internal storage type. It seems that the implementation details of that type can be shared with
move_only_function, but it doesn't seem possible to directly usemove_only_functionbecausemove_only_functionis only available since C++23.vNext note: Resolving this issue will require breaking binary compatibility. We won't be able to accept pull requests for this issue until the vNext branch is available. See #169 for more information.