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Section: 22.10.5 [func.invoke] Status: Resolved Submitter: Zhihao Yuan Opened: 2016-03-25 Last modified: 2021-06-12
Priority: Not Prioritized
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Discussion:
In N4169 the author dropped the invoke<R> support by claiming that it's an unnecessary cruft in TR1, obsoleted by C++11 type inference. But now we have some new business went to *INVOKE*(f, t1, t2, ..., tN, R), that is to discard the return type when R is void. This form is very useful, or possible even more useful than the basic form when implementing a call wrapper. Also note that the optional R support is already in std::is_callable and std::is_nothrow_callable.
[2016-07-31, Tomasz Kamiński comments]
The lack of invoke<R> was basically a result of the concurrent publication of the never revision of the paper and additional special semantics of INVOKE(f, args..., void).
In contrast to existing std::invoke function, the proposed invoke<R> version is not SFINAE friendly, as elimination of the standard version of invoke is guaranteed by std::result_of_t in the result type that is missing for proposed invoke<R> version. To provide this guarantee, following remarks shall be added to the specification:Remarks: This function shall not participate in overload resolution unless is_callable_v<F(Args...), R> is true.
[2016-08-01, Tomasz Kamiński and Zhihao Yuan update the proposed wording]
Previous resolution [SUPERSEDED]:
This wording is relative to N4606.
Modify 22.10 [function.objects]/2, header <functional> synopsis, as indicated:
namespace std { // 20.12.3, invoke: template <class F, class... Args> result_of_t<F&&(Args&&...)> invoke(F&& f, Args&&... args); template <class R, class F, class... Args> R invoke(F&& f, Args&&... args);Add the following sequence of paragraphs after 22.10.5 [func.invoke]/1 as indicated:
template <class R, class F, class... Args> R invoke(F&& f, Args&&... args);-?- Returns: INVOKE(std::forward<F>(f), std::forward<Args>(args)..., R) (22.10.4 [func.require]).
-?- Remarks: This function shall not participate in overload resolution unless is_callable_v<F(Args...), R> is true.
[2016-09-04, Tomasz Kamiński comments and improves wording]
The usage of is_callable_v<F(Args...), R> causes problem in situation when either F or Args is an abstract type and the function type F(Args...) cannot be formed or when one of the args is cv-qualified, as top-level cv-qualification for function parameters is dropped by language rules. It should use is_callable_v<F&&(Args&&...), R> instead.
Previous resolution [SUPERSEDED]:
This wording is relative to N4606.
Modify 22.10 [function.objects]/2, header <functional> synopsis, as indicated:
namespace std { // 20.12.3, invoke: template <class F, class... Args> result_of_t<F&&(Args&&...)> invoke(F&& f, Args&&... args); template <class R, class F, class... Args> R invoke(F&& f, Args&&... args);Add the following sequence of paragraphs after 22.10.5 [func.invoke]/1 as indicated:
template <class R, class F, class... Args> R invoke(F&& f, Args&&... args);-?- Returns: INVOKE(std::forward<F>(f), std::forward<Args>(args)..., R) (22.10.4 [func.require]).
-?- Remarks: This function shall not participate in overload resolution unless is_callable_v<F&&(Args&&...), R> is true.
[2018-08-22, Zhihao Yuan provides improved wording]
Previous resolution [SUPERSEDED]:
This wording is relative to N4762.
Modify 22.10.2 [functional.syn], header <functional> synopsis, as indicated:
namespace std { // 22.10.5 [func.invoke], invoke template<class F, class... Args> invoke_result_t<F, Args...> invoke(F&& f, Args&&... args) noexcept(is_nothrow_invocable_v<F, Args...>); template <class R, class F, class... Args> R invoke(F&& f, Args&&... args) noexcept(is_nothrow_invocable_r_v<R, F, Args...>);Add the following sequence of paragraphs after 22.10.5 [func.invoke]/1 as indicated:
template <class R, class F, class... Args> R invoke(F&& f, Args&&... args) noexcept(is_nothrow_invocable_r_v<R, F, Args...>);-?- Constraints: is_invocable_r_v<R, F, Args...>.
-?- Returns: INVOKE<R>(std::forward<F>(f), std::forward<Args>(args)...) (22.10.4 [func.require]).
[2021-06-12; Resolved by accepting P2136R3.]
Proposed resolution:
This wording is relative to N4849.
Modify 22.10.2 [functional.syn], header <functional> synopsis, as indicated:
namespace std {
// 22.10.5 [func.invoke], invoke
template<class F, class... Args>
constexpr invoke_result_t<F, Args...> invoke(F&& f, Args&&... args)
noexcept(is_nothrow_invocable_v<F, Args...>);
template <class R, class F, class... Args>
constexpr R invoke(F&& f, Args&&... args)
noexcept(is_nothrow_invocable_r_v<R, F, Args...>);
Add the following sequence of paragraphs after 22.10.5 [func.invoke]/1 as indicated:
template <class R, class F, class... Args> constexpr R invoke(F&& f, Args&&... args) noexcept(is_nothrow_invocable_r_v<R, F, Args...>);-?- Constraints: is_invocable_r_v<R, F, Args...>.
-?- Returns: INVOKE<R>(std::forward<F>(f), std::forward<Args>(args)...) (22.10.4 [func.require]).