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std::is_invocable, std::is_invocable_r, std::is_nothrow_invocable, std::is_nothrow_invocable_r

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Defined in header <type_traits>
template< class Fn, class... ArgTypes >
struct is_invocable;
 (1) (since C++17)
template< class R, class Fn, class... ArgTypes >
struct is_invocable_r;
 (2) (since C++17)
template< class Fn, class... ArgTypes >
struct is_nothrow_invocable;
 (3) (since C++17)
template< class R, class Fn, class... ArgTypes >
struct is_nothrow_invocable_r;
 (4) (since C++17)
1) Determines whether INVOKE(std::declval<Fn>(), std::declval<ArgTypes>()...) is well formed when treated as an unevaluated operand.
2) Determines whether INVOKE<R>(std::declval<Fn>(), std::declval<ArgTypes>()...) is well formed when treated as an unevaluated operand.
3) Determines whether INVOKE(std::declval<Fn>(), std::declval<ArgTypes>()...) is well formed when treated as an unevaluated operand, and is known not to throw any exceptions.
4) Determines whether INVOKE<R>(std::declval<Fn>(), std::declval<ArgTypes>()...) is well formed when treated as an unevaluated operand, and is known not to throw any exceptions.

If Fn, R or any type in the parameter pack ArgTypes is not a complete type, (possibly cv-qualified) void, or an array of unknown bound, the behavior is undefined.

If an instantiation of a template above depends, directly or indirectly, on an incomplete type, and that instantiation could yield a different result if that type were hypothetically completed, the behavior is undefined.

If the program adds specializations for any of the templates described on this page, the behavior is undefined.

Contents

[edit] Helper variable templates

Defined in header <type_traits>
template< class Fn, class... ArgTypes >

inline constexpr bool is_invocable_v =

    std::is_invocable<Fn, ArgTypes...>::value;
 (1) (since C++17)
template< class R, class Fn, class... ArgTypes >

inline constexpr bool is_invocable_r_v =

    std::is_invocable_r<R, Fn, ArgTypes...>::value;
 (2) (since C++17)
template< class Fn, class... ArgTypes >

inline constexpr bool is_nothrow_invocable_v =

    std::is_nothrow_invocable<Fn, ArgTypes...>::value;
 (3) (since C++17)
template< class R, class Fn, class... ArgTypes >

inline constexpr bool is_nothrow_invocable_r_v =

    std::is_nothrow_invocable_r<R, Fn, ArgTypes...>::value;
 (4) (since C++17)
[edit]

Inherited from std::integral_constant

Member constants

value
[static]
 true if (for overload (1)) INVOKE(std::declval<Fn>(), std::declval<ArgTypes>()...) is well formed when treated as an unevaluated operand, false otherwise
(public static member constant)

Member functions

operator bool
 converts the object to bool, returns value
(public member function)
operator()
(C++14)
 returns value
(public member function)

Member types

Type Definition
value_type bool
type std::integral_constant<bool, value>

[edit] Notes

Feature-test macro Value Std Feature
__cpp_lib_is_invocable 201703L (C++17) std::is_invocable, std::invoke_result

[edit] Examples

Run this code
#include <type_traits>
 
auto func2(char) -> int (*)()
{
    return nullptr;
}
 
int main()
{
    static_assert(std::is_invocable_v<int()>);
    static_assert(not std::is_invocable_v<int(), int>);
    static_assert(std::is_invocable_r_v<int, int()>);
    static_assert(not std::is_invocable_r_v<int*, int()>);
    static_assert(std::is_invocable_r_v<void, void(int), int>);
    static_assert(not std::is_invocable_r_v<void, void(int), void>);
    static_assert(std::is_invocable_r_v<int(*)(), decltype(func2), char>);
    static_assert(not std::is_invocable_r_v<int(*)(), decltype(func2), void>);
}

[edit] See also

(C++17)(C++23)
 invokes any Callable object with given arguments and possibility to specify return type(since C++23)
(function template) [edit]
(C++11)(removed in C++20)(C++17)
 deduces the result type of invoking a callable object with a set of arguments
(class template) [edit]
(C++11)
 obtains a reference to an object of the template type argument for use in an unevaluated context
(function template) [edit]
(C++20)
 specifies that a callable type can be invoked with a given set of argument types
(concept) [edit]
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