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

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 Fn can be invoked with the arguments ArgTypes.... Formally, determines whether INVOKE(declval<Fn>(), declval<ArgTypes>()...) is well formed when treated as an unevaluated operand, where INVOKE is the operation defined in Callable.

2) Determines whether Fn can be invoked with the arguments ArgTypes... to yield a result that is convertible to R. Formally, determines whether INVOKE<R>(declval<Fn>(), declval<ArgTypes>()...) is well formed when treated as an unevaluated operand, where INVOKE is the operation defined in Callable

3) Determines whether Fn is callable with the arguments ArgTypes... (same as (1)), and that such call is known not to throw any exceptions.

4) Determines whether Fn can be invoked with the arguments ArgTypes... to yield a result that is convertible to R (same as (2)), and that such call (including the conversion) is known not to throw any exceptions.

Fn, R and all types in the parameter pack ArgTypes shall each be a complete type, (possibly cv-qualified) void, or an array of unknown bound. Otherwise, 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.

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)

Inherited from std::integral_constant

Member constants

value

[static]

true if INVOKE<R>(declval<Fn>(), 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>

Examples

Run this code

#include <type_traits>
 
auto func2(char) -> int (*)()
{
    return nullptr;
}
 
int main()
{
    static_assert( std::is_invocable<int()>::value );
    static_assert( std::is_invocable_r<int, int()>::value );
    static_assert( std::is_invocable_r<void, void(int), int>::value );
    static_assert( std::is_invocable_r<int(*)(), decltype(func2), char>::value );
}

Language
Standard Library Headers
Freestanding and hosted implementations
Named requirements
Language support library
Concepts library (C++20)
Diagnostics library
Utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Filesystem library (C++17)
Technical Specifications

invoke (C++17)	invokes any Callable object with given arguments (function template)
result_ofinvoke_result (C++11)(removed in C++20)(C++17)	deduces the result type of invoking a callable object with a set of arguments (class template)
declval (C++11)	obtains a reference to its argument for use in unevaluated context (function template)