std::is_invocable,std::is_invocable_r,std::is_nothrow_invocable,(3)

NAME
std::is_invocable,std::is_invocable_r,std::is_nothrow_invocable, - std::is_invocable,std::is_invocable_r,std::is_nothrow_invocable,

Synopsis
   Defined in header <type_traits>
   template <class Fn, class... ArgTypes> (1) (since C++17)
   struct is_invocable;
   template <class R, class Fn, class... ArgTypes> (2) (since C++17)
   struct is_invocable_r;
   template <class Fn, class... ArgTypes> (3) (since C++17)
   struct is_nothrow_invocable;
   template <class R, class Fn, class... ArgTypes> (4) (since C++17)
   struct is_nothrow_invocable_r;
   1) Determines whether Fn can be invoked with the arguments ArgTypes.... Formally,
   determines whether INVOKE(std::declval<Fn>(), std::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
   and the implicit conversion does not bind a reference to a temporary object
   (since C++23). If R is cv void, the result can be any type. Formally, determines
   whether INVOKE<R>(std::declval<Fn>(), std::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
   and the implicit conversion does not bind a reference to a temporary object
   (since C++23) (same as (2)), and that such call (including the conversion of the
   parameters and the result) is known not to throw any exceptions. If R is cv void,
   the result can be any type (same as (2)). The converson of the parameters and the
   call itself still has to be 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.
   The behavior of a program that adds specializations for any of the templates
   described on this page is undefined.
  Helper variable templates
   Defined in header <type_traits>
   template <class Fn, class... ArgTypes>
   inline constexpr bool is_invocable_v = (1) (since C++17)
   std::is_invocable<Fn, ArgTypes...>::value;
   template <class R, class Fn, class... ArgTypes>
   inline constexpr bool is_invocable_r_v = (2) (since C++17)
   std::is_invocable_r<R, Fn, ArgTypes...>::value;
   template <class Fn, class... ArgTypes>
   inline constexpr bool is_nothrow_invocable_v = (3) (since C++17)
   std::is_nothrow_invocable<Fn, ArgTypes...>::value;
   template <class R, class Fn, class... ArgTypes>
   inline constexpr bool is_nothrow_invocable_r_v = (4) (since C++17)
   std::is_nothrow_invocable_r<R, Fn, ArgTypes...>::value; Inherited from std::integral_constant

Member constants
   value true if INVOKE<R>(declval<Fn>(), declval<ArgTypes>()...) is well formed
   [static] 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() returns value
   (C++14) (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_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> );
 }

See also
   invoke invokes any Callable object with given arguments
   invoke_r and possibility to specify return type
   (C++17) (since C++23)
   (C++23) (function template)
   result_of deduces the result type of invoking a callable object with
   invoke_result a set of arguments
   (C++11)(removed in C++20) (class template)
   (C++17)
   declval obtains a reference to its argument for use in unevaluated
   (C++11) context
                        (function template)
   invocable specifies that a callable type can be invoked with a given
   regular_invocable set of argument types
   (C++20) (concept)