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

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< cpp‎ | types

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.

### 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 (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>

### Notes

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

### 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

invokeinvoke_r (C++17)(C++23) | invokes any Callable object with given arguments and possibility to specify return type(since C++23) (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 an object of the template type argument for use in an unevaluated context (function template)
invocableregular_invocable (C++20) | specifies that a callable type can be invoked with a given set of argument types (concept)

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