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

Notes

Examples

#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