operator==(std::move_only_function)

friend bool operator==( const std::move_only_function& f, std::nullptr_t ) noexcept;

(since C++23)

Checks whether the wrapper f has a callable target by formally comparing it with std::nullptr_t. Empty wrappers (that is, wrappers without a target) compare equal, non-empty functions compare non-equal.

This function is not visible to ordinary unqualified or qualified lookup, and can only be found by argument-dependent lookup when std::move_only_function<FunctionType> is an associated class of the arguments.

The != operator is synthesized from operator==.

Parameters

f - std::move_only_function to compare

Return value

!f.

Example

#include <functional>
#include <iostream>
#include <utility>

using SomeVoidFunc = std::move_only_function<void(int) const>;

class C {
public:
    C() = default;
    C(SomeVoidFunc func) : void_func_(std::move(func)) {}

    void default_func(int i) const { std::cout << i << '\n'; };

    void operator()() const
    {
        if (void_func_ == nullptr) // specialized compare with nullptr
            default_func(7);
        else
            void_func_(7);
    }

private:
    SomeVoidFunc void_func_{};
};

void user_func(int i)
{
    std::cout << (i + 1) << '\n';
}

int main()
{
    C c1;
    C c2(user_func);
    c1();
    c2();
}

Output:

7
8

operator bool	checks if the `std::move_only_function` has a target (public member function)
operator==operator!= (removed in C++20)	compares a `std::function` with `nullptr` (function template)

operator==(std::move_only_function)

Parameters

Return value

Example

See also