operator==,!=(std::function) - cppreference.com

From cppreference.com

Defined in header `<functional>`
`template< class R, class... ArgTypes > bool operator==( const std::function<R(ArgTypes...)>& f, std::nullptr_t ) noexcept;`	(1)	(since C++11)
`template< class R, class... ArgTypes > bool operator==( std::nullptr_t, const std::function<R(ArgTypes...)>& f ) noexcept;`	(2)	(since C++11) (until C++20)
`template< class R, class... ArgTypes > bool operator!=( const std::function<R(ArgTypes...)>& f, std::nullptr_t ) noexcept;`	(3)	(since C++11) (until C++20)
`template< class R, class... ArgTypes > bool operator!=( std::nullptr_t, const std::function<R(ArgTypes...)>& f ) noexcept;`	(4)	(since C++11) (until C++20)

Compares a std::function with a null pointer. Empty functions (that is, functions without a callable target) compare equal, non-empty functions compare non-equal.

The != operator is synthesized from operator==.

(since C++20)

Parameters

f - std::function to compare

Return value

1,2) !f

3,4) (bool) f

Example

#include <functional>
#include <iostream>

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

class C
{
public:
    C(SomeVoidFunc void_func = nullptr) : void_func_(void_func)
    {
        if (void_func_ == nullptr) // specialized compare with nullptr
            void_func_ = std::bind(&C::default_func, this, std::placeholders::_1);
        void_func_(7);
    }

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

private:
    SomeVoidFunc void_func_;
};

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

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

Output:

Parameters

Return value

Example

See also