operator==,!=(std::function)

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)
template< class R, class... ArgTypes > bool operator!=( const std::function<R(ArgTypes...)>& f, std::nullptr_t ) noexcept;	(3)	(since C++11)
template< class R, class... ArgTypes > bool operator!=( std::nullptr_t, const std::function<R(ArgTypes...)>& f ) noexcept;	(4)	(since C++11)

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.

Parameters

f - std::function to compare

Return value

1-2) !f

3-4) (bool) f

Example

Run this code

#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:

7
8

Language
Headers
Freestanding and hosted implementations
Named requirements
Language support library
Concepts library (C++20)
Diagnostics library
Utilities library
Strings library
Containers library
Algorithms library
Iterators library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Filesystem library (C++17)
Technical Specifications

Member functions
function::function
function::~function
function::operator=
function::swap
function::assign (until C++17)
function::operator bool
function::operator()
function::target_type
function::target
Non-member functions
operator==operator!=
std::swap(std::function)
Helper classes
std::uses_allocator<std::function> (until C++17)
Deduction guides(C++17)