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

template< class R, class... ArgTypes > bool operator==( const function<R(ArgTypes...)>& f, std::nullptr_t );	(1)
template< class R, class... ArgTypes > bool operator==( std::nullptr_t, const function<R(ArgTypes...)>& f );	(2)
template< class R, class... ArgTypes > bool operator!=( const function<R(ArgTypes...)>& f, std::nullptr_t );	(3)
template< class R, class... ArgTypes > bool operator!=( std::nullptr_t, const function<R(ArgTypes...)>& f );	(4)

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.

[edit] Parameters

f - std::function to compare

[edit] Return value

1-2) !f

3-4) (bool) f

[edit] 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
Standard library headers
Concepts
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)

Type support (basic types, RTTI, type traits)
Dynamic memory management
Error handling
Program utilities
Variadic functions
Date and time
Function objects
initializer_list (C++11)
bitset
hash (C++11)
Relational operators
rel_ops::operator!=rel_ops::operator>rel_ops::operator<=rel_ops::operator>=
Pairs and tuples
pair
tuple (C++11)
piecewise_construct_t (C++11)
piecewise_construct (C++11)
Swap, forward and move
swap
forward (C++11)
move (C++11)
move_if_noexcept (C++11)
declval (C++11)

Member functions
function::function
function::~function
function::operator=
function::swap
function::assign
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>