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

From cppreference.com


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>
 
typedef std::function<void(int)> SomeVoidFunc;
 
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)