W3cubDocs

/C++

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

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:

7
8

See also

(C++23)
 compares a std::move_only_function with nullptr
(function)

© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
https://en.cppreference.com/w/cpp/utility/functional/function/operator_cmp