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

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C++
Utilities library
General utilities
Relational operators (deprecated in C++20)
Function objects
Function invocation
(C++17) (C++23)
Identity function object
(C++20)
Old binders and adaptors
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( until C++17* )
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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

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

See also

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