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

Since C++11

Header: <functional>

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.

# Declarations

template< class R, class... ArgTypes >
bool operator==( const std::function<R(ArgTypes...)>& f,
std::nullptr_t ) noexcept;

(since C++11)

template< class R, class... ArgTypes >
bool operator==( std::nullptr_t,
const std::function<R(ArgTypes...)>& f ) noexcept;

(since C++11) (until C++20)

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

(since C++11) (until C++20)

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

(since C++11) (until C++20)

# Parameters

f: std::function to compare

# 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);
}

# See also

operator==