operator==(std::move_only_function)

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std::move_only_function
friend bool operator== ( const std::move_only_function & f, std::nullptr_t ) noexcept ;
(since C++23)

Checks whether the wrapper f has a callable target by formally comparing it with std::nullptr_t

This function is not visible to ordinary unqualified or qualified lookup, and can only be found by argument-dependent lookup when std::move_only_function<FunctionType>

The != operator is synthesized from operator==.

Parameters

f - std::move_only_function to compare

Return value

!f.

Example

Run this code
#include <functional>
#include <iostream>
#include <utility>
 
using SomeVoidFunc = std::move_only_function<void(int) const>;
 
class C {
public:
    C() = default;
    C(SomeVoidFunc func) : void_func_(std::move(func)) {}
 
    void default_func(int i) const { std::cout << i << '\n'; };
 
    void operator()() const
    {
        if (void_func_ == nullptr) // specialized compare with nullptr
            default_func(7);
        else
            void_func_(7);
    }
 
private:
    SomeVoidFunc void_func_{};
};
 
void user_func(int i)
{
    std::cout << (i + 1) << '\n';
}
 
int main()
{
    C c1;
    C c2(user_func);
    c1();
    c2();
}

Output: 

7
8

See also

 checks if the std::move_only_function has a target
(public member function)
(removed in C++20)
 compares a std::function with nullptr
(function template)
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