friend bool operator==( const move_only_function& f, std::nullptr_t ) noexcept; | (since C++23) |
Compares a std::move_only_function with std::nullptr_t. Empty wrappers (that is, wrappers without a callable target) compare equal, non-empty functions compare non-equal.
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> is an associated class of the arguments.
The != operator is synthesized from operator==.
| f | - | std::move_only_function to compare |
!f.
#include <functional>
#include <utility>
#include <iostream>
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
|
(removed in C++20) | compares a std::function with nullptr (function template) |
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