operator==(std::move_only_function)
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< cpp | utility | functional | move only function
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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 . Empty wrappers (that is, wrappers without a 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==.
Contents
[edit] Parameters
f
-
std::move_only_function to compare
[edit] Return value
!f.
[edit] 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