22 General utilities library [utilities]

22.10 Function objects [function.objects]

22.10.17 Polymorphic function wrappers [func.wrap] Copyable wrapper [func.wrap.copy] Class template copyable_function [func.wrap.copy.class]

namespace std { template<class R, class... ArgTypes> class copyable_function<R(ArgTypes...) cv ref noexcept(noex)> { public: using result_type = R; // [func.wrap.copy.ctor], constructors, assignments, and destructors copyable_function() noexcept; copyable_function(nullptr_t) noexcept; copyable_function(const copyable_function&); copyable_function(copyable_function&&) noexcept; template<class F> copyable_function(F&&); template<class T, class... Args> explicit copyable_function(in_place_type_t<T>, Args&&...); template<class T, class U, class... Args> explicit copyable_function(in_place_type_t<T>, initializer_list<U>, Args&&...); copyable_function& operator=(const copyable_function&); copyable_function& operator=(copyable_function&&); copyable_function& operator=(nullptr_t) noexcept; template<class F> copyable_function& operator=(F&&); ~copyable_function(); // [func.wrap.copy.inv], invocation explicit operator bool() const noexcept; R operator()(ArgTypes...) cv ref noexcept(noex); // [func.wrap.copy.util], utility void swap(copyable_function&) noexcept; friend void swap(copyable_function&, copyable_function&) noexcept; friend bool operator==(const copyable_function&, nullptr_t) noexcept; private: template<class VT> static constexpr bool is-callable-from = see below; // exposition only }; }
The copyable_function class template provides polymorphic wrappers that generalize the notion of a callable object ([func.def]).
These wrappers can store, copy, move, and call arbitrary callable objects, given a call signature.
Recommended practice: Implementations should avoid the use of dynamically allocated memory for a small contained value.
[Note 1: 
Such small-object optimization can only be applied to a type T for which is_nothrow_move_constructible_v<T> is true.
— end note]