20 General utilities library [utilities]

20.14 Function objects [function.objects]

20.14.17 Polymorphic function wrappers [func.wrap]

20.14.17.3 Class template function [func.wrap.func]

20.14.17.3.1 General [func.wrap.func.general]

namespace std { template<class> class function; // not defined template<class R, class... ArgTypes> class function<R(ArgTypes...)> { public: using result_type = R; // [func.wrap.func.con], construct/copy/destroy function() noexcept; function(nullptr_t) noexcept; function(const function&); function(function&&) noexcept; template<class F> function(F); function& operator=(const function&); function& operator=(function&&); function& operator=(nullptr_t) noexcept; template<class F> function& operator=(F&&); template<class F> function& operator=(reference_wrapper<F>) noexcept; ~function(); // [func.wrap.func.mod], function modifiers void swap(function&) noexcept; // [func.wrap.func.cap], function capacity explicit operator bool() const noexcept; // [func.wrap.func.inv], function invocation R operator()(ArgTypes...) const; // [func.wrap.func.targ], function target access const type_info& target_type() const noexcept; template<class T> T* target() noexcept; template<class T> const T* target() const noexcept; }; template<class R, class... ArgTypes> function(R(*)(ArgTypes...)) -> function<R(ArgTypes...)>; template<class F> function(F) -> function<see below>; // [func.wrap.func.nullptr], null pointer comparison operator functions template<class R, class... ArgTypes> bool operator==(const function<R(ArgTypes...)>&, nullptr_t) noexcept; // [func.wrap.func.alg], specialized algorithms template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&) noexcept; }
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The function class template provides polymorphic wrappers that generalize the notion of a function pointer.
Wrappers can store, copy, and call arbitrary callable objects, given a call signature, allowing functions to be first-class objects.
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A callable type F is Lvalue-Callable for argument types ArgTypes and return type R if the expression INVOKE<R>(declval<F&>(), declval<ArgTypes>()...), considered as an unevaluated operand, is well-formed ([func.require]).
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The function class template is a call wrapper whose call signature is R(ArgTypes...).
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[Note 1:
The types deduced by the deduction guides for function might change in future revisions of C++.
— end note]