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std::is_function(3) C++ Standard Libary std::is_function(3) NAME std::is_function - std::is_function Synopsis Defined in header <type_traits> template< class T > (since C++11) struct is_function; Checks whether T is a function type. Types like std::function, lamb- das, classes with overloaded operator() and pointers to functions don't count as func- tion types. Provides the member constant value which is equal to true, if T is a function type. Otherwise, value is equal to false. The behavior of a program that adds specializations for is_function or is_function_v (since C++17) is undefined. Template parameters T - a type to check Helper variable template template< class T > (since C++17) inline constexpr bool is_function_v = is_function<T>::value; Inherited from std::integral_constant Member constants value true if T is a function type , false otherwise [static] (public static member constant) Member functions operator bool converts the object to bool, returns value (public member function) operator() returns value (C++14) (public member function) Member types Type Definition value_type bool type std::integral_constant<bool, value> Notes std::is_function can be implemented in much simpler ways. Implemen- tations similar to the following one are used by new versions of libc++, libstdc++ and MS STL: template<class T> struct is_function : std::integral_constant< bool, !std::is_const<const T>::value && !std::is_reference<T>::value > {}; The implementation shown below is for pedagogical purposes, since it exhibits the myriad kinds of function types. Possible implementation // primary template template<class> struct is_function : std::false_type { }; // specialization for regular functions template<class Ret, class... Args> struct is_function<Ret(Args...)> : std::true_type {}; // specialization for variadic functions such as std::printf template<class Ret, class... Args> struct is_function<Ret(Args......)> : std::true_type {}; // specialization for function types that have cv-qualifiers template<class Ret, class... Args> struct is_function<Ret(Args...) const> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) volatile> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) const volatile> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) const> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) volatile> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) const volatile> : std::true_type {}; // specialization for function types that have ref-qualifiers template<class Ret, class... Args> struct is_function<Ret(Args...) &> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) const &> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) volatile &> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) const volatile &> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) &> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) const &> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) volatile &> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) const volatile &> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) &&> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) const &&> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) volatile &&> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) const volatile &&> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) &&> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) const &&> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) volatile &&> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) const volatile &&> : std::true_type {}; // specializations for noexcept versions of all the above (C++17 and later) template<class Ret, class... Args> struct is_function<Ret(Args...) noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) const noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) volatile noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) const volatile noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) const noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) volatile noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) const volatile noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) & noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) const & noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) volatile & noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) const volatile & noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) & noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) const & noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) volatile & noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) const volatile & noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) && noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) const && noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) volatile && noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args...) const volatile && noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) && noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) const && noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) volatile && noexcept> : std::true_type {}; template<class Ret, class... Args> struct is_function<Ret(Args......) const volatile && noexcept> : std::true_type {}; Example // Run this code #include <iostream> #include <type_traits> struct A { int fun() const&; }; template<typename> struct PM_traits {}; template<class T, class U> struct PM_traits<U T::*> { using member_type = U; }; int f(); int main() { std::cout << std::boolalpha; std::cout << "#1 " << std::is_function_v<A> << '\n'; std::cout << "#2 " << std::is_function_v<int(int)> << '\n'; std::cout << "#3 " << std::is_function_v<decltype(f)> << '\n'; std::cout << "#4 " << std::is_function_v<int> << '\n'; using T = PM_traits<decltype(&A::fun)>::member_type; // T is int() const& std::cout << "#5 " << std::is_function_v<T> << '\n'; } Output: #1 false #2 true #3 true #4 false #5 true See also is_invocable is_invocable_r checks if a type can be invoked (as if by std::invoke) with is_nothrow_invocable the given argument types is_nothrow_invocable_r (class template) (C++17) is_object checks if a type is an object type (C++11) (class template) is_class checks if a type is a non-union class type (C++11) (class template) http://cppreference.com 2022.07.31 std::is_function(3)
NAME | Synopsis | Template parameters | Member constants | Member functions | Member types | Notes | Possible implementation | Example | Output: | See also
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