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std::is_function

Defined in header <type_traits> 
template< class T >
struct is_function;
 (since C++11)

Checks whether T is a function type. Types like std::function, lambdas, classes with overloaded operator() and pointers to functions don't count as function types. Provides the member constant value which is equal to true, if T is a function type. Otherwise, value is equal to false.

Template parameters

T - a type to check

Helper variable template 

template< class T >
inline constexpr bool is_function_v = is_function<T>::value;
 (since C++17)

Inherited from std::integral_constant

Member constants

value
[static]
 true if T is a function type , false otherwise
(public static member constant)

Member functions

operator bool
 converts the object to bool, returns value
(public member function)
operator()
(C++14)
 returns value
(public member function)

Member types

Type Definition
value_type bool
type std::integral_constant<bool, value>

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

#include <iostream>
#include <type_traits>
 
struct A {};
 
int f()
{
    return 1;
}
 
int main() 
{
    std::cout << std::boolalpha;
    std::cout << std::is_function<A>::value << '\n';
    std::cout << std::is_function<int(int)>::value << '\n';
    std::cout << std::is_function<decltype(f)>::value << '\n';
    std::cout << std::is_function<int>::value << '\n';
}

Output:

false
true
true
false

See also

(C++17)
 checks if a type can be invoked (as if by std::invoke) with the given argument types
(class template)
(C++11)
 checks if a type is object type
(class template)
(C++11)
 checks if a type is a non-union class type
(class template)

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