std::is_function

< cpp‎ | types

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

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

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 << 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';
 
    using T = PM_traits<decltype(&A::fun)>::member_type; // T is int() const&
    std::cout << std::is_function<T>::value << '\n';
}

Output:

false
true
true
false
true

Language
Standard Library Headers
Freestanding and hosted implementations
Named requirements
Language support library
Concepts library (C++20)
Diagnostics library
Utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Filesystem library (C++17)
Technical Specifications

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