# std::is_function

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< 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

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

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

### See also

is_invocable is_invocable_r is_nothrow_invocable is_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)

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