# std::is_function

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

`std::is_function` is an UnaryTypeTrait.

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.

The behavior of a program that adds specializations for `std::is_function` or `std::is_function_v` (since C++17) is undefined.

### 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>

### Notes

`std::is_function` can be implemented in much simpler ways. Implementations 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 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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