std::is_function
template< class T >
struct is_function;
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(since C++11) |
std::is_function is a 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 is undefined.
Template parameters
Helper variable template
template< class T >
inline constexpr bool is_function_v = is_function<T>::value;
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(since C++17) |
Member constants
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value
[static]
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true if T is a function type, false otherwise
(public static member constant) |
Member functions
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operator bool
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converts the object to bool, returns value
(public member function) |
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operator()
(C++14)
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returns value
(public member function) |
Member types
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 {};
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Example
#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
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(C++17)
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checks if a type can be invoked (as if by std::invoke) with the given argument types
(class template) |
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(C++11)
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checks if a type is an object type
(class template) |
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(C++11)
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checks if a type is a non-union class type
(class template) |