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std::is_class

From cppreference.com
< cpp‎ | types
 
 
Metaprogramming library
Type traits
Type categories
(C++11)
(C++14)  
(C++11)
(C++11)
(C++11)
(C++11)
is_class
(C++11)
(C++11)
(C++11)
Type properties
(C++11)
(C++11)
(C++14)
(C++11)
(C++11)(until C++20)
(C++11)(deprecated in C++20)
(C++11)
Type trait constants
Metafunctions
(C++17)
Supported operations
Relationships and property queries
Type modifications
(C++11)(C++11)(C++11)
Type transformations
(C++11)(deprecated in C++23)
(C++11)(deprecated in C++23)
(C++11)
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(C++11)(until C++20)(C++17)
Compile-time rational arithmetic
Compile-time integer sequences
 
Defined in header <type_traits>
template< class T >
struct is_class;
(since C++11)

Checks whether T is a non-union class type. Provides the member constant value which is equal to true, if T is a class type (but not union). Otherwise, value is equal to false.

The behavior of a program that adds specializations for is_class or is_class_v (since C++17) is undefined.

Contents

[edit] Template parameters

T - a type to check

[edit] Helper variable template

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

Inherited from std::integral_constant

Member constants

value
[static]
true if T is a non-union class 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>

[edit] Possible implementation

namespace detail {
template <class T>
std::integral_constant<bool, !std::is_union<T>::value> test(int T::*);
 
template <class>
std::false_type test(...);
}
 
template <class T>
struct is_class : decltype(detail::test<T>(nullptr))
{};

[edit] Example

#include <iostream>
#include <type_traits>
 
struct A {};
 
class B {};
 
enum class E {};
 
union U { class UC {}; };
static_assert(not std::is_class_v<U>);
static_assert(std::is_class_v<U::UC>);
 
int main() 
{
    std::cout << std::boolalpha;
    std::cout << std::is_class<A>::value << ": A\n";
    std::cout << std::is_class_v<B> << ": B \n";
    std::cout << std::is_class_v<B*> << ": B* \n";
    std::cout << std::is_class_v<B&> << ": B& \n";
    std::cout << std::is_class_v<const B> << ": const B \n";
    std::cout << std::is_class<E>::value << ": E\n";
    std::cout << std::is_class_v<int> << ": int\n";
    std::cout << std::is_class_v<struct S> << ": struct S (incomplete)\n";
    std::cout << std::is_class_v<class C> << ": class C (incomplete)\n";
}

Output:

true: A
true: B 
false: B* 
false: B& 
true: const B 
false: E
false: int
true: struct S (incomplete)
true: class C (incomplete)

[edit] See also

(C++11)
checks if a type is an union type
(class template) [edit]