NAME¶

std::is_polymorphic - std::is_polymorphic

Synopsis¶

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

If T is a polymorphic class (that is, a non-union class that declares or inherits at
least one virtual function), provides the member constant value equal to true. For
any other type, value is false.

If T is a non-union class type, T shall be a complete type; otherwise, the behavior
is undefined.

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

Template parameters¶

T - a type to check

Helper variable template

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

Inherited from std::integral_constant

Member constants¶

value true if T is a polymorphic class type , false otherwise
[static] (public static member constant)

Member functions¶

operator bool converts the object to bool, returns value
(public member function)
operator() returns value
(C++14) (public member function)

Member types¶

Type Definition
value_type bool
type std::integral_constant<bool, value>

Possible implementation¶

namespace detail {

template <class T>
std::true_type detect_is_polymorphic(
decltype(dynamic_cast<const volatile void*>(static_cast<T*>(nullptr)))
);
template <class T>
std::false_type detect_is_polymorphic(...);

} // namespace detail

template <class T>
struct is_polymorphic : decltype(detail::detect_is_polymorphic<T>(nullptr)) {};

Example¶

// Run this code

#include <iostream>
#include <type_traits>

struct A {
int m;
};

struct B {
virtual void foo();
};

struct C : B {};

struct D {
virtual ~D() = default;
};

int main()
{
std::cout << std::boolalpha
<< std::is_polymorphic<A>::value << ' '
<< std::is_polymorphic<B>::value << ' '
<< std::is_polymorphic<C>::value << ' '
<< std::is_polymorphic<D>::value << '\n';
}

Output:¶

false true true true

See also¶

is_class checks if a type is a non-union class type
(C++11) (class template)
is_abstract checks if a type is an abstract class type
(C++11) (class template)
has_virtual_destructor checks if a type has a virtual destructor
(C++11) (class template)