std::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

Helper variable template

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

#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';
}

false true true true

See also