std:: is_polymorphic

std::is_polymorphic is a UnaryTypeTrait .

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 an incomplete non-union class type, the behavior is undefined.

If the program adds specializations for std::is_polymorphic or std::is_polymorphic_v , the behavior 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 <type_traits>
 
struct A { int m; };
static_assert(!std::is_polymorphic_v<A>);
 
struct B { virtual void foo(); };
static_assert(std::is_polymorphic_v<B>);
 
struct C : B {};
static_assert(std::is_polymorphic_v<C>);
 
struct D { virtual ~D() = default; };
static_assert(std::is_polymorphic_v<D>);
 
// Uses inheritance, but not the virtual keyword:
struct E : A {};
static_assert(!std::is_polymorphic_v<E>);
 
struct F : virtual A {};
static_assert(!std::is_polymorphic_v<F>);
 
struct AX : A {};
struct AY : A {};
struct XY : virtual AX, virtual AY {};
static_assert(!std::is_polymorphic_v<XY>);
 
int main() {}

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also