std::is_polymorphic

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

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

a type to check

Helper variable template

template< class T >
constexpr bool is_polymorphic_v = is_polymorphic<T>::value;

(since C++17)

Inherited from std::integral_constant

Member constants

value

[static]

true if T is a polymorphic 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>

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>);
 
// Virtual inheritance does not make a type polymorphic.
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.

DR	Applied to	Behavior as published	Correct behavior
LWG 2015	C++11	the behavior was undefined if `T` is an incomplete union type	the base characteristic is `std::false_type` in this case

is_class (C++11)	checks if a type is a non-union class type (class template)
is_abstract (C++11)	checks if a type is an abstract class type (class template)
has_virtual_destructor (C++11)	checks if a type has a virtual destructor (class template)
is_polymorphic_type (C++26)	checks if reflected type is a polymorphic class type (function)