NAME¶

std::is_polymorphic - std::is_polymorphic

Synopsis¶

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

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 <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.

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

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)