std::is_polymorphic

 

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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

T

-

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

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 T is an incomplete union type	the base characteristic is std::false_type in this case

See also

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)