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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.

Contents

[edit] Template parameters

T - a type to check

[edit] Helper variable template

template< class T >
constexpr bool is_polymorphic_v = is_polymorphic<T>::value;
 (since C++17)
[edit]

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>

[edit] 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)) {};

[edit] 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() {}

[edit] 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

[edit] See also

(C++11)
 checks if a type is a non-union class type
(class template) [edit]
(C++11)
 checks if a type is an abstract class type
(class template) [edit]
(C++11)
 checks if a type has a virtual destructor
(class template) [edit]
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