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Showing content from https://en.cppreference.com/w/cpp/language/../error/error_code/../../types/is_polymorphic.html below:

std::is_polymorphic - cppreference.com

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.

[edit] Template parameters [edit] 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 true if T is a polymorphic class type, false otherwise
(public static member constant) Member functions converts the object to bool, returns value
(public member function) returns value
(public member function) Member types [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

#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

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