std::is_polymorphic - cppreference.com (original) (raw)

| | | | | -------------------------------------------- | | ------------- | | 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

• 1 Template parameters
• 2 Helper variable template
• 3 Inherited from std::integral_constant
  • 3.1 Member constants
  • 3.2 Member functions
  • 3.3 Member types
  • 3.4 Possible implementation
  • 3.5 Example
  • 3.6 Defect reports
  • 3.7 See also

[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 std::true_type detect_is_polymorphic( decltype(dynamic_cast<const volatile void*>(static_cast<T*>(nullptr))) ); template std::false_type detect_is_polymorphic(...); } // namespace detail   template struct is_polymorphic : decltype(detail::detect_is_polymorphic(nullptr)) {};

[edit] Example

#include   struct A { int m; }; static_assert(!std::is_polymorphic_v);   struct B { virtual void foo(); }; static_assert(std::is_polymorphic_v);   struct C : B {}; static_assert(std::is_polymorphic_v);   struct D { virtual ~D() = default; }; static_assert(std::is_polymorphic_v);   // Uses inheritance, but not the virtual keyword: struct E : A {}; static_assert(!std::is_polymorphic_v);   struct F : virtual A {}; static_assert(!std::is_polymorphic_v);   struct AX : A {}; struct AY : A {}; struct XY : virtual AX, virtual AY {}; static_assert(!std::is_polymorphic_v);   int main() {}

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit] See also