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

| | | | | ------------------------------------------------- | | ------------- | | template< class T > struct is_standard_layout; | | (since C++11) |

std::is_standard_layout is a UnaryTypeTrait.

If T is a standard-layout type, provides the member constant value equal to true. For any other type, value is false.

If std::remove_all_extents_t<T> is an incomplete type and not (possibly cv-qualified) void, the behavior is undefined.

If the program adds specializations for std::is_standard_layout or std::is_standard_layout_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 Notes
  • 3.5 Example
  • 3.6 Defect reports
  • 3.7 See also

[edit] Template parameters

[edit] Helper variable template

| template< class T > constexpr bool is_standard_layout_v = is_standard_layout<T>::value; | | (since C++17) | | ----------------------------------------------------------------------------------------------- | | ------------- |

Inherited from std::integral_constant

Member constants

| | true if T is a standard-layout 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] Notes

A pointer to a standard-layout class may be converted (with reinterpret_cast) to a pointer to its first non-static data member (see empty base optimization) and vice versa.

If a standard-layout union holds two or more standard-layout structs, it is permitted to inspect the common initial part of them.

The macro offsetof is only guaranteed to be usable with standard-layout classes.

[edit] Example

#include   struct A { int m; }; static_assert(std::is_standard_layout_v == true);   class B: public A { int m; }; static_assert(std::is_standard_layout_v == false);   struct C { virtual void foo(); }; static_assert(std::is_standard_layout_v == false);   int main() {}

[edit] Defect reports

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

[edit] See also