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

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

std::is_empty is a UnaryTypeTrait.

If T is an empty type (that is, a non-union class type with no non-static data members other than bit-fields of size 0, no virtual functions, no virtual base classes, and no non-empty base classes), 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_empty or std::is_empty_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_empty_v = is_empty<T>::value; | | (since C++17) | | ------------------------------------------------------------------------- | | ------------- |

Inherited from std::integral_constant

Member constants

| | true if T is an empty 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] Notes

Inheriting from empty base classes usually does not increase the size of a class due to empty base optimization.

std::is_empty<T> and all other type traits are empty classes.

[edit] Example

#include #include   struct A {}; static_assert(std::is_empty_v == true);   struct B { int m; }; static_assert(std::is_empty_v == false);   struct C { static int m; }; static_assert(std::is_empty_v == true);   struct D { virtual ~D(); }; static_assert(std::is_empty_v == false);   union E {}; static_assert(std::is_empty_v == false);   struct F { [[no_unique_address]] E e; };   struct G { int:0; // C++ standard allow "as a special case, an unnamed bit-field with a width of zero // specifies alignment of the next bit-field at an allocation unit boundary. // Only when declaring an unnamed bit-field may the width be zero." }; static_assert(std::is_empty_v); // holds only unnamed bit-fields of zero width   int main() { std::cout << std::boolalpha; std::cout << "F: " << std::is_empty_v << '\n'; // the result is ABI-dependent }

Possible output:

[edit] Defect reports

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

[edit] See also

| | checks if a type is a non-union class type (class template) [edit] | | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |