std::is_empty (3) - Linux Manuals

std::is_empty: std::is_empty

NAME

std::is_empty - std::is_empty

Synopsis

Defined in header <type_traits>
template< class T > (since C++11)
struct is_empty;

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 true. For any other type, value is false.
If T is a non-union class type, T shall be a complete type; otherwise, the behavior is undefined.

Template parameters

T - a type to check

Helper variable template

template< class T > (since C++17)
inline constexpr bool is_empty_v = is_empty<T>::value;

Inherited from std::integral_constant

Member constants

value true if T is an empty class type , false otherwise
         (public static member constant)
[static]

Member functions

              converts the object to bool, returns value
operator bool (public member function)

operator() returns value
              (public member function)
(C++14)

Member types

Type Definition
value_type bool
type std::integral_constant<bool, value>

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.

Example

// Run this code

  #include <iostream>
  #include <type_traits>

  struct A {};

  struct B {
      int m;
  };

  struct C {
      static int m;
  };

  struct D {
      virtual ~D();
  };

  union E {};

  struct F {
      [[no_unique_address]] E e;
  };

  int main()
  {
      std::cout << std::boolalpha;
      std::cout << "A " << std::is_empty<A>::value << '\n';
      std::cout << "B " << std::is_empty<B>::value << '\n';
      std::cout << "C " << std::is_empty<C>::value << '\n';
      std::cout << "D " << std::is_empty<D>::value << '\n';
      std::cout << "E " << std::is_empty<E>::value << '\n';
      std::cout << "F " << std::is_empty<F>::value << '\n'; // the result is ABI-dependent

  }

Possible output:

  A true
  B false
  C true
  D false
  E false
  F true