C++

Reference

<type_traits>

class template
<type_traits>

std::is_empty

template <class T> struct is_empty;
Is empty class

Trait class that identifies whether T is an empty class.

An empty class is a class that stores no data, either cv-qualified or not.
is_empty identifies as such all classes (declared with class or struct, but no union) that fulfill all of the following:
  • has no non-static members other than bit-fields of length 0,
  • has no virtual functions and no virtual base class, and
  • has no base class for which is_empty is false_type.

is_empty inherits from integral_constant as being either true_type or false_type, depending on whether T is an empty type.

Template parameters

T
A complete type, or void (possible cv-qualified), or an array of unknown bound of a complete element type.
A type.
If a class type, it shall be a complete type or a union (for unions, this trait is always false).

Member types

Inherited from integral_constant:
member typedefinition
value_typebool
typeeither true_type or false_type

Member constants

Inherited from integral_constant:
member constantdefinition
valueeither true or false

Member functions

Inherited from integral_constant:
operator bool
Returns value (public member function)

Example

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// is_empty example
#include <iostream>
#include <type_traits>

struct A { };
struct B { void fn(){} };
struct C { int x; };

int main() {
  std::cout << std::boolalpha;
  std::cout << "is_empty:" << std::endl;
  std::cout << "int: " << std::is_empty<int>::value << std::endl;
  std::cout << "A: " << std::is_empty<A>::value << std::endl;
  std::cout << "B: " << std::is_empty<B>::value << std::endl;
  std::cout << "C: " << std::is_empty<C>::value << std::endl;
  return 0;
}

Output:
is_empty:
int: false
A: true
B: true
C: false


See also

is_class
Is non-union class (class template)
is_trivial
Is trivial type (class template)
is_abstract
Is abstract class (class template)