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Showing content from https://cplusplus.com/reference/type_traits/is_empty/ below:

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 constants Inherited from integral_constant:
member constant definition value either true or false
Member functions Inherited from integral_constant:

operator bool: Returns value (public member function)

Example

// 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