is_empty template in C++

In this article we will be discussing the working, syntax and examples of std::is_empty template in C++ STL.

is_empty is a template which comes under the <type_traits> header file. This template is used to check whether the given class T is an empty class or not.

What is an empty class?

A class is known as empty, when there is no data stored in a class. Empty class satisfies the following −

• Must have no non-static members other than bit fields of length 0.
• Must have no virtual base class or virtual functions.
• Must have no base class.

Syntax

template <class T>is_empty;

Parameters

The template can have only parameter of class T, and check whether class T is an empty class or not.

Return value

It returns a Boolean value, true if the given type is an empty class, and false if the given type is not an empty class.

Example

Input: class A{};
   is_empty<A>::value;
Output: true

Input: class B{ void fun() {} };
   is_empty<B>::value;
Output: true

Example

 Live Demo

#include <iostream>
#include <type_traits>
using namespace std;
class TP_1 {
};
class TP_2 {
   int var;
};
class TP_3 {
   static int var;
};
class TP_4 {
   ~TP_4();
};
int main() {
   cout << boolalpha;
   cout << "checking for is_empty template for a class with no variable: "<< is_empty<TP_1>::value;
   cout <<"\nchecking for is_empty template for a class with one variable: "<< is_empty<TP_2>::value;
   cout <<"\nchecking for is_empty template for a class with one static variable: "<< is_empty<TP_3>::value;
   cout <<"\nchecking for is_empty template for a class with constructor: "<< is_empty<TP_4>::value;
   return 0;
}

Output

If we run the above code it will generate the following output −

checking for is_empty template for a class with no variable: true
checking for is_empty template for a class with one variable: false
checking for is_empty template for a class with one static variable: true
checking for is_empty template for a class with constructor: true

Example

 Live Demo

#include <iostream>
#include <type_traits>
using namespace std;
struct TP_1 {
};
struct TP_2 {
   int var;
};
struct TP_3 {
   static int var;
};
struct TP_4 {
   ~TP_4();
};
int main() {
   cout << boolalpha;
   cout << "checking for is_empty template for a structure with no variable: "<< is_empty<TP_1>::value;
   cout <<"\nchecking for is_empty template for a structure with one variable: "<< is_empty<TP_2>::value;
   cout <<"\nchecking for is_empty template for a structure with one static variable: "<< is_empty<TP_3>::value;
   cout <<"\nchecking for is_empty template for a structure with constructor: "<< is_empty<TP_4>::value;
   return 0;
}

Output

If we run the above code it will generate the following output −

checking for is_empty template for a structure with no variable: true
checking for is_empty template for a structure with one variable: false
checking for is_empty template for a structure with one static variable: true
checking for is_empty template for a structure with constructor: true