is_empty template in C++

The std::is_empty template of C++ STL is used to check whether the given type is empty or not. This method returns a boolean value showing whether the given type is empty or not.

Syntax:

template < class T > struct is_empty;

Parameters: This template contains single parameter T (Trait class) that identifies whether T is a empty type.

Return Value: This template returns a boolean value as shown below:

  • True: if the type is a empty.
  • False: if the type is not empty.

Below programs illustrate the std::is_empty template in C++ STL:

Program 1: Using Structs

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// C++ program to illustrate
// std::is_empty template
  
#include <iostream>
#include <type_traits>
using namespace std;
  
// empty struct
struct GFG1 {
};
  
// struct with local variable
struct GFG2 {
    int variab;
};
  
// Struct with global variable
struct GFG3 {
    static int variab;
};
  
// Struct with virtual destructor
struct GFG4 {
    virtual ~GFG4();
};
  
// Driver code
int main()
{
    cout << boolalpha;
  
    cout << "Is GFG1 empty: "
         << is_empty<GFG1>::value
         << '\n';
    cout << "Is GFG2 empty: "
         << is_empty<GFG2>::value
         << '\n';
    cout << "Is GFG3 empty: "
         << is_empty<GFG3>::value
         << '\n';
    cout << "Is GFG4 empty: "
         << is_empty<GFG4>::value
         << '\n';
  
    return 0;
}

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Output:

Is GFG1 empty: true
Is GFG2 empty: false
Is GFG3 empty: true
Is GFG4 empty: false

Program 2: Using classes

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// C++ program to illustrate
// std::is_empty template
  
#include <iostream>
#include <type_traits>
using namespace std;
  
// empty class
class GFG1 {
};
  
// class with local variable
class GFG2 {
    int variab;
};
  
// class with global variable
class GFG3 {
    static int variab;
};
  
// class with virtual destructor
class GFG4 {
    virtual ~GFG4();
};
  
int main()
{
    cout << boolalpha;
    cout << "Is GFG1 empty: "
         << is_empty<GFG1>::value
         << '\n';
    cout << "Is GFG2 empty: "
         << is_empty<GFG2>::value
         << '\n';
    cout << "Is GFG3 empty: "
         << is_empty<GFG3>::value
         << '\n';
    cout << "Is GFG4 empty: "
         << is_empty<GFG4>::value
         << '\n';
  
    return 0;
}

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Output:

Is GFG1 empty: true
Is GFG2 empty: false
Is GFG3 empty: true
Is GFG4 empty: false


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