How to add reference of an object in Container Classes

Last Updated : 12 Jun, 2020

We all are familiar with an alias in C++. An alias means another name for some entity. So, a reference variable is an alias that is another name for an existing variable/object etc.

Below is the program for adding reference to a variable:

// C++ program to illustrate 
// aliasing in variable 
  
#include <bits/stdc++.h> 
using namespace std; 
  
void aliasing(int N) 
{ 
    // Adding reference variable to 
    // N using & 
    int& a = N; 
  
    // Print the value pointed by 
    // reference variable 
    cout << "Value of a: " << a << endl; 
  
    // Update the value of N using 
    // reference variable 
    a = 100; 
  
    cout << "After Update:" << endl; 
  
    // Print the value of a and N 
    cout << "Value of a :" << a << endl; 
    cout << "Value of N :" << N << endl; 
} 
  
// Driver Code 
int main() 
{ 
    // Given number 
    int N = 9; 
  
    // Function Call 
    aliasing(N); 
    return 0; 
} 

Output:

Value of a: 9
After Update:
Value of a :100
Value of N :100

Explanation: In the above program, a variable a is an alias of variable N that means we have given another name to variable N. So what ever we are doing with a it will effect N also and vice-versa.
Therefore, when we change the value of a to 100, then, value of N also changed to be 100.

Reference of an object in Container Classes:
The above method is correct to give an alias to any variable but in the case of containers the above method will throw a Compilation Error because containers directly can’t store the references, But there is an alternative way of doing the same. The template std::reference_wrapper in C++ STL is used to give reference to any containers in C++. The std::reference_wrapper is a class template that wraps a reference in a copyable, assignable object. It is frequently used as a mechanism to store references inside standard containers(like in vector, list, etc) which cannot normally hold references.

Below is the program for adding a reference of an object in container class:

// C++ program to illustrate aliasing 
// in list containers in C++ 
#include <bits/stdc++.h> 
using namespace std; 
class gfg { 
private: 
    int a; 
  
public: 
    gfg(int a) 
    { 
        this->a = a; 
    } 
    void setValue(int a) 
    { 
        this->a = a; 
    } 
    int getValue() 
    { 
        return this->a; 
    } 
}; 
  
// Driver Code 
int main() 
{ 
    // Declare list with reference_wrapper 
    list<reference_wrapper<gfg> > l; 
  
    // Object of class gfg 
    gfg obj(5); 
  
    l.push_back(obj); 
  
    // Print the value of a 
    cout << "Value of a for object obj is "
         << obj.getValue() << endl; 
  
    cout << "After Update" << endl; 
  
    // Change the value of a for Object obj 
    // using member function 
    obj.setValue(700); 
  
    // Print the value of a after Update 
    cout << "Value of a for object obj is "
         << obj.getValue() << endl; 
  
    cout << "\nValue stored in the list is "; 
    for (gfg i : l) 
        cout << i.getValue() << endl; 
    return 0; 
} 

Output:

Value of a for object obj is 5
After Update
Value of a for object obj is 700

Value stored in the list is 700

Explanation:
In the above program, when an object is created of class gfg, the constructor is called and the value of variable a is initialized to 5. We have stored the reference of the object in the list and then we have changed the value of the variable a to 700 by calling the member function setValue(). Now, when we see the value of the property a of the object whose reference we had stored in the list. The value stored is 700.