how to pass an object by reference in c++

// C++ program to show passing 
// of objects to a function 
  
#include <bits/stdc++.h> 
using namespace std; 
  
class Example { 
public: 
    int a; 
  
    // This function will take 
    // an object as an argument 
    void add(Example E) 
    { 
        a = a + E.a; 
    } 
}; 
  
// Driver Code 
int main() 
{ 
  
    // Create objects 
    Example E1, E2; 
  
    // Values are initialized for both objects 
    E1.a = 50; 
    E2.a = 100; 
  
    cout << "Initial Values \n"; 
    cout << "Value of object 1: " << E1.a 
         << "\n& object 2: " << E2.a 
         << "\n\n"; 
  
    // Passing object as an argument 
    // to function add() 
    E2.add(E1); 
  
    // Changed values after passing 
    // object as argument 
    cout << "New values \n"; 
    cout << "Value of object 1: " << E1.a 
         << "\n& object 2: " << E2.a 
         << "\n\n"; 
  
    return 0; 
}