References in C++ - GeeksforGeeks

When a variable is declared as reference, it becomes an alternative name for an existing variable. A variable can be declared as reference by putting ‘&’ in the declaration.

#include<iostream> 
using namespace std; 
  
int main() 
{ 
  int x = 10; 
  
  // ref is a reference to x. 
  int& ref = x; 
  
  // Value of x is now changed to 20 
  ref = 20; 
  cout << "x = " << x << endl ; 
  
  // Value of x is now changed to 30 
  x = 30; 
  cout << "ref = " << ref << endl ; 
  
  return 0; 
} 

Output:

 x = 20
ref = 30

Applications :

Modify the passed parameters in a function : If a function receives a reference to a variable, it can modify the value of the variable. For example, in the following program variables are swapped using references.

#include<iostream> 
using namespace std; 
  
void swap (int& first, int& second) 
{ 
    int temp = first; 
    first = second; 
    second = temp; 
} 
  
int main() 
{ 
    int a = 2, b = 3; 
    swap( a, b ); 
    cout << a << " " << b; 
    return 0; 
} 

Output:

3 2

Avoiding copy of large structures : Imagine a function that has to receive a large object. If we pass it without reference, a new copy of it is created which causes wastage of CPU time and memory. We can use references to avoid this.

struct Student { 
   string name; 
   string address; 
   int rollNo; 
} 
  
// If we remove & in below function, a new 
// copy of the student object is created.  
// We use const to avoid accidental updates 
// in the function as the purpose of the function 
// is to print s only. 
void print(const Student &s) 
{ 
    cout << s.name << "  " << s.address << "  " << s.rollNo; 
} 

In For Each Loops to modify all objects : We can use references in for each loops to modify all elements

#include <bits/stdc++.h>  
using namespace std;  
  
int main()  
{  
    vector<int> vect{ 10, 20, 30, 40 };  
  
    // We can modify elements if we  
    // use reference 
    for (int &x : vect)  
        x = x + 5; 
  
    // Printing elements 
    for (int x : vect)  
       cout << x << " ";  
  
    return 0;  
}  

In For Each Loops to avoid copy of objects : We can use references in for each loops to avoid copy of individual objects when objects are large.

#include <bits/stdc++.h>  
using namespace std;  
  
int main()  
{  
    vector<string> vect{"geeksforgeeks practice",  
                     "geeksforgeeks write", 
                     "geeksforgeeks ide"};  
  
    // We avoid copy of the whole string 
    // object by using reference. 
    for (const auto &x : vect)  
       cout << x << endl;  
  
    return 0;  
} 

References vs Pointers
Both references and pointers can be used to change local variables of one function inside another function. Both of them can also be used to save copying of big objects when passed as arguments to functions or returned from functions, to get efficiency gain.
Despite above similarities, there are following differences between references and pointers.

A pointer can be declared as void but a reference can never be void
For example

int a = 10;
void* aa = &a;. //it is valid
void &ar = a; // it is not valid

Thanks to Shweta Bansal for adding this point.

References are less powerful than pointers

1) Once a reference is created, it cannot be later made to reference another object; it cannot be reseated. This is often done with pointers.
2) References cannot be NULL. Pointers are often made NULL to indicate that they are not pointing to any valid thing.
3) A reference must be initialized when declared. There is no such restriction with pointers

Due to the above limitations, references in C++ cannot be used for implementing data structures like Linked List, Tree, etc. In Java, references don’t have above restrictions, and can be used to implement all data structures. References being more powerful in Java, is the main reason Java doesn’t need pointers.

References are safer and easier to use:
1) Safer: Since references must be initialized, wild references like wild pointers are unlikely to exist. It is still possible to have references that don’t refer to a valid location (See questions 5 and 6 in the below exercise )
2) Easier to use: References don’t need dereferencing operator to access the value. They can be used like normal variables. ‘&’ operator is needed only at the time of declaration. Also, members of an object reference can be accessed with dot operator (‘.’), unlike pointers where arrow operator (->) is needed to access members.

Together with the above reasons, there are few places like copy constructor argument where pointer cannot be used. Reference must be used pass the argument in copy constructor. Similarly references must be used for overloading some operators like ++.

Exercise:
Predict the output of following programs. If there are compilation errors, then fix them.

Question 1

#include<iostream> 
using namespace std; 
  
int &fun() 
{ 
    static int x = 10; 
    return x; 
} 
int main() 
{ 
    fun() = 30; 
    cout << fun(); 
    return 0; 
} 

Question 2

#include<iostream> 
using namespace std; 
  
int fun(int &x) 
{ 
    return x; 
} 
int main() 
{ 
    cout << fun(10); 
    return 0; 
} 

Question 3

#include<iostream> 
using namespace std; 
  
void swap(char * &str1, char * &str2) 
{ 
  char *temp = str1; 
  str1 = str2; 
  str2 = temp; 
} 
  
int main() 
{ 
  char *str1 = "GEEKS"; 
  char *str2 = "FOR GEEKS"; 
  swap(str1, str2); 
  cout<<"str1 is "<<str1<<endl; 
  cout<<"str2 is "<<str2<<endl; 
  return 0; 
} 

Question 4

#include<iostream> 
using namespace std; 
  
int main() 
{ 
   int x = 10; 
   int *ptr = &x; 
   int &*ptr1 = ptr; 
} 

Question 5

#include<iostream> 
using namespace std; 
  
int main() 
{ 
   int *ptr = NULL; 
   int &ref = *ptr; 
   cout << ref; 
} 

Question 6

#include<iostream> 
using namespace std; 
  
int &fun() 
{ 
    int x = 10; 
    return x; 
} 
int main() 
{ 
    fun() = 30; 
    cout << fun(); 
    return 0; 
} 

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