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C++ references allow you to give a variable a second name that you can use to read or modify the original data stored in that variable. Once a reference has been initialized with a variable, it can be referred to using either the variable name or the reference name. For instance,

int x=19;
int &a=x; //here a is a reference to int x.

A variable can be declared as a reference by including ‘&’ in the declaration, and there is no need to use the ‘*’ to dereference a reference variable.

Pointers vs. References:

References and pointers are frequently confused, but there are three major differences between the two.

• NULL references are not permitted. You must always be able to assume that a reference refers to a valid piece of storage.
• Once a reference is set to an object, it cannot be changed to another object. At any time, pointers can be directed to another object.
• When a reference is created, it must be initialized. At any time, pointers can be initialized.

Functions with references:

When a function receives a variable reference, it can change the variable’s value.

Note: When we return a reference from a function, whatever the reference is connected to should not be out of scope when the function ends. Make that global or static.

Example:

#include <iostream>
using namespace std;

void swap(int& x, int& y)
{
 int temp = x;
 x = y;
 y = temp;
}

int main()
{
 int a = 2, b = 3;
 cout <<"Before swapping: "<< a << " " << b;
 swap(a, b);
 cout<<"After swapping: " << a << " " << b;
 return 0;
}

Output:

Before swapping: 2 3
After swapping: 3 2

Const Reference:

Consider a function that must accept a large object. If we pass it without a reference, a new copy is created, wasting CPU time and memory. To avoid this, we can use const references. For instance,

void func(const int& x)
{ 
    x++; 
}   // ERROR

int main()
{
    int i=10;
    func(i);
}

Output:

/tmp/RXDruFSvQc.cpp: In function 'void func(const int&)':
/tmp/RXDruFSvQc.cpp:3:5: error: increment of read-only reference 'x'
    3 |     x++;
      |     ^

Because the argument is passed as a const reference, we cannot change it in the function.

Note:

• References are less powerful than pointers
• References are safer and easier to use

Note: also read about Mutable keyword in C++

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