Difference Between Pointer And Reference In C++ (With Examples)

References and pointers are two very important concepts for those who want to write efficient, correct code. In this article, we will discuss the difference between pointer and reference in C++ in complete detail to help you understand the nuances and write better codes. But first, let's take a look at the basics of the concepts themselves.

What is Reference?

A reference is an alias for an already existing variable. After the reference variable is initialized, it is possible to use a reference name to refer to that variable. Reference in C++ is created by storing the address of another variable. A reference variable can be considered a constant pointer with automatic indirection, i.e., the compiler will apply an indirection operator (*) for you. The declaration of a reference variable contains the base type followed by the ‘&’ sign and then the variable name.

The Syntax for Reference in C++

<data-type> &<reference-variable-name> = <existing variable of same type>

In the syntax above, the data type on the right side should be the same as the assignment operator. For example-

int &a = i; //for reference we use & sign

Code Example 

#include <iostream>

using namespace std;

int main(){

int temp = 10; //initializing temp named variable

int &a = temp; //creating a reference variable to it 

cout<<"The value of temp is:"<<a;

return 0;

}

Output:

The value of temp is: 10;

Explanation:

• We started by declaring the header file, then moved to the main function.
• In the main function, we first declare an int variable temp and assign it a value of 10.
• Then we declared a reference to temp and named it 'a'.
• In the next line, when we print the value of 'a', it shows the value of the variable where it is referencing.

What is Pointer?

A pointer is a programming language object that stores the address of another variable. In other words, it stores the direct address of the memory location. A pointer should also be declared before it is used, unlike other variables. It can be dereferenced with the help of the '*' operator to access the memory location to which the pointer points.

The size of the int pointer is 4 bytes. (which can be determined using the operation sizeof())

The syntax for pointer in C++

The declaration of the pointer variable contains the base data type followed by the ‘*’ sign. For example,

int *a; //

Here we use a '*' operator for defining a pointer 

Code Example

#include <iostream>

using namespace std;

int main(){

int temp = 10; //initializing temp named variable

int *ptr = &temp; //assigning the address to pointer ptr 

cout<<"The value of temp is:"<<a;

return 0;

}

Output:

The value of temp is: 10;

Explanation:

In the C++ program above, 

• We started by declaring the header file, then move to the main function.
• In the main function, we first declare an int variable temp and assign it a value of 10.
• Then we create a pointer variable 'ptr', which stores/points to the address of the temp variable (i.e., we have used &temp, which indicates the address of the temp variable).
• In the next line, when we print the value of 'ptr', it shows the value of the variable where it is pointing.

To know about the history and timeline of C++ read- History Of C++ | Detailed Explanation (With Timeline Infographic)

Comparison Table Of C++ Pointer Vs. Reference

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Differences Between Reference And Pointer: A Detailed Explanation

Some of the key differences between Pointers and references are given below-

1. Methos of Initialization

The initialization of a pointer in C++ is as given below:

//at the time of declaration only

int a = 10;

int *ptr = &a;

//after the declaration

int *ptr;

ptr = &a;

The way to initialize the reference variable is as shown below- 

//here we have to initialize and declare at the same time only

int a = 10;

int &ref = a;

2. Functionality

A pointer holds the address of the variable to which it is pointing to, while a reference variable is an alias for a variable that already exists.

3. Operator

The pointer operator, i.e., the operator used to declare the pointer is the star operator (*), while the reference operator is an ampersand, i.e., (&).

Example for the pointer: 

int a = 10;

int *ptr = &a; // use the * operator for declaring the pointer in C++

Example for reference:

int a = 10;

int &ref = a; //uses the & operator for reference 

4. Null value

In C++, we can have a NULL pointer, which can be used in logical operations. However, we cannot have NULL references.

Code Example:

#include<stdio.h>

using namespace std;

int main(){

int *ptr = NULL;

cout<<"The value of NULL Pointer is:"<<ptr;

return 0;

}

Output:

The value of NULL pointer is: 0

Here we have created a NULL pointer in C++ 'ptr' and assigned it to NULL. When we print the value of NULL it shows '0'.

5. Memory Address

A pointer has its own memory address and size in the stack. In contrast, a reference does not have its own separate memory location. Instead, it shares with the variable memory only but takes some space on the stack.

Example:

int *ptr = a;

cout<<"Address of a pointer ptr:"<<&ptr<<endl;

cout<<"Address of variable where Pointer points:" << &a<endl;

Explanation:

The first line will print the address of the pointer, while the other line prints the address of the variable. You will observe that both addresses are different

6. Dereferencing

A pointer variable can be dereferenced with the help of the '*' operator. In contrast, the reference variable requires no operator for dereferencing. This is because it can be used like a normal variable.

7. Reassignments

Reassignment is the property of a variable to reassign its value again. A pointer can be re-assigned, i.e., a pointer can be re-assigned again to a different variable. This property of the pointer makes it useful in many data structures, such as a Linked list, trees, etc. 

Code Example:

#include<stdio.h>

using namespace std;

int main(){

int a = 10;

int *ptr = &a;

cout<<"The value of pointer is:" <<ptr;

int b = 20;

ptr = &b; //re-assign value

cout<<"the value of pointer is:"<<ptr;

return 0;

}

Output:

The value of pointer is: 10
The value of pointer is: 20

Explanation:

• Initially, we started the code by including the header file, and then we start the main function.
• We declare and initialize an int variable 'a' and assign it a value of 10.
• Next, a pointer is initialized and assigned to 'a'.
• Using the cout statement, we print the value of the pointer, which came out to be 10.
• Then again, we declare and initialize a different variable b, and assign a value of 20.
• Next, we create a pointer that points to b, and using the cout statement, we print the value of the pointer again, which comes out to be 20.

So we can see that we can re-assigned the value of the pointer. While on the other hand, references cannot be re-assigned. That is, it has to be assigned at the time of initialization only.

Example:

int a  =10;
int &ref = a;
int b = 20;
int &ref = b; // this line will throw an error stating "Multiple declarations is not allowed"

The example above throws the error as the reassignment operation is not valid in references.

8. Arithmetic operations

Various arithmetic operations can be performed on the pointer, which is also known as pointer arithmetic. For example, increment/decrement, addition, subtraction, etc. But there are no such arithmetic operations in reference. 

9. Indirection

You can have multiple levels of indirection in a pointer, such as a pointer to a pointer (known as a double pointer) and others. In contrast, references only offer one level of indirection.

Code Example for indirection in pointers:

#include<stdio.h>
using namespace std;
int main(){
int a = 10;
int *ptr = &a;
int **ptr2 = &ptr;
return 0;
}