C++ Dereferencing

Prerequisites:  

• Pointers in C++
• References in C++

Pointers are symbolic representations of addresses. They enable programs to simulate call-by-reference as well as to create and manipulate dynamic data structures. Iterating over elements in arrays or other data structures is one of the main use of pointers. Pointers and references work simultaneously together, where the pointer uses references to store the address of another variable.

Syntax:

datatype *var_name;

Pointer Working shown in the Image

Dereference Operator

Dereferencing is the method where we are using a pointer to access the element whose address is being stored. We use the * operator to get the value of the variable from its address.

Example:

int a;

// Referencing 
int *ptr=&a; 

// Dereferencing
int b=*ptr;

Below is the implementation of the above example:

C++

#include <bits/stdc++.h>
using namespace std;

int main()
{
    int a = 10, b = 20;
    int* pt;
    pt = &a;

    cout << "The address where a is stored is: " << pt
         << endl;
    cout << "The value stored at the address by "
            "dereferencing the pointer is: "
         << *pt << endl;
}

Output

The address where a is stored is: 0x7ffdcae26a0c
The value stored at the address by dereferencing the pointer is: 10

Example :  We Can Directly assign value to a pointer using Dereference : 

C++

#include <iostream>
using namespace std;

int main() {

    int *p;
    *p = 80;  //Assiging value to pointer p by using asterisk
    cout << *p<<endl;
    return 0;
}

Output

80

In Above code we directly assign value to pointer p using asterisk (*)  and using dereference we print its pointed value that is 80.

Changing the value of the Pointer

There are 2 methods to change the value of pointers 

1. Change the address of the stored variable
2. Change the value in the stored variable

When do you want to assign another address to the pointer?

One of the functionalities of the pointer is that it can store some other variable address. If we want to change the address to another variable in that case we can just reference the variable to another variable.

Example:

C++

// C++ Program to changing the
// address pointed by pointer
#include <iostream>
using namespace std;

// Driver code
int main()
{
    int a = 10, b = 20;
    int* pt;

    // Referencing to the pointer
    pt = &a;

    cout << "The address where a is stored is: " << pt
         << endl;

    // dereference the pointer
    cout << "The value stored at the address by "
            "dereferencing the pointer is: "
         << *pt << endl;

    // Referening the address to same pointer
    pt = &b;

    // dereference the pointer
    cout << "Pointer is now pointing at: " << pt << endl;
    cout << "New value the pointer is pointing to is: "
         << *pt << endl;

    return 0;
}

Output

The address where a is stored is: 0x7ffe14972128
The value stored at the address by dereferencing the pointer is: 10
Pointer is now pointing at: 0x7ffe1497212c
New value the pointer is pointing to is: 20

When you want to change the value of the variable by dereference operator?

In a few conditions, we can change the value of the actual variable when we need it rather than referencing another variable. In this, case we can just the dereferenced value to make actual changes.

Example:

int a=5;

// Referencing
int *ptr=&a;

// Dereferencing and changing value
*ptr=6;

Below is the implementation of the above example:

C++

// C++ Program to change the value
// of variable using dereferncing of
// pointers
#include <iostream>
using namespace std;

int main()
{
    int a = 5, b = 6;

    // Pointer declared
    int* pt;

    // Referencing
    pt = &a;

    cout << "The address where a is stored is: " << pt
         << endl;
    cout << "The value stored at the address by "
            "dereferencing the pointer is: "
         << *pt << endl;

    // Changing value of variable
    // Address stored in the pointer will not
    // be effected by this
    *pt = b;
    cout << "Pointer is still pointing at: " << pt << endl;
    cout << "The new value stored at the address by "
            "dereferencing the pointer is: "
         << *pt << endl;
    cout << "Now the value of a is: " << a << endl;

    return 0;
}

Output

The address where a is stored is: 0x7ffc6d36859c
The value stored at the address by dereferencing the pointer is: 5
Pointer is still pointing at: 0x7ffc6d36859c
The new value stored at the address by dereferencing the pointer is: 6
Now the value of a is: 6

Using Array with Pointers

An array is the collection of elements with similar data types, also the memory blocks where elements are continuous. We can use ptr to point to the elements of the array. Let’s check how:

Example:

int arr=[1,2,3,4,5];

// Referencing with array
int *ptr=arr;

// Dereferencing the array 
// 1 will be printed
cout<< *ptr <<" ";

Below is the implementation of the above example:

C++

// C++ Program to implement
// Array using Pointer
#include <iostream>
using namespace std;

int main()
{
    // Array Declared
    int arr[] = { 1, 2, 3, 4, 5 };

    // Referencing array with pointer
    int* ptr = arr;

    // Using dereferencing to print
    // elements of array
    cout << "Elements of array:";
    for (int i = 0; i < 5; i++) {
        cout << *(ptr + i) << " ";
    }
    cout << endl;

    return 0;
}

Output

Elements of array:1 2 3 4 5