C++ Pointers

C++ Pointers

In this tutorial, we will learn about pointers in C++ and their working with the help of examples.

In C++, pointers are variables that store the memory addresses of other variables.

Address in C++

If we have a variable var in our program, &var will give us its address in the memory. For example,

Example 1: Printing Variable Addresses in C++

#include <iostream>
using namespace std;

int main()
{
    // declare variables
    int var1 = 3;
    int var2 = 24;
    int var3 = 17;

    // print address of var1
    cout << "Address of var1: "<< &var1 << endl;

    // print address of var2
    cout << "Address of var2: " << &var2 << endl;

    // print address of var3
    cout << "Address of var3: " << &var3 << endl;
}

Output

Address of var1: 0x7fff5fbff8ac
Address of var2: 0x7fff5fbff8a8
Address of var3: 0x7fff5fbff8a4

Here, 0x at the beginning represents the address is in the hexadecimal form.

Notice that the first address differs from the second by 4 bytes and the second address differs from the third by 4 bytes.

This is because the size of an int variable is 4 bytes in a 64-bit system.

Note: You may not get the same results when you run the program.


C++ Pointers

As mentioned above, pointers are used to store addresses rather than values.

Here is how we can declare pointers.

int *pointVar;

Here, we have declared a pointer pointVar of the int type.

We can also declare pointers in the following way.

int* pointVar; // preferred syntax

Let's take another example of declaring pointers.

int* pointVar, p;

Here, we have declared a pointer pointVar and a normal variable p.

Note: The * operator is used after the data type to declare pointers.


Assigning Addresses to Pointers

Here is how we can assign addresses to pointers:

int* pointVar, var;
var = 5;

// assign address of var to pointVar pointer
pointVar = &var;

Here, 5 is assigned to the variable var. And, the address of var is assigned to the pointVar pointer with the code pointVar = &var.


Get the Value from the Address Using Pointers

To get the value pointed by a pointer, we use the * operator. For example:

int* pointVar, var;
var = 5;

// assign address of var to pointVar
pointVar = &var;

// access value pointed by pointVar
cout << *pointVar << endl;   // Output: 5

In the above code, the address of var is assigned to the pointVar pointer. We have used the *pointVar to get the value stored in that address.

When * is used with pointers, it's called the dereference operator. It operates on a pointer and gives the value pointed by the address stored in the pointer. That is, *pointVar = var.

Note: In C++, pointVar and *pointVar is completely different. We cannot do something like *pointVar = &var;


Example 2: Working of C++ Pointers

#include <iostream>
using namespace std;
int main() {
    int var = 5;

    // declare pointer variable
    int* pointVar;

    // store address of var
    pointVar = &var;

    // print value of var
    cout << "var = " << var << endl;

    // print address of var
    cout << "Address of var (&var) = " << &var << endl
         << endl;

    // print pointer pointVar
    cout << "pointVar = " << pointVar << endl;

    // print the content of the address pointVar points to
    cout << "Content of the address pointed to by pointVar (*pointVar) = " << *pointVar << endl;
    
    return 0;
}

Output

var = 5
Address of var (&var) = 0x61ff08

pointVar = 0x61ff08
Content of the address pointed to by pointVar (*pointVar) = 5
Working of C++ Pointers
Working of C++ pointers

Changing Value Pointed by Pointers

If pointVar points to the address of var, we can change the value of var by using *pointVar.

For example,

int var = 5;
int* pointVar;

// assign address of var
pointVar = &var;

// change value at address pointVar
*pointVar = 1;

cout << var << endl; // Output: 1

Here, pointVar and &var have the same address, the value of var will also be changed when *pointVar is changed.


Example 3: Changing Value Pointed by Pointers

#include <iostream>
using namespace std;
int main() {
    int var = 5;
    int* pointVar;

    // store address of var
    pointVar = &var;

    // print var
    cout << "var = " << var << endl;

    // print *pointVar
    cout << "*pointVar = " << *pointVar << endl
         << endl;

    cout << "Changing value of var to 7:" << endl;

    // change value of var to 7
    var = 7;

    // print var
    cout << "var = " << var << endl;

    // print *pointVar
    cout << "*pointVar = " << *pointVar << endl
         << endl;

    cout << "Changing value of *pointVar to 16:" << endl;

    // change value of var to 16
    *pointVar = 16;

    // print var
    cout << "var = " << var << endl;

    // print *pointVar
    cout << "*pointVar = " << *pointVar << endl;
    return 0;
}

Output

var = 5
*pointVar = 5

Changing value of var to 7:
var = 7
*pointVar = 7

Changing value of *pointVar to 16:
var = 16
*pointVar = 16

Common mistakes when working with pointers

Suppose, we want a pointer varPoint to point to the address of var. Then,

int var, *varPoint;

// Wrong! 
// varPoint is an address but var is not
varPoint = var;

// Wrong!
// &var is an address
// *varPoint is the value stored in &var
*varPoint = &var;

// Correct! 
// varPoint is an address and so is &var
varPoint = &var;

 // Correct!
// both *varPoint and var are values
*varPoint = var;

Recommended Readings: