C++ Pointer to Pointer

In C++, a pointer to pointer is a type of pointer that stores the address of another pointer. This allows for multiple levels of indirection, which can be useful in situations like dynamic memory allocation, multi-dimensional arrays, or when working with complex data structures.

A pointer to pointer is declared using two asterisks (**) and is often referred to as a “double pointer.”

Syntax

</>

Copy

data_type** pointer_name;

data_type: The type of data that the final pointer will point to.
pointer_name: The name of the pointer to pointer variable.

Examples

Example 1: Basic Pointer to Pointer

This example demonstrates how to declare, initialize, and access a pointer to pointer.

</>

Copy

#include <iostream>
using namespace std;

int main() {
    int num = 42;
    int* ptr = &num;     // Pointer to an integer
    int** ptr2 = &ptr;   // Pointer to pointer

    cout << "Value of num: " << num << endl;
    cout << "Value of num using *ptr: " << *ptr << endl;
    cout << "Value of num using **ptr2: " << **ptr2 << endl;

    return 0;
}

Output:

Value of num: 42
Value of num using *ptr: 42
Value of num using **ptr2: 42

Explanation:

The variable num is declared and initialized to 42.
The pointer ptr is assigned the address of num using the address-of operator (&).
The pointer to pointer ptr2 is assigned the address of ptr.
*ptr dereferences the first pointer, accessing the value of num.
**ptr2 dereferences the second pointer, which accesses num through ptr.

Example 2: Modifying a Value Using Pointer to Pointer

This example demonstrates how a pointer to pointer can be used to modify the value of a variable.

</>

Copy

#include <iostream>
using namespace std;

void modifyValue(int** ptr2) {
    **ptr2 = 100; // Modify the value pointed by the pointer
}

int main() {
    int num = 42;
    int* ptr = &num;     // Pointer to an integer
    int** ptr2 = &ptr;   // Pointer to pointer

    cout << "Before modification: " << num << endl;

    modifyValue(ptr2);   // Pass the pointer to pointer

    cout << "After modification: " << num << endl;

    return 0;
}

Output:

Before modification: 42
After modification: 100

Explanation:

The variable num is declared and initialized to 42.
The pointer ptr is assigned the address of num.
The pointer to pointer ptr2 is assigned the address of ptr.
The function modifyValue is called with ptr2 as an argument. Inside the function, **ptr2 dereferences the pointer twice to access and modify the value of num.
After the function call, the value of num is updated to 100.

Key Points to Remember about Pointer to Pointer

A pointer to pointer stores the address of another pointer.
It allows for multiple levels of indirection, enabling access to the value indirectly.
To access the value, the pointer must be dereferenced multiple times (e.g., **ptr2).
Pointers to pointers are commonly used in dynamic memory allocation, passing arrays to functions, and working with complex data structures.

TutorialKart

C++ Pointer to Pointer (Double Pointer)