🔴 Advanced · Lesson 34

Pointer to Pointer

What is Pointer to Pointer?

A pointer stores the address of a variable. A pointer to pointer stores the address of another pointer.

Basic Idea

int x = 10;
int *p = &x;
int **pp = &p;

Syntax

Expression	Meaning
`x`	value of variable
`&x`	address of x
`p`	address stored in p
`*p`	value of x
`pp`	address of p
`**pp`	value of x

Example

C Language

#include <stdio.h>

int main() {
    int x = 10;
    int *p = &x;
    int **pp = &p;

    printf("x = %d\n", x);
    printf("*p = %d\n", *p);
    printf("**pp = %d\n", **pp);

    **pp = 50;
    printf("Updated x = %d", x);
    return 0;
}

x = 10 *p = 10 **pp = 10 Updated x = 50

Function Use

Double pointers are useful when a function needs to change a pointer itself.

C Language

#include <stdio.h>
#include <stdlib.h>

void allocate(int **p) {
    *p = (int*) malloc(sizeof(int));
    **p = 100;
}

int main() {
    int *ptr = NULL;
    allocate(&ptr);
    printf("Value = %d", *ptr);
    free(ptr);
    return 0;
}

Summary

int **pp stores address of int * pointer
**pp accesses the final value
Useful in dynamic memory allocation
Useful for modifying pointer inside function
Also used in arrays of strings and command line arguments

Pointer to Pointer क्या है?

Pointer variable किसी normal variable का address store करता है। Pointer to pointer किसी pointer variable का address store करता है।

C Language

int x = 10;
int *p = &x;
int **pp = &p;

Syntax

Expression	Meaning
`x`	variable की value
`*p`	x की value
`pp`	p का address
`**pp`	x की value

Example

C Language

int x = 10;
int *p = &x;
int **pp = &p;
**pp = 50;
printf("%d", x);   // 50

Function Use

जब function के अंदर pointer को ही change करना हो, तब double pointer useful होता है।

C Language

void allocate(int **p) {
    *p = malloc(sizeof(int));
    **p = 100;
}

सारांश

Double pointer pointer का address store करता है
**pp final value access करता है
Dynamic memory में useful है
Function में pointer update करने के लिए use होता है
Command line arguments में भी concept आता है

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