Lesson 40: Pointer to Pointer (Double Pointers)

A double pointer (or pointer to a pointer) is a variable that stores the address of another pointer variable.

Declaration

A double pointer uses two asterisks (**).

c int x = 10; int *p = &x; // p stores address of x int **pp = &p; // pp stores address of p

Dereferencing Double Pointers

1. pp: Holds the address of p.
2. *pp: Dereferences once, resulting in the content of p (which is the address of x).
3. **pp: Dereferences twice, resulting in the content of x (which is the value 10).

c printf("Value of x via **pp: %d\n", **pp); // 10

// We can change x using the double pointer **pp = 20; printf("New value of x: %d\n", x); // 20

Applications of Double Pointers

1. Passing a Pointer by Reference

This allows a function to change the actual address held by a pointer variable in the calling function (e.g., initializing a structure pointer or performing a dynamic array resize).

2. Implementing 2D Dynamic Arrays

Dynamically creating a 2D matrix requires allocating an array of pointers, where each pointer points to a dynamically allocated row (array of data).

c // Allocation for 10 rows (pointers), each pointing to 20 columns (integers) int **dynamic_matrix = (int **) malloc(10 * sizeof(int *)); for (int i = 0; i < 10; i++) { dynamic_matrix[i] = (int *) malloc(20 * sizeof(int)); }