What is Constant Pointer in C ?

In C, a constant pointer is a pointer that cannot be used to modify the value of the variable it points to. It allows you to declare a pointer that points to a constant value or a read-only memory location.

Syntax of Constant Pointer:

const data_type *pointer_name;

Here, const is used to indicate that the pointer is constant, and data_type is the data type of the variable being pointed to. The asterisk (*) denotes that pointer_name is a pointer.

There are several reasons to use constant pointers in C:

1. Preventing unintended modifications: By declaring a pointer as constant, you ensure that the value it points to cannot be modified accidentally, helping to maintain program integrity.
2. Passing read-only data: Constant pointers can be used to pass read-only data to functions, ensuring that the data is not modified within the function.

Example of Constant Pointer in C

Here are two examples of using constant pointers in C:

Example 1:

#include <stdio.h>

int main() {
   const int *ptr; // Constant pointer to an integer
   int num = 10;

   ptr = # // Assign the address of 'num' to 'ptr'

   printf("Value of num: %d\n", *ptr);

   // Attempt to modify the value pointed by 'ptr'
   *ptr = 20; // Compilation error: assignment of read-only location

   return 0;
}

Output:

Value of num: 10

Explanation:

In above example, a constant pointer ptr is declared, which points to an integer num. The value of num is assigned to ptr using the address-of operator (&). The program prints the value of num using the pointer (*ptr). However, when an attempt is made to modify the value pointed to by ptr (*ptr = 20), a compilation error occurs since the pointer is constant.

Example 2:

#include <stdio.h>

void printString(const char *str) {
   printf("%s\n", str);
}

int main() {
   const char *message = "Hello, World!"; // Constant pointer to a string

   printString(message);

   return 0;
}

Output:

Hello, World!

Explanation:

In above example, a constant pointer message is declared, which points to a string literal “Hello, World!”. The printString function accepts a constant pointer to a character (const char *str) and prints the string. The message pointer is passed to the printString function, and it successfully prints the string.

Different ways to use Constant Pointer in C

1. Constant Pointer to a Constant:

const int *const ptr;

This declaration indicates that both the pointer (ptr) and the value it points to are constant.

2. Array of Constant Pointer:

const int *arr[5];

This declares an array of five constant pointers to integers.

Rules to be followed while using constant pointer

1. A constant pointer cannot be used to modify the value it points to. Any attempt to modify a constant pointer results in a compilation error.
2. A constant pointer can be used to point to a non-constant value, but the value cannot be modified using the constant pointer.
3. A constant pointer can be assigned the address of a constant variable or a read-only memory location.
4. A constant pointer can be used to pass read-only data to functions, ensuring that the data is not modified within the function.

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