C constant pointers

What is Constant Pointers [ Const Pointer ] in C Programming :

  • As name suggests , Constant Pointers Cannot be modified .
  • Modification in Integer to which it Points to is Allowed
  • Modification made in Pointer is Not Allowed

Syntax Declaration :

data_type * const Pointer_name = Initial_Address;

Sample Code Snippet : 

int num = 20;
 int * const ptr = &num ; // Declare Constant Pointer
 *ptr = 20 ;              // Valid Statement
 ptr ++ ;                 // Invalid Statement

Explanation :

  1. In the Diagram Shown The at 2001 we have Store the value 15 .
  2. Constant Pointer is Pointer to that Variable [Pointer Stores address 2001]
  3. We cannot Change the Address stored in Pointer Variable i.e 2001 is fixed
  4. But we can Change the Value at 2001 i.e we can change value 15 to 20

Example 1: Illustrate const pointer in C

#include<stdio.h>
int main()
{
int num[2] = {10,20};
int* const ptr = &num[0];
printf("%d",*ptr);
return(0);
}

Output :

10

Explanation :

int* const ptr = &num[0];

We must initialize const pointer at the time of declaration. Otherwise it will cause error.

int* const ptr;
ptr = &num[0];

above statements will cause error.

Example 2 : We cannot change address stored inside Pointer

#include<stdio.h>
int main()
{
int num[2] = {10,20};
int* const ptr = &num[0];
ptr++;
printf("%d",*ptr);
return(0);
}

Output :

Example 3 : We can change value stored at address.

#include<stdio.h>
int main()
{
int num[2] = {10,20};
int* const ptr = &num[0];
*ptr = 30;
printf("%d",*ptr);
return(0);
}

Output :

30

Summary

We can DO following operations -

  • Assigning Value at Address
  • Printing Address or Value
  • Assigning Address at the time of declaration.

We Can’t DO following operations on const pointer -

  • Adding Integer to const Pointer
  • Subtracting Integer to const Pointer.
  • Any operation that can change address of pointer.