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Constant Pointer and Pointer to constants


Constant Pointer:-
It is type of pointer whose address cannot be modified after first time initialization.

Ex:-
 #include<stdio.h>
   int main()
   {
       int var = 10, var2 = 20;
       int *const ptr = &var;
       ptr = &var2;                /* error cant change read only ptr is constant*/
       *ptr = 30;                  /* value can be changed */
       printf(“ptr  = %d\n”,*ptr);
      return 0;
  }

In this program ptr is constant which cannot hold the other addresses but value can be modified.

Pointer to a constant:-

It is type of pointer in which the data  pointed cannot be modified.

Ex:-
#include<stdio.h>
     int main()
   {
       int var = 10, var2 = 20;
       const int* ptr = &var;
       ptr = &var2;                /* can change the address */
       *ptr = 30;                  /* error data is constant  */
       printf(“ptr  = %d\n”,*ptr);
      return 0;
  }

In this program ptr points to constant data which cannot be modified,but address can be modified.

Constant pointer to a constant:-

This is combination of above to here data and address are constants.

Ex:-

#include<stdio.h>
   int main()
 {
     int var = 10, var2 = 20;
     const int* const ptr = &var;
      ptr = &var2;                /* error address is constant */
      *ptr = 30;                  /* error data is constant  */
      printf(“ptr  = %d\n”,*ptr);
      return 0;
 }

Easy way to remember:-

const int * ptr :- if * comes after const then data is constant.
int * const  ptr :-  if * comes before const then address is constant.
cons int * const ptr :- address and data both are constant.
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