Const Qualifier in C

const qualifier in C/C++ can be applied to the declaration of any variable to specify that its value will not be changed.  const keyword applies to whatever is immediately to its left. If there is nothing to its left, it applies to whatever is immediately to its right.

Pointer to const Int

Syntax to declares a pointer to const int type. So one can modify ptr itself but the object pointed to by ptr shall not be modified.

// ptr is a pointer to constant int 
const int* ptr;

Below example shows allowed operation on pointer to constant integer.

const int a = 10;
const int* ptr = &a;
*ptr = 5; // Wrong
ptr++;    // Right

const pointer to int

Syntax to declare constant pointer to integer

// ptr is a constant pointer to int
int * const ptr;

Below example shows the use of const pointer to int type. You are not allowed to modify ptr but the object pointed to by ptr.

int a = 10;
int *const ptr = &a; 
*ptr = 5; // Right
ptr++;    // Wrong

const pointer to const int

Syntax to declare constant pointer to constant integer is

// constant pointer to constant integer.
const int * const ptr;

For const pointer to const integer, you can neither change the value pointed by ptr nor the pointer ptr.

int a =10, b =20; 
const int *const ptr = &a; 
ptr = &b; // illegal statement 
*ptr = b; // illegal statement