Constant pointer vs Pointer to constant [duplicate]

Answer 1

int i;
int j;

int * const ptr1 = &i;

The compiler will stop you changing ptr1.

const int * ptr2 = &i;

The compiler will stop you changing *ptr2.

ptr1 = &j; // error
*ptr1 = 7; // ok

ptr2 = &j; // ok
*ptr2 = 7; // error

Note that you can still change *ptr2, just not by literally typing *ptr2:

i = 4;
printf("before: %d\n", *ptr2); // prints 4
i = 5;
printf("after: %d\n", *ptr2); // prints 5
*ptr2 = 6; // still an error

You can also have a pointer with both features:

const int * const ptr3 = &i;

ptr3 = &j; // error
*ptr3 = 7; // error

Answer 2

Referncing from This Thread

Constant Pointers

Lets first understand what a constant pointer is. A constant pointer is a pointer that cannot change the address its holding. In other words, we can say that once a constant pointer points to a variable then it cannot point to any other variable.

A constant pointer is declared as follows :
<type of pointer> * const <name of pointer>
An example declaration would look like :
int * const ptr;
Lets take a small code to illustrate these type of pointers :

#include<stdio.h>

int main(void)
{
    int var1 = 0, var2 = 0;
    int *const ptr = &var1;
    ptr = &var2;
    printf("%d\n", *ptr);

    return 0;
} 

In the above example :

• We declared two variables var1 and var2
  
• A constant pointer ‘ptr’ was declared and made to point var1
  
• Next, ptr is made to point var2.
  
• Finally, we try to print the value ptr is pointing to.
  

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Pointer to Constant

As evident from the name, a pointer through which one cannot change the value of variable it points is known as a pointer to constant. These type of pointers can change the address they point to but cannot change the value kept at those address.

A pointer to constant is defined as : const <type of pointer>* <name of pointer> An example of definition could be : const int* ptr; Lets take a small code to illustrate a pointer to a constant :

 #include<stdio.h>

int main(void)
{
    int var1 = 0;
    const int* ptr = &var1;
    *ptr = 1;
    printf("%d\n", *ptr);

    return 0;
} 

In the code above :

• We defined a variable var1 with value 0
• we defined a pointer to a constant which points to variable var1
• Now, through this pointer we tried to change the value of var1
• Used printf to print the new value.