changing const value in C

Question

I find that in the following code snippet

const int i = 2;  
const int* ptr1= &i;  
int* ptr2 = (int*)ptr1;  
*ptr2 =3;

i's value changes to 3. What I could like to know is why is this allowed. What are the situations in which this could become helpful?

Answer 1

It's allowed because you have overruled the constness of ptr1 by casting it to a non-const pointer. This is why casts can be very dangerous.

Note that some compilers, such as GCC, will not allow you to cast away const status like this.

Answer 2

You have broken the constantness guarantee by playing pointer tricks. This is not guaranteed to work all the time and could invoke almost any behavior depending on the system/OS/compiler that you throw it at.

Don't do that.

Or at least don't do that unless you really know what you are doing and even then understand that it is not in the least portable.