Undefined behaviour of const

Question

I never thought I will be going to ask this question but I have no idea why this happens.

const int a = 3; 
int *ptr;
ptr = (int*)( &a );
printf( \"A=%d\         


        

Answer 1

The problem is that the type of ptr is "pointer to int" not "pointer to const int". You are then casting the address of 'a' (a const int) to be of type "pointer to int" and storing that address in ptr. The effect of this is that you are casting away the const-ness of a const variable.

This results in undefined behavior so your results may vary from compiler to compiler.

It is possible for the compiler to store 'a' in program ROM since it knows 'a' is a const value that can never be changed. When you lie to the compiler and cast away the const-ness of 'a' so that you can modify it through ptr, it may be invalid for ptr to actually modify the value of 'a' since that data may be stored in program ROM. Instead of giving you a crash, this compiler this time decided to point ptr to a different location with a different value this time. But anything could have happened since this behavior is undefined.