It is undefined behaviour to modify objects declared as const (except if those objects have mutable members, in which case the mutable members can be modified).

const int a = 1;

If you try to const_cast away the const of x to change its value, you have undefined behaviour. As for pointers, if you make a const pointer to a non-const object then all it means is that you cannot modify the object via that pointer, not that the object will never be modified.

int a = 1; int b = &a; const int c = &a;

a = 2;

Both b and c are now 2 and this is absolutely fine, and the compiler won't try to optimise out any reads of *c and assume its value is still 1 as const/non-const pointer aliasing is allowed (C has the restrict keyword, it's not in C++).