In this case, n is a pointer to a constant int. When you cast it to int* you remove the const qualifier, and so the operation is allowed.
If you tell the compiler to remove the const qualifier, it will happily do so. The compiler will help ensure that your code is correct, if you let it do its job. By casting the const-ness away, you are telling the compiler that you know that the target of n is non-constant and you really do want to change it.
If the thing that your pointer points to was in fact declared const in the first place, then you are invoking undefined behavior by attempting to change it, and anything could happen. It might work. The write operation might not be visible. The program could crash. Your monitor could punch you. (Ok, probably not that last one.)
void const_is_a_lie(const char * c) {
*((char *)c) = '5';
}
int main() {
const char * text = "12345";
const_is_a_lie(text);
printf("%s\n", text);
return 0;
}
Depending on your specific environment, there may be a segfault (aka access violation) in const_is_a_lie since the compiler/runtime may store string literal values in memory pages that are not writable.
The Standard has this to say about modifying const objects.
7.1.6.1/4 The cv-qualifiers [dcl.type.cv]
Except that any class member declared mutable (7.1.1) can be modified, any attempt to modify a const object during its lifetime (3.8) results in undefined behavior
"Doctor, it hurts when I do this!" "So don't do that."
