Is it safe to test relations between pointers for iteration?

Question

Please consider an array of Thing. It's a kind of stack.

It is pointed to by thingsBottom, and its empty end is pointed to by thingsTop.

EDIT: Every time I want push something onto the list, I do *thingsTop = newThing; thingsTop++;.

I'd like to iterate it from the end to the beginning using pointers, like so:

for (Thing* thing = thingsTop - 1; thing >= thingsBottom; thing--) {
    doSomething(*thing);
}

Is this guaranteed to always work, regardless of the specific C implementation used?

Is it safe to say thing >= thingsBottom?

Answer 1

Is this guaranteed to always work, regardless of the specific C implementation used?

Is it safe to say thing >= thingsBottom?

No, and not unconditionally.

The problem with your approach is that it produces undefined behavior to compute a pointer value that would be before the beginning of the array on which that pointer is based, and pointer comparisons are valid only for pointers into, or just past the end of, the same array. "Just before the beginning" does not have any special status or provision.

You can write such a loop; you just need to test before you decrement:

for (Thing* thing = thingsTop; thing > thingsBottom; ) {
    thing--;
    doSomething(*thing);
}