Is it legal to compare dangling pointers?

Question

Is it legal to compare dangling pointers?

int *p, *q;
{
    int a;
    p = &a;
}
{
    int b;
    q = &b;
}
std::cout << (p == q) << \'\\         


        

Answer 1

Historically, there have been some systems where using a pointer as an rvalue might cause the system to fetch some information identified by some bits in that pointer. For example, if a pointer could contain the address of an object's header along with an offset into the object, fetching a pointer could cause the system to also fetch some information from that header. If the object has ceased to exist, the attempt to fetch information from its header could fail with arbitrary consequences.

That having been said, in the vast majority of C implementations, all pointers that were alive at some particular moment in time will forever hold the same relationships with regard to the relational and subtraction operators as they had at that particular time. Indeed, in most implementations if one has char *p, one may determine whether it identifies part of an object identified by char *base; size_t size; by checking whether (size_t)(p-base) < size; such comparison will work even retrospectively if there is any overlap in the objects' lifetime.

Unfortunately, the Standard defines no means by which code can indicate that it requires any of the latter guarantees, nor is there a standard means by which code can ask whether a particular implementation can promise any of the latter behaviors and refuse compilation if it does not. Further, some hyper-modern implementations will regard any use of relational or subtraction operators on two pointers as a promise by the programmer that the pointers in question will always identify the same live object, and omit any code which would only be relevant if that assumption didn't hold. Consequently, even though many hardware platforms would be able to offer guarantees that would be useful to many algorithms, there's no safe way by which code can exploit any such guarantees even if code will never need to run on hardware which does not naturally provide them.