It is not always defined. It's defined to take an address to a pointer with an offset within the object's space plus one. Subtracting two pointers to distinct o

It is not always defined. It's defined to take an address to a pointer with an offset within the object's space plus one. Subtracting two pointers to distinct objects is undefined. This is to allow for segmented memory architectures. It's also to allow implementation of garbage collectors by the compiler