How can pointers be totally ordered?

Question

Pointers in C++ may in general only be compared for equality. By contrast, less-than comparison is only allowed for two pointers that point to subobjects of the same complet

Answer 1

The problem is segmented architectures, where a memory address has two parts: a segment and an offset. It's "easy enough" to turn those pieces into some sort of linear form, but that takes extra code, and the decision was to not impose that overhead for operator<. For segmented architectures, operator< can simply compare the offsets. This issue was present for earlier versions of Windows.

Note that "easy enough" is a systems programmer's perspective. Different segment selectors can refer to the same memory block, so producing a canonical ordering requires pawing through details of segment mapping, which is platform-dependent and may well be slow.