[ub] Justification for < not being a total order on pointers?

Jeffrey Yasskin jyasskin at google.com
Mon Aug 26 18:00:40 CEST 2013


The library's running into problems because < is unspecified on
pointers that don't point to the same object. (C++14CD[expr.rel]p4)
Naively, this restriction makes a bit of sense, since we can't give a
particular answer for any two pointers, but it's causing problems for
some generic library components. Specifically:

std::map<T*> works because std::less has a special case for pointers.
([comparisons]p14)

std::map<std::tuple<T*>> gives undefined behavior because
std::less<tuple> uses tuple's operator<, which uses T*'s operator<.

std::map<std::optional<T*>> goes back to defined behavior because we
special-cased optional's operator< to use std::less:
[optional.relops]p4


Could someone explain why we need to allow operator<(T*) to be a non-order?

Thanks,
Jeffrey


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