c++ - comparing iterators from different containers

Question

Is it legal to compare iterators from different containers?

std::vector<int> foo;
std::vector<int> bar;

Does the expression foo.begin() == bar.begin() yield false or undefined behavior?

(I am writing a custom iterator and stumbled upon this question while implementing operator==.)

question from:https://stackoverflow.com/questions/4657513/comparing-iterators-from-different-containers

Answer 1

If you consider the C++11 standard (n3337):

§ 24.2.1 — [iterator.requirements.general#6]

An iterator j is called reachable from an iterator i if and only if there is a finite sequence of applications of the expression ++i that makes i == j. If j is reachable from i, they refer to elements of the same sequence.

§ 24.2.5 — [forward.iterators#2]

The domain of == for forward iterators is that of iterators over the same underlying sequence.

Given that RandomAccessIterator must satisfy all requirements imposed by ForwardIterator, comparing iterators from different containers is undefined.

The LWG issue #446 talks specifically about this question, and the proposal was to add the following text to the standard (thanks to @Lightness Races in Orbit for bringing it to attention):

The result of directly or indirectly evaluating any comparison function or the binary - operator with two iterator values as arguments that were obtained from two different ranges r1 and r2 (including their past-the-end values) which are not subranges of one common range is undefined, unless explicitly described otherwise.