C++ nested iterators

Question

Is it OK to have a nested iterator like the following?

for (vector<type>::iterator i = list.begin(); i != list.end(); ++i) {
    for (vector<type>::iterator j = i; j != list.end(); ++j) {
        ...
    }
}

Note that j starts at i, and not list.begin(). Since the iterator is random access, can I guarantee that both i and j will have the same order? is there a better way of doing this?

You're not nesting iterators (which doesn't make sense in itself). You're nesting loops, and copying iterators. There's nothing strange here. — Lightness Races in Orbit, Feb 17 '13 at 18:40

score 4 · Accepted Answer · edited May 23 '17 at 11:58

4

Your code is correct.

Both iterators will have the same order and incrementing j doesn't affect i as long as you don't make any operation that invalidates iterators (for example erasing from or pushing to vector).

edited May 23 '17 at 11:58

Community

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answered Feb 17 '13 at 18:39

Rafał Rawicki

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There isn't really any "the documentation" for C++, but see http://stackoverflow.com/questions/6438086/iterator-invalidation-rules. – Lightness Races in Orbit Feb 17 '13 at 18:41
@LightnessRacesinOrbit Thanks for the link! You are right, I simplified too much writing that sentence. – Rafał Rawicki Feb 17 '13 at 18:45

score 3 · Answer 2 · answered Feb 17 '13 at 18:39

3

This is absolutely fine as long as you don't do anything inside the loops that might invalidate iterators.

(As an aside, list is a questionable name for a std::vector in my opinion.)

answered Feb 17 '13 at 18:39

us2012

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+1 for mentioning the choice of the name of a standard container as the name of a variable. I've been seeing this very often lately (`array`, `list`, `map`, etc.). – Andy Prowl Feb 17 '13 at 18:41
the actual name is of course not list, just shortened it so that it would be easier to read in this forum. – nbubis Feb 17 '13 at 18:49

score 3 · Answer 3 · answered Feb 17 '13 at 18:40

That's perfectly fine. Random access doesn't mean random order. It means that you can jump through the container by using additive operators (+ and -) on your iterator. For example, with a random access iterator it, you can do it + 10. For an non-random access iterator, you would have to do it++ 10 times to achieve the same effect. (the std::advance function will encapsulate this for you though)

score 1 · Answer 4 · answered Feb 17 '13 at 18:40

1

This should work fine. Vector stores elements in order, and both iterators will obey this order.

answered Feb 17 '13 at 18:40

Mikhail Vladimirov

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C++ nested iterators

4 Answers4

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