Is std::advance on an iterator of std::multiset done in constant time?

Question

I would like to know if calling std::advance on an iterator of std::multiset is done in constant time.

This question shows no sign of effort made to find the solution yourself. Just looking up cppreference would have sufficed. — Walter, Nov 06 '18 at 14:42
Oh dear @Walter, I only wish I could show you my browsing history. cppreference doesn't say what kind of iterator can be obtained from std::multiset, Hence, I wasn't sure if std::advance can be done in constant time. It would be great if didn't make assumptions on the nature of questions asked here. You know, most of us (like me) who use the internet aren't so well versed with C++ and are making an effort to learn. Getting a -1 will only discourage us further. — Gautham, Nov 06 '18 at 19:32

score 5 · Answer 1 · edited Nov 06 '18 at 14:40

5

No, only random access iterators can advance in constant time. Otherwise, it's linear time on the distance.

edited Nov 06 '18 at 14:40

Jeffrey

answered Nov 06 '18 at 14:37

Matthieu Brucher

Could you provide a reference ? – Jeffrey Nov 06 '18 at 14:39
Sure, I added the cpp reference link. – Matthieu Brucher Nov 06 '18 at 14:39
Well, at worst linear. In theory one could write a multiset with log-speed advance iterators. It would, however, cost some performance (not standards-breaking, just a constant factor) in other operations, and I am unaware of any std implementation that does this. – Yakk - Adam Nevraumont Nov 06 '18 at 15:13
But how do I know if std::multiset returns a random access iterator? cppreference page only says that the "begin" method for multiset returns only iterator (doesn't mention which kind) - https://en.cppreference.com/w/cpp/container/multiset/begin – Gautham Nov 07 '18 at 13:59
1

As you can see in the members of multiset in https://en.cppreference.com/w/cpp/container/multiset, the iterators are bidirectional, not random. – Matthieu Brucher Nov 07 '18 at 14:13

score 4 · Accepted Answer · edited Jun 20 '20 at 09:12

From std::advance, we see that

Complexity

Linear. However, if InputIt additionally meets the requirements of RandomAccessIterator, complexity is constant.

As any template from the std::*map and std::*set family doesn't fulfill the RandomAccessIterator requirements, it's linear. From e.g. here:

Member types

[...]

iterator Constant BidirectionalIterator
const_iterator Constant BidirectionalIterator

2 Answers2