Which one is better when select the last entry of an ordered map in c++

Question

auto& myKey = myMap.rbegin()->first;
auto& myKey = std::prev(myMap.end())->first;

myMap is a constant ordered map. Both of the approaches are with constant complexity. rbegin()uses reverse iterator, while std::prevworks on bidirectional iterator. Any efficiency difference between them?

This would seem to be more of a personal preference than anything else. Personally 2 is easier to follow for me. — Claudiu, Mar 12 '16 at 06:29
"`rbegin()` usese reverse iterator, while [`myMap.end()`] works on bidirectional iterator" - I think this statement confuses iterator types with iterator categories. These are both iterators with bidirectional traversal capabilities, iterating over logically-different ranges. I'm mentioning this as the statement might be understood as one call returning an iterator that is more complex than the other one. — Ami Tavory, Mar 12 '16 at 06:41
Don't know the bigger problem, but you might want to use a `std::map` with inverted ordering instead, i.e. using a comparison functor that calls `!std::less(...)`. — Christian Hackl, Mar 12 '16 at 10:49

score 2 · Answer 1 · edited Mar 12 '16 at 08:45

The typical implementation for rbegin():

reverse_iterator rbegin()
{
    return reverse_iterator(end());
}

This is essentially the same as doing std::prev(myMap.end()). So, theoretically they would be identical. However, the reverse_iterator tends to be a little tougher on compiler optimizers. If you care much, I would suggest tracing through the actual release-mode assembly (or generate assembly output) in your compiler and see if there is a difference.

Which one is better when select the last entry of an ordered map in c++

1 Answers1