Last key in a std::map

Yes. Map is a sorted container, the reverse iterator must return the elements in reverse (i.e. decreasing) order of their keys.

[Edit: as Charles Bailey points out in his answer, your code gives the greatest key if it exists - i.e. if the map is non-empty]

Yes, but remember to check that map.rbegin() != map.rend().

You can use following method :-

if(!map.empty())
    (--map.end())->first;