Returning the greatest key strictly less than the given key in a C++ Map

Yeah, lower_bound can be used for that, i’ve seen it before and used it like that.

map_type::iterator it = map.lower_bound(2.3);
if(it != map.begin()) {
    --it;
    // it now points at the right element
}

Would actually return the greatest, yet smaller (if it != map.begin() was true) one. If it was at .begin, then there is no smaller key. Nice idea from the comments is to return .end if there is no element that’s less and pack this stuff into a function:

template<typename Map> typename Map::const_iterator 
greatest_less(Map const& m, typename Map::key_type const& k) {
    typename Map::const_iterator it = m.lower_bound(k);
    if(it != m.begin()) {
        return --it;
    }
    return m.end();
}

template<typename Map> typename Map::iterator 
greatest_less(Map & m, typename Map::key_type const& k) {
    typename Map::iterator it = m.lower_bound(k);
    if(it != m.begin()) {
        return --it;
    }
    return m.end();
}

The template should work for std::set too.