[C++] Explaining std::set<>.insert with hints...
 

Okay, it's time to really use this forum!

Now, there is a method, insert(), defined for std::set that takes a std::set::iterator as a hint to where to insert a value, as well as the value, itself. My question is two-fold:

First, what if my hint is not particularly good? Will it still be able to properly insert the value? With what efficiency will it do this?

Secondly, very peculiarly, this version of std::set::insert, unlike it's ordinary counter-part, doesn't return a std::pair<>. Instead, it returns a single iterator to where the element was inserted. Why is that? What does that mean? Inserting can't fail? How am I supposed to know whether the value was inserted or if the value already existed? Why does the standard believe that I wouldn't want to know this?

If you can answer these questions (particularly the latter one), I'll be very impressed!