c++ back_inserter for a set?

Question

I guess this is a simple question. I need to do something like this:

std::set<int> s1, s2;
s1 = getAnExcitingSet();
transform(s1.begin(), s1.end(), std::back_inserter(s2), ExcitingUnaryFunctor());

Of course, back_inserter doesn't work since there's no push_back. std::inserter also needs an iterator? I haven't used inserter so I'm not sure what to do. Does anyone have an idea?

Of course, my other option is to use a vector for s2, and then just sort it later. Maybe that's better?

Answer 1

set doesn't have push_back because the position of an element is determined by the comparator of the set. Use std::inserter and pass it .begin():

std::set<int> s1, s2;
s1 = getAnExcitingSet();
transform(s1.begin(), s1.end(), 
          std::inserter(s2, s2.begin()), ExcitingUnaryFunctor());

The insert iterator will then call s2.insert(s2.begin(), x) where x is the value passed to the iterator when written to it. The set uses the iterator as a hint where to insert. You could as-well use s2.end().