changing vector while processing one of its elements?

Hi,

I observed a behavior that I didn't expect: I have a vector of sets. I
iterate over the first of these sets, and while I iterate over it, I
add another set at the end of the vector. I thought that shouldn't
affect the first set I am iterating over, but it does, and I get a
segmentation fault.

See the example program below.

It works fine if I make set0 a *copy* of v[0], instead of a reference
or a pointer, but I would rather not copy it (this routine is called
very often in my program and I don't see why I should make an
expensive copy).

How can I fix this? Thanks!

int main(int argc, char** argv){
  std::vector<std::set<int> > v;
  std::set<int> s;
  s.insert(1);
  s.insert(2);
  v.push_back(s);
  std::set<int>& set0 = v[0]; // using reference because we don't want
to copy to local var (too expensive)
  for(std::set<int>::const_iterator it = set0.begin(); it !=
set0.end(); ++it){
    std::cout << *it << std::endl;
    std::set<int> tmp;
    tmp.insert(10);
    v.push_back(tmp); // will be v[1], so it shouldn't change set0 or
its
iterator?
  }
  return 0;
}

As output I get:
1
10
1
10
1
10
....
Segmentation fault