I've seen a number of posts on SO about how to define custom iterators, but nothing that seems to exactly answers my question, which is...

How do I create an iterator that hides a nested for loop?

For instance, I have a class Foo, inside of the Foo is a Bar, and inside of the Bar is a string. I could write

for (const Foo& foo : foo_set)
  for (const Bar& bar : foo.bar_set)
    if (bar.my_string != "baz")
      cout << bar.my_string << endl;

but instead I want to be able to do something like:

for (const string& good : foo_set)
  cout << good << endl;

How do I do something like this?