Const iterators
There's just one more complication to consider, before we abandon this list iterator example. Notice that I quietly changed our count_if function template so that it takes Container& instead of const Container&! That's because the begin() and end() member functions we provided are non-const member functions; and that's because they return iterators whose operator* returns non-const references to the elements of the list. We'd like to make our list type (and its iterators) completely const-correct--that is, we'd like you to be able to define and use variables of type const list_of_ints, but prevent you from modifying the elements of a const list.
The standard library generally deals with this issue by giving each standard container two different kinds of iterator: bag::iterator and bag::const_iterator. The non-const member function bag::begin() returns an iterator and the bag::begin() const member function returns a const_iterator. The underscore is all-important! Notice...
