Question

How do I acquire a const_iterator (of some container class) from an iterator (of that container class) in C++? What about a const_iterator from an insert_iterator? The resulting iterator should point at the same spot as the original does.

Answer 1

Containers are required to provide iterator as a type convertible to const_iterator, so you can convert implicitly:

Container::iterator it = /* blah */;
Container::const_iterator cit = it;

std::insert_iterators are output iterators. This gives no way to convert them to a regular Container::iterator which must be a forward iterator.

Another kind of insert iterator may allow such a thing, but those obtained from the standard functions don’t.

I guess you can write your own wrapper around std::insert_iterator that exposes the protected member iter, though:

template <typename Container>
class exposing_insert_iterator : public std::insert_iterator<Container> {
public:
    exposing_insert_iterator(std::insert_iterator<Container> it)
    : std::insert_iterator<Container>(it) {}
    typename Container::iterator get_iterator() const {
        return std::insert_iterator<Container>::iter;
    }
};

// ...
std::insert_iterator<Container> ins_it;
exposing_insert_iterator<Container> exp_it = ins_it;
Container::iterator it = exp_it.get_iterator();