title: C++ iterator to const_iterator
body: <p>How do I acquire a <code>const_iterator</code> (of some container class) from an <code>iterator</code> (of that container class) in C++? What about a <code>const_iterator</code> from an <code>insert_iterator</code>? The resulting <code>iterator</code> should point at the same spot as the original does.</p>