The Boost.Iterator Library
New-Style Iterators
The iterator categories defined in C++98 are extremely limiting because they bind together two orthogonal（正交） concepts: traversal and element access. For example, because a random access iterator is required to return a reference (and not a proxy) when dereferenced, it is impossible to capture the capabilities of vector<bool>::iterator using the C++98 categories. This is the infamous "vector<bool> is not a container, and its iterators aren't random access iterators", debacle about which Herb Sutter wrote two papers for the standards comittee (n1185 and n1211), and a Guru of the Week.
NOTE: 上面这段话的意思是：traversal 和 element access是正交的，两者应该是独立的，应该分开来，但是C++98的iterator却将两者杂糅在一起了，这段话中以random access iterator为例对此进行了说明，random access iterator显然是具备traversal 的概念的，要求它return a reference (and not a proxy) when dereferenced则是要求它具备element access的概念。
New-style iterators go well beyond patching up vector<bool>, though: there are lots of other iterators already in use which can't be adequately represented by the existing concepts. For details about the new iterator concepts, see our