[1] Output Iterators have neither a distance type nor a value type; in many ways, in fact, Output Iterators aren't really iterators. Output iterators do not have a value type, because it is impossible to obtain a value from an output iterator but only to write a value through it. They do not have a distance type, similarly, because it is impossible to find the distance from one output iterator to another. Finding a distance requires a comparison for equality, and output iterators do not support operator==.

[2] The iterator_traits class relies on a C++ feature known as partial specialization. Many of today's compilers don't implement the complete standard; in particular, many compilers do not support partial specialization. If your compiler does not support partial specialization, then you will not be able to use iterator_traits, and you will have to continue to use the older iterator tag functions.

[3] Note that Trivial Iterator does not appear in this list. The Trivial Iterator concept is introduced solely for conceptual clarity; the STL does not actually define any Trivial Iterator types, so there is no need for a Trivial Iterator tag. There is, in fact, a strong reason not to define one: the C++ type system does not provide any way to distinguish between a pointer that is being used as a trivial iterator (that is, a pointer to an object that isn't part of an array) and a pointer that is being used as a Random Access Iterator into an array.