Introduction

The new iterator categories are intended to correspond to the old iterator categories, as specified in a diagram in N1550. For example, an iterator categorized as a mutable Forward Iterator under the old scheme is now a Writable, Lvalue, and Foward Traversal iterator. However, there is a problem with this correspondance, the new iterator categories place requirements on the iterator_traits<X>::reference type whereas the standard iterator requirements say nothing about the reference type . In particular, the new Readable Iterator requirements say that the return type of *a must be iterator_traits<X>::reference and the Lvalue Iterator requirements says that iterator_traits<X>::reference must be T& or const T&.

Proposed Resolution

Change the standard requirements to match the requirements of the new iterators. (more details to come)

Rationale

The lack of specification in the standard of the reference type is certainly a defect. Without specification, it is entirely useless in a generic function. The current practice in the community is generally to assume there are requirements on the reference type, such as those proposed in the new iterator categories.

There is some danger in adding requirements to existing concepts. This will mean that some existing iterator types will no longer meet the iterator requirements. However, we feel that the impact of this is small enough to warrant going ahead with this change.

An alternative solution would be to leave the standard requirements as is, and to remove the requirements for the reference type in the new iterator concepts. We are not in favor of this approach because it extends what we see as a defect further into the future.