Dereference-assignment to a doubly incremented OutputIterator

Question

Per the (excellent) question C++ OutputIterator post-increment requirements, we observe that for a dereferenceable and incrementable value r of OutputIter

Answer 1

This issue was raised in 2004 as defect 485, and the wording in n3066 clarifies the issue, requiring that an output iterator need only support a sequence of alternating increments and dereference/assignments. So in your example, r need not be incrementable after the first ++r, unless there is an intervening dereference/assignment. This behavior is also required by SGI's STL (see footnote 3). As you mentioned above, n3225 appeared without the fixes from n3066, so defect 2035 was raised; but alas the fix did not make it into the published version of C++11 (ISO/IEC 14882:2011).

Furthermore, defect 2035 says that a (from X a(r++);) cannot be used like *a = 0:

"After this operation [i.e., ++r] r is not required to be incrementable and any copies of the previous value of r are no longer required to be dereferenceable or incrementable."

There are situations where this may aid the implementation (in terms of simplicity): see e.g. this question on ostream_iterator, where such (invalid) double increments are ignored simply returning *this; only a dereference/assignment causes the ostream_iterator to actually increment.