Re: [Boost-users] A forward iterator need not be default-constructible

on Thu Sep 29 2011, Brian Allison wrote:

On Thu, Sep 29, 2011 at 9:50 AM, Dave Abrahams wrote:

I'm sorry - but regarding the statement "every valid iterator is-a singular iterator in some sense"... I thought the Standard (2003) explicitly stated that singular values for an iterator had all but one operation as Undefined Behavior, and the only defined operation on an iterator with a singular value was to assign it a non-singular value. (24.1, para 5)

Yes; this is a problem with negative requirements statements. Just like you can't decrement a forward iterator, you can't dereference a singular iterator. However, a bidirectional iterator, which you can decrement, is-a forward iterator. In the same way, a valid iterator is-a singular iterator.

Results of most expressions are undefined for singular values; the only exception is an assignment of a non-singular value to an iterator that holds a singular value. In this case the singular value is overwritten the same way as any other value. Dereferenceable values are always nonsingular. Doesn't the last sentence make it true that "every valid iterator is-a nonsingular iterator"?

(it doesn't, even by logic, since some valid iterators are not dereferenceable, but that aside...). In the sense I'm using is-a, a nonsingular iterator is-a singular iterator. How's that for fun logic? :-) When the standard says "that's a singular iterator" it's saying you can only assume it supports two operations. That doesn't mean it can't support more operations. It's a constraint on the user, not on the iterator.

I also thought it was the case that the latest standard defines singular to be the same as uninitialized with respect to iterators?

There's still no definition of singular other than the text you cited.

If that's the case then we cannot call a valid iterator a singular iterator in any sense.

In the sense of concept requirements, you can. Any valid iterator supports a superset of the requirements on singular iterators The point is that the OP claimed every default-constructed iterator is singular. The only way that could be true is if you take the term "is-a" in the sense I'm using it here. That is, I can easily create an iterator that, when default-constructed, supports a strict superset of the required operations for singular iterators. -- Dave Abrahams BoostPro Computing http://www.boostpro.com