Andrey Semashev skrev:
On Sat, Nov 22, 2008 at 8:13 PM, Mathias Gaunard < mathias.gaunard@ens-lyon.org> wrote:
Andrey Semashev wrote:
Yes, in general default-constructed
iterators are singular and they cannot be used reliably. However, not all iterators behave that way. Pointers, for example, are NULL on default-initialization (which is used in the default constructor of the iterator_range class). I can well have my own iterators that are default-constructible and allow comparison in this state. And I don't understand why an iterator range of these iterators suddenly restricts me from being able to compare these iterators in empty(), for example.
Iterators where default-construction leads to a valid past-the-end iterator for all sequences are a fairly special case of iterators, even if they happen to be quite common. However, that is certainly not guaranteed by the iterator concepts. Therefore, when you deal with iterators in a generic way, you should not assume such a thing.
iterator_range being a generic tool, it wants to maintain the invariants which are necessary to make it work whatever its parameters are, even if those invariants happen not to be necessary for some parameters.
I think this is the case when excessive invariant checking does more harm than good. I hate to reiterate, but I think it should be up to user whether to rely on the ability to compare default-constructed iterators or not. The library, being as you rightfully say, a generic tool, must not impose excessive restrictions on the use model of the underlying iterators. Right now the library does so, which basically cancels a considerable set of valid use cases. I would say, it contradicts the term "a generic tool".
Your views have some merits, and I think we should have a more conservative debugging policy in iterator_range. -Thorsten