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

Subject: Re: [Boost-users] A forward iterator need not be default-constructible
From: Brian Allison (brian.w.allison_at_[hidden])
Date: 2011-09-30 12:02:09

On Fri, Sep 30, 2011 at 11:22 AM, Dave Abrahams <dave_at_[hidden]> wrote:

>
> on Thu Sep 29 2011, Brian Allison <brian.w.allison-AT-gmail.com> wrote:
>
> > On Thu, Sep 29, 2011 at 3:59 PM, Dave Abrahams <dave_at_[hidden]>
> > wrote:
> >
> > Your ontology seems to have no basis in the ontology of the standard.
> > The last sentence I quoted:
> >
> >     Dereferenceable values are always nonsingular.
> >
> > This sentence doesn't leave room for a statement like "every valid
> > iterator is-a singular iterator in some sense", given that
> > dereferenceable iterators are valid, and the standard explicitly
> > states that they are non-singular. Your definition of "in a sense"
> > and "in teh same way" are then in contradiction to the standard.
>
> Not if a nonsingular iterator is-a singular iterator.  That way of using
> is-a is the only sense that makes the OP's claim true.  Either you have
> to accept that way of using is-a or you have to reject the OP's claim.
> I'm able to use and think about "is-a" that way, and if you chafe at the
> idea that a nonsingular iterator is-a singular iterator, that's fine
> with me.  I think that means you have to reject the claim that all
> default-constructed iterators are singular, which is also fine with me.

It's not an issue of chafing, but accepting the OPs claim to be true
requires the quoted sentence to be nonsensical. I'm not sure of any useful
algebra where a member can be both X and non-X simultaneously. If there is
one, please share?

However, refuting the OPs claim has not even an apparent contradiction
either to the standard or to my own understanding of fundamental logic.