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

Subject: Re: [Boost-users] A forward iterator need not be default-constructible
From: Krzysztof Czainski (1czajnik_at_[hidden])
Date: 2011-09-29 09:23:34

Hi,

W dniu 26 wrze¶nia 2011 19:48 u¿ytkownik Krzysztof ¯elechowski <
giecrilj_at_[hidden]> napisa³:
[snip]

> The standard C++ library defined iterators to serve and additional task to
> serve as abstract pointers that may be singular, i.e. not corresponding to
> any object and distinct from any other iterator.  To this end, the standard
> requires that interators may be constructed out of thin air, and that such
> an iterator is singular.

Chris, if I understand correctly, you claim that default-constructed
iterators are required by the standard to be singular?

W dniu 29 wrze¶nia 2011 11:02 u¿ytkownik Krzysztof ¯elechowski <
giecrilj_at_[hidden]> napisa³:

> Dave Abrahams wrote:
> > I'd also like to point out that there's no rule saying
> > default-constructed iterators must be singular.
>
> A default-constructed iterator must be singular not because the government
> says so but because of logic.  You usually get wet when it rains, although
> there is no rule saying that you must.  Standard iterators are all singular
> by default.
>

I disagree. I think requiring default-constructed iterstors to be singular
would make plain old pointers not meet the requirements. For example:
vector<int>::iterator a; // (1)
vector<int>::iterator b = a; // (2)

Correct me if I'm wrong: line (1) is legal, and the value of a may be
singular, or a may be uninitialized. Therefore line (2) is undefined
behavior.

However, if we change the example a bit to make a value-initialized:
vector<int>::iterator a = vector<int>::iterator(); // (3)
vector<int>::iterator b = a; // (4)

Now for vactor<int>::iterator == int* the value of a is guaranteed to be
singular: NULL, and so line (4) is ok.

But my question is, does the standard require singularity for
iterators value-initialized like on line (3)? If I understand Dave's point
correctly, there's no such requirement in the standard.

So Chris, in my opinion you may have a point there, that it could make sense
to:
1) add a requirement, that value-initialized default-constructed iterators
be singular;
2) allow using such singular-value iterators to compare, but not dereference
-- just like vector<int>::end() may be used.
- Note, that I'm *not* suggesting to make vector<int>::end() to return a
value equal to vector<int>::iterator().

Regards,
Kris