From: David Abrahams (dave_at_[hidden])
Date: 2004-04-23 06:36:06
Vladimir Prus <ghost_at_[hidden]> writes:
> Okay. BTW, what guarantees that ++r does not invalidate any copies for
> forward/bidirectional/random iterator?
None, I think.
>>> If single pass iterator requirements
>>> - would add the same note about ++r making iterators dereferencable
>> Probably a good idea.
>>> - retain the same requirements for operator++(int)
>> Really you mean requirements on the expression "*r++", I think.
> No, on operator++(int)
What requirements do you mean, specifically?
>> ...then what?
No answer? You started a phrase with if (condition) but there was no
"body", if you will.
>>> The result of r++ is not required even to be dereferencable
>> Not required by which concept?
> By nothing I could find in new iterator requirements. Requirements on
> operator++(int) say:
> X tmp = r;
> return tmp;
Right. Traversal and access are supposed to be orthogonal.
> operator++ is allowed to invalidate 'tmp', and nothing explicitly requires
> return value from operator++(int) to be dereferencable.
Right. That's why we need to somehow retain the requirements for
>>> The solutions I see are:
>>> 1) require that ++r does not makes any copies dereferencable, or
>> I think you mean "not require that any copies are dereferencable after
> Nope, I meant what written. If ++r is required to keep the copies
That's the opposite of what you wrote. "does not makes any copies
dereferencable" means, "doesn't change any copies of r from
non-dereferenceable to dereferenceable."
> then *r++ will be guaranteed to work. OTOH, this would require
> storing value in iterator which as you say is not indented by current
Yes, and it mean that not all readable single-pass iterators are input
iterators, so I'm against it.
>>> 2) allow returning proxy from operator++(int)
>> That doesn't allow all readable single-pass iterators to be input
>> iterators. I'm against it.
> It's possible to require that return value from operator++(int) is
> some type with operator* and applicatqion of operator* returns the
> same value as the *it before incrementing.
Ah, whoops. OK, that solution is compatible with input iterator and
output iterator, so I favor it.
>>> 3) require that result of r++ is dereferencable and is equivivalent to
>>> the dereferencing of the previous value of 'r'.
Well, that requirement is equivalent to input iterator's requirement
on "*r++". The question is, in which concept does that requirement
go? It's neither a pure access nor a pure traversal concept.
>> I think we need want 1&3.
> Will 3) require extra storage in iterator?
Not if accompanied by 2.
> Now, transform_iterator can store only wrapped iterator and a
> functor. If 3) is required it should additionally store either
> value, or a flag telling there's a undereferenced copy (as you've
I don't think the flag will work, actually, because of this
requirement on input iterator:
(void)r++ equivalent to (void)++r
> Besides, In the second case it should be stated that return of r++
> is dereferencable untill you call operator*() or operator++() on
> original iterator.
>> > The variant 2) would be most convenient for directory_iterator...
>> Yeah, but it would break interoperability with old algorithms.
> Why? Input iterator requirements only say that *r++ should return T. They
> don't say anything about type of r++.
You're right. It's 2&3.
-- Dave Abrahams Boost Consulting http://www.boost-consulting.com
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