Vladimir Prus <ghost@cs.msu.su> 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 "non-"----------^ 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; ++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 *r++.
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 "++r"?
Nope, I meant what written. If ++r is required to keep the copies deferencable
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 input_iterator.
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.
Now 2&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 suggested).
I don't think the flag will work, actually, because of this requirement on input iterator: operation semantics --------- ----------------------- (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