riccibruno marked 2 inline comments as done.
riccibruno added inline comments.
================
Comment at: include/clang/AST/Expr.h:5103
+ using reference = AssociationTy<Const>;
+ using pointer = AssociationTy<Const>;
+ AssociationIteratorTy() = default;
----------------
dblaikie wrote:
> aaron.ballman wrote:
> > riccibruno wrote:
> > > aaron.ballman wrote:
> > > > Carrying over the conversation from D57098:
> > > >
> > > > >> @aaron.ballman Cute, but I suspect this may come back to bite us at
> > > > >> some point. For instance, if someone thinks they're working with a
> > > > >> real pointer, they're likely to expect pointer arithmetic to work
> > > > >> when it won't (at least they'll get compile errors though).
> > > > > @riccibruno Hmm, but pointer is just the return type of operator-> no
> > > > > ? Is it actually required to behave like a pointer ? The only
> > > > > requirement I can find is that It->x must be equivalent to (*It).x,
> > > > > which is true here.
> > > >
> > > > I double-checked and you're right, this is not a requirement of the STL.
> > > >
> > > > > Also looking at the requirements for forward iterators I think that
> > > > > this iterator should actually be downgraded to an input iterator,
> > > > > because of the requirement that reference = T&.
> > > >
> > > > My concern is that with the less functional iterators, common
> > > > algorithms get more expensive. For instance, `std::distance()`,
> > > > `std::advance()` become more expensive without random access, and
> > > > things like `std::prev()` become impossible.
> > > >
> > > > It seems like the view this needs to provide is a read-only access to
> > > > the `AssociationTy` objects (because we need to construct them on the
> > > > fly), but not the data contained within them. If the only thing you can
> > > > get back from the iterator is a const pointer/reference/value type,
> > > > then we could store a local "current association" object in the
> > > > iterator and return a pointer/reference to that. WDYT?
> > > I am worried about lifetime issues with this approach. Returning a
> > > reference/pointer to an `Association` object stored in the iterator means
> > > that the reference/pointer will dangle as soon as the iterator goes out
> > > of scope. This is potentially surprising since the "container" (that is
> > > the `GenericSelectionExpr`) here will still be in scope. Returning a
> > > value is safer in this aspect.
> > >
> > > I believe it should be possible to make the iterator a random access
> > > iterator, at least
> > > if we are willing to ignore some requirements:
> > >
> > > 1.) For forward iterators and up, we must have `reference = T&` or `const
> > > T&`.
> > > 2.) For forward iterators and up, `It1 == It2` if and only if `*It1` and
> > > `*It2` are bound to the same object.
> > > I am worried about lifetime issues with this approach. Returning a
> > > reference/pointer to an Association object stored in the iterator means
> > > that the reference/pointer will dangle as soon as the iterator goes out
> > > of scope. This is potentially surprising since the "container" (that is
> > > the GenericSelectionExpr) here will still be in scope. Returning a value
> > > is safer in this aspect.
> >
> > That's valid.
> >
> > > I believe it should be possible to make the iterator a random access
> > > iterator, at least if we are willing to ignore some requirements:
> > >
> > > 1.) For forward iterators and up, we must have reference = T& or const T&.
> > > 2.) For forward iterators and up, It1 == It2 if and only if *It1 and *It2
> > > are bound to the same object.
> >
> > Yes, but then passing these iterators to STL algorithms will have UB
> > because we claim to meet the requirements for random access iteration but
> > don't actually meet the requirements. I am not certain what problems might
> > arise from violating these requirements.
> >
> > @dblaikie, @mclow.lists: do you have opinions on whether it's okay to not
> > meet these requirements or suggestions on what we could do differently with
> > the design?
> My vote is usually "yeah, have a member inside the iterator you return a
> reference/pointer to" to meet the iterator requirements. Yes, it means if you
> keep a pointer/reference to it, that is invalidated when you increment the
> iterator. But that's well established in iterator semantics.
>
> (might be shooting from the hip here/not fully understanding the
> nuances/tradeoffs in this case)
I believe there are 3 possibilities here:
Option 1 : Make the iterator an input iterator, and make `operator*` return a
value.
Pros : Safe, compliant with the spec.
Cons : Morally we can do all of the operations of a random iterator.
Option 2 : Make the iterator a random access iterator, and make `operator*`
return a value.
Pros : Safe wrt lifetime issues.
Cons : Do not complies with the two requirement of forward iterators mentioned
above.
Option 3 : Make the iterator a random access iterator, and make `operator*`
return a
reference to an object stored inside the iterator.
Pros : Compliant with the spec.
Cons : Nasty lifetime issues (see below)
I believe that option 3 is problematic. An example:
```
AssociationIterator It = ...;
const Association &Assoc = *It++;
// oops, Assoc is dangling since It++ returns a value,
// which goes out of scope at the end of the full expression.
// The same problem do not exists when operator* returns a
// value since the lifetime of the returned Association will be
// extended when bound to the reference.
```
Probably the safe thing to do is to go with option 1.
================
Comment at: include/clang/AST/Expr.h:5084
+ /// storage of Stmt * and TypeSourceInfo * in GenericSelectionExpr.
+ template <bool Const> class AssociationIteratorTy {
+ friend class GenericSelectionExpr;
----------------
dblaikie wrote:
> Worth using any of the iterator helpers LLVM has? (iterator_facade or the
> like)
I did try to use `iteratore_facade` but for some reason I was getting strange
overload resolution failures with it.
In the end it did not save much and so I just rewrote the boiler-plate
(especially given that if we end up going with an input iterator there is not
going to be much boiler-plate).
Repository:
rC Clang
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D57106/new/
https://reviews.llvm.org/D57106
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