aaron.ballman added inline comments.
================
Comment at: include/clang/AST/Expr.h:5103
+ using reference = AssociationTy<Const>;
+ using pointer = AssociationTy<Const>;
+ AssociationIteratorTy() = default;
----------------
riccibruno wrote:
> 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.
> Probably the safe thing to do is to go with option 1.
In the interests of solving the problem at hand, I think we should go with the
safest option and can put a FIXME near the `iterator_category` that explains
why it's an input iterator and that we'd like it to have stronger guarantees
someday. It should meet our needs for the AST dumping refactoring; we can solve
the harder problems if/when they arise in practice.
================
Comment at: lib/AST/ASTDumper.cpp:1465
- for (unsigned I = 0, N = E->getNumAssocs(); I != N; ++I) {
+ for (auto Assoc : E->associations()) {
dumpChild([=] {
----------------
I believe `const auto &` should be safe here even with the call to
`dumpChild()` because the lambda captures `Assoc` by copy (even if the iterator
does not return a reference, the value will be lifetime extended because of the
const reference). Same applies elsewhere.
Repository:
rC Clang
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D57106/new/
https://reviews.llvm.org/D57106
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