dexonsmith added inline comments.
================
Comment at: llvm/include/llvm/ADT/SCCIterator.h:165-170
+ SCCProxy operator*() const {
assert(!CurrentSCC.empty() && "Dereferencing END SCC iterator!");
return CurrentSCC;
}
+ SCCProxy operator->() const { return operator*(); }
----------------
dblaikie wrote:
> rusyaev-roman wrote:
> > dblaikie wrote:
> > > I always forget in which cases you're allowed to return a proxy object
> > > from an iterator - I thought some iterator concepts (maybe random access
> > > is the level at which this kicks in?) that required something that
> > > amounts to "there has to be a real object that outlives the iterator"
> > >
> > > Could you refresh my memory on that/on why proxy objects are acceptable
> > > for this iterator type? (where/how does this iterator declare what
> > > concept it models anyway, since this removed the facade helper?)
> > A proxy object is allowed to be returned while dereferencing an `input
> > iterator` (https://en.cppreference.com/w/cpp/named_req/InputIterator#Notes)
> >
> > ```
> > The reference type for an input iterator that is not also a
> > LegacyForwardIterator does not have to be a reference type: dereferencing
> > an input iterator may return a proxy object or value_type itself by value
> > ```
> >
> > For our case (that's `forward iterator`) we need to satisfy the following
> > thing:
> > ```
> > The type std::iterator_traits<It>::reference must be exactly
> > ...
> > * const T& otherwise (It is constant),
> >
> > (where T is the type denoted by std::iterator_traits<It>::value_type)
> > ```
> > I'll also update the patch according to this point. Other things are ok for
> > using a proxy object.
> Thanks for doing the legwork/quotations there.
>
> so what's the solution here, if we're going to meet the forward iterator
> requirements but want a proxy object?
IIRC, one solution is to store a proxy object inside the iterator, make `using
value_type = ProxyT`, update what the stored proxy points at on `operator*()`,
and return `ProxyT&` when dereferencing. But maybe I'm misremembering. (I'm
pretty sure `iterator_facade_base` has machinery to help with this stuff,
either way; might be worth looking at other uses of it.)
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D131448/new/
https://reviews.llvm.org/D131448
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