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Re: [std-proposals] about incrementing a pointer of a type with pure virtual function

From: Edward Catmur <ecatmur_at_[hidden]>
Date: Mon, 23 Jan 2023 09:49:05 +0000
On Sun, 22 Jan 2023 at 08:32, Thiago Macieira via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> On Saturday, 21 January 2023 06:20:25 PST Edward Catmur via Std-Proposals
> wrote:
> > Right. The existence of that unary array is somewhat questionable, since
> it
> > would not be possible to create such an array within the program. That
> > aside, you are correct which means that the UB occurs in the next
> > iteration, when the past the end iterator is indirected.
> Which points to a valid use-case of sizeof in an abstract class and
> pointer
> math with it.
> template <typename It, typename L> void apply(It begin, It end, L lambda)
> {
> for ( ; begin <= end; ++begin)
> lambda(*begin);
> }
> void f(I *ptr)
> {
> apply(ptr, ptr+1, [](auto p) { p->something(); });
> }

That only uses pointer arithmetic, not `sizeof`?

I'm not saying this is good code, but it's perfectly valid. I know I've
> used
> the "any object is an array of 1" technique.
> https://gitlab.com/thiagomacieira/qtbase/-/blob/master/src/corelib/tools/
> qgenericarray.h#L310-313
> <https://gitlab.com/thiagomacieira/qtbase/-/blob/master/src/corelib/tools/qgenericarray.h#L310-313>
> (this code is not in production, it was a prototype)

 You aren't going to be able to create a QGenericArray of abstract class
`I`, though?

I feel like we should be moving away from 1-element array views now we have
iterator-sentinel pairs (e.g. for std::views::single). I guess you might
want to erase down to a pointer pair, though.

Also, there's the possibility you might want to inspect the memory of an
abstract base class, e.g. as_bytes(span(static_cast<I*>(&obj), 1)). You
need either sizeof or pointer arithmetic to do that, so I don't think we
can talk about making them ill-formed.

Still, a warning flag would be good to experiment with: -Wabstract-sizeof,

Received on 2023-01-23 09:49:18