On Mon, 23 Jan 2023 at 16:34, Thiago Macieira <thiago_at_[hidden]> wrote:

> On Monday, 23 January 2023 01:49:05 PST Edward Catmur wrote:
> > > void f(I *ptr)
> > > {
> > > apply(ptr, ptr+1, [](auto p) { p->something(); });
> > > }
> >
> > That only uses pointer arithmetic, not `sizeof`?
>
> It's implicit there in the pointer math.
>

At runtime and if the machine is byte-addressed, sure, although this is
splitting hairs. We can do pointer arithmetic without sizeof in constant
evaluation:

struct S {}; // MSVC allows S to be incomplete (??)
extern S a[4];
template<auto> int i;
int& j = i<&a[1]>;

> >
> 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?
>
> No, you wouldn't, but the fact that this is a generic array is not
> important.
>
> > 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.
>
> That code was a prototype from 2012 that still needed to support C++98.
> This
> container didn't have emplacement functions. The full production version
> does,
> so the trick wasn't necessary.
>

Right, but as the offending code becomes less necessary, the weighting
towards adding warnings (as QoI, as you say) improves since the likelihood
of false positives decreases or points towards replacement with safer
idioms.

> 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.
>
> Yes, good point.
>
> > Still, a warning flag would be good to experiment with:
> -Wabstract-sizeof,
> > -Wabstract-pointer-arithmetic.
>
> QoI.
>
> --
> Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
> Software Architect - Intel DCAI Cloud Engineering
>
>
>
>