On Sat, 21 Jan 2023, 12:34 , <language.lawyer_at_[hidden]> wrote:

> On 21/01/2023 17:31, Edward Catmur wrote:
> > On Sat, 21 Jan 2023, 12:25 language.lawyer--- via Std-Proposals, <
> > std-proposals_at_[hidden]> wrote:
> >
> >> On 20/01/2023 12:42, Jens Maurer via Std-Proposals wrote:
> >>> On 20/01/2023 07.55, Julien Allali via Std-Proposals wrote:
> >>>> Hi everyone,
> >>>>
> >>>> I checked the following code from one of my student:
> >>>>
> >>>> class I{
> >>>> public:
> >>>> virtual ~I(){}
> >>>> virtual void print(){
> >>>> printf("hello %p\n",this);
> >>>> }
> >>>> virtual void f()=0;
> >>>> };
> >>>>
> >>>> class B : public virtual I{
> >>>> int i,j;
> >>>> public:
> >>>> ~B(){}
> >>>> virtual void f(){
> >>>> printf("B %p\n",this);
> >>>> }
> >>>> };
> >>>>
> >>>>
> >>>> void f(I *i){
> >>>> for(int j=0;j<5;++j)
> >>>> {
> >>>> i->print();
> >>>> i->f();
> >>>> printf("i=%p ",i);
> >>>> i++; // PROBLEM IS HERE
> >>>> printf(" => %p \n",i);
> >>>> }
> >>>> }
> >>>>
> >>>> int main(){
> >>>> B b[]={B(),B()};
> >>>> printf("B: %p %p\n",b,b+1);
> >>>> f(b);
> >>>> }
> >>>>
> >>>> As you can see, I is a non instantiable type as it has one pure
> virtual
> >>>> method. My concern is about the line "i++": I believed it will lead to
> >>>> an error or at least a warning (g++ 11.3.0)... Indeed, I can not
> imagine
> >>>> a valid case where doing arithmetic on a pointer of a type with pure
> >>>> virtual function can be valid.... Shouldn't the standard forbid such
> >>>> arithmetic?
> >>>
> >>> Why is the code iterating over 5 elements of a two-element array to
> >>> start with?
> >>>
> >>> Anyway, the "i++" is undefined behavior per [expr.add] p6:
> >>>
> >>>> For addition or subtraction, if the expressions P or Q have type
> >> “pointer to cv T”,
> >>>> where T and the array element type are not similar (7.3.6), the
> >> behavior is undefined.
> >>
> >> In f(b), first we get array-to-pointer conversion to get a pointer to
> the
> >> first array element, and then conversion to pointer to its I base class
> >> subobject, so the pointer type and the object it points to are aligned.
> >> Which means there is no UB per [expr.add]/6.
> >>
> >
> > "similar" isn't specified in terms of alignment.
>
> "aligned" == "coherent/matching/in agreement"
>

I have no idea what you mean by that. Are you saying that the array element
type and the base class type are similar?

>