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"