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

From: Jens Maurer <jens.maurer_at_[hidden]>
Date: Fri, 20 Jan 2023 08:42:35 +0100
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);
> 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.

This is a rather broad prohibition that can, in general, not be checked
by a compiler (halting problem). I agree it can be checked in the narrow
case of a pointer to an abstract class, but that might not be worth the
bother. If you feel you want a guaranteed diagnostic in that case, submit
a short paper to EWG: https://isocpp.org/std/submit-a-proposal

The paper should contain good rationale why diagnosing a comparatively
rare single case among the many undefined behavior cases in this area is
worth the (standardization and implementation) effort.


Received on 2023-01-20 07:42:38