Re: Preventing type extensions

[Cyrille <comar@eu.adacore.com>]

On Oct 5, 7:02 pm, "J-P. Rosen" <ro...@adalog.fr> wrote:
> Hmmm... pessimistic testing is certainly impractical for C++, but I
> wonder if it applies to Ada as well.

I don't see any reason why it would be more practical in one language
than the other. It depends on the amount of dispatching calls and the
size of your class hierarchy.

> >> To conclude about differentiating T and T'Class, the trick you suggest
> >>> here is easily implementable in other OO languages. There is nothing
> >>> magic in creating a wrapper around a given dispatching call and use
> >>> this wrapper at each dispatch point.
>
> Not at all, in other languages either a subprogram is a method (part of
> the dispatch table), or it applies to a single type. Class-wide
> operations are not dispatched, but still can be applied to a whole
> hierarchy. OK, I dismiss "void *" parameters...

Well my C++ is a bit rusty but I think this is exactly the other way
around. a subprogram is a method or it applies to the whole class
hierarchy by default  and not a single type as you seem to think. Here
is an example in C++, 'subp' has a parameter that is a ref to the
specific class C1 and you can pass it an object of any descendant of
C1 without any problems. Looks like what you call a classwide
operations to me... In the body you can even choose between
dispatching or not dispatching using a slightly different syntax as
shown. So really, classwide programming is not specific to Ada... only
the explicit notation is.

<<
#include <iostream>
using namespace std;
class C1 {
public:
  virtual int m() {return a;}
  C1 () { a = 1; }
  int a;
};

class C2: public C1 {
public:
  virtual int m() {return b;}
  C2 () { a = 2; b = 3;}
  int b;
};

void Subp (C1 &c) {
  cout << "dispatching to m " << c.m() << endl;
  cout << "not dispatching to C1::m "  <<  c.C1::m () << endl;

}

int  main () {

    C1 c1;
    C2 c2;
    Subp (c1);
    Subp (c2);
}

>>

>
> If you (or anyone else on this list, or even in outer space ;) ) have
> good ideas about what to put in that profile, by all means, come to the
> workshop, or make a proposal (private mail to me, I'll gather proposals).
>

We definitely have ideas and are planning to work on such a profile in
the near future but not in the schedule of the workshop so I'm afraid
we will have to synchronize at a later date.