Re: Question about OO programming in Ada

[Chad Bremmon <bremmon@acm.org>]

Dmitry A. Kazakov wrote:

> On Mon, 08 Dec 2003 08:42:28 GMT, Hyman Rosen <hyrosen@mail.com>
> wrote:
> 
> 
>>Chad Bremmon wrote:
> 
> 
>>>Because Ada 95 is strongly typed, the compiler can figure out at compile 
>>>time which function to call, based on the type.  This ensures that 
>>>dispatching is done during compile time.  You still have the look and 
>>>feel of polymorphism, without the non-deterministic nature of runtime 
>>>dispatching.
>>
>>What makes you think that C++ is any different in this regard?
> 
> 
> C++ does not distinguish class-wide and specific types. The same
> object is treated as a class-wide or specific depending on the
> run-time call context. For example:
> 
> class X 
> {
> public :
>    virtual void Foo ();
>    virtual void Baz ()
>    {
>       Foo ();
>    }
>    ...
> 
> Is the call to Foo from Baz dispatching? The answer is, well,
> sometimes it will. Arguably C++ is not strongly typed because no type
> could be addressed to implicit "this" in Baz, and so to the actual
> type [*).

This function would NEVER dispatch in Ada95.  Unless the parameter to a 
class is of a Tagged_Type'Class, there is no possibility for dynamic 
runtime dispatcing, period.  End of story. You can't get there!

If you still disagree, code it up and show it to me!

Even if you use the TaggedType'Class as a parameter to a primitive 
operation, the compiler will do it's damndest to get the dispatching 
figured out at compile time.

The above C++ code snipped shows my point.  In C++ you can NEVER know if 
the compiler is going to throw in a dispatching table and another level 
of non-deterministic indirection waiting to get that function called.
> 
> In Ada one can always statically determine whether a call is
> dispatching. Unfortunately, this advantage remains largely unused
> because of redispatching support and embedded tags.
> 
> -------
> [*]  "this" is a class-wide pointer when Baz was not directly or
> indirectly called from a constructor/destructor. It is a specific
> pointer otherwise.
> 
> --
> Regards,
> Dmitry Kazakov
> http://www.dmitry-kazakov.de