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From: "Ludovic Brenta" <ludovic@ludovic-brenta.org>
Newsgroups: comp.lang.ada
Subject: Re: procedural vs object oriented
Date: 27 Apr 2006 06:20:42 -0700
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Date: 2006-04-27T06:20:42-07:00
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Dmitry A. Kazakov wrote:
> On 27 Apr 2006 03:42:10 -0700, Ludovic Brenta wrote:
>> Dynamic dispatching, by definition, means that you don't know which
>> subprogram you call at run-time. The compiler guarantees that the call
>> will succeed (i.e. that there exists a subprogram to dispatch to),
>> but there is uncertainty about which one it is.
>
> (I have an impression that it will not be true in Ada 200Y.)

No, nothing changes in this respect, just synctactic sugar.

> OK, but this is the same uncertainty as in X + 1. You don't know X, so the
> result selected by "+" is uncertain (to you.) Are programs using "+"
> certifiable?

Yes, programs using "+" are certifiable. To obtain the certification,
you prove that X+1 is within range and that the operation does not
raise an exception. Ada's type safety features go a long way to help us
there, so we only have to test for a few boundary cases. SPARK takes
that to its logical conclusion, i.e. static proof.

>> A static procedure call has no uncertainty: when you read the program
>> source, you know exactly which subprogram is called, even in the
>> presence of overloading.
>
> Well, no:
>
> if Read (File) then
>    Foo;
> else
>    Bar;
> end if;
>
> The uncertainty of a dispatching call is one of the context, exactly as in
> the example above.

Nothing in your example is uncertain. Whatever path execution takes,
you know exactly what's happening.

> Provided, that there is nothing uncertain in how
> dispatching works or what potential targets do.

But with compiler-provided dynamic dispatching, there *is* uncertainty
as to how dispatching works and what the targets are.

-- 
Ludovic Brenta.