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From: boschg@cs.utwente.nl (Geert Bosch)
Subject: Re: Extending A Generic Signature Package
Date: 1997/03/21
Message-ID: <5gtvbv$1nj@pandora.cs.utwente.nl>#1/1
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References: <5gkv23INN3rn@thalamus.cis.ohio-state.edu>
 <E78yEu.15.0.-s@inmet.camb.inmet.com> <5gn90o$gm1@sutton.cs.columbia.edu>
X-Server-Date: 21 Mar 1997 12:36:15 GMT
Organization: University of Twente, Dept. of Computer Science
Newsgroups: comp.lang.ada
Date: 1997-03-21T12:36:15+00:00
List-Id: <comp.lang.ada>


Alexander V. Konstantinou (akonstan@news.cs.columbia.edu) wrote:
: OO people will tell you to use inheritance, but there is
: really no reason to pay for the overhead when your abstraction
: can be served by a compile-time check.

You shouldn't confuse using tagged types with dynamic dispatching. 
Suppose you have this situation:

   type Base is tagged record ... end record;
   procedure Op1 (X : in out Base);
   procedure Op2 (X : in out Base);

   type Extended is new Base with ...;
   procedure Op1 (X : in out Extended);
   procedure Op3 (X : in out Extended);

In this case you can just declare an object of type Extended and use
the operations Op1, Op2 and Op3 without any overhead due to dispatching. 
Dispatching occurs when you use (pointers to) class-wide types like
in the following case:

   declare
       type Object_Ptr is access all Base'Class;
      X : Object_Ptr := new Base;
   begin
      Op1 (X.all);
   end;
   
So whether a call to a primitive operation on a tagged type is dispatching or
not is determined at the call-site. I don't know why you wouldn't want to use
tagged types in your situation. The per-object space overhead shouldn't be
more than one or to pointers in most compilers. 

Regards,
   Geert