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From: tmoran@bix.com
Subject: Re: Naming of Tagged Types and Associated Packages
Date: 1998/07/26
Message-ID: <6pfs36$8rh@lotho.delphi.com>#1/1
X-Deja-AN: 375084698
Organization: Delphi Internet Services <http://www.delphi.com/>
Newsgroups: comp.lang.ada
Date: 1998-07-26T00:00:00+00:00
List-Id: <comp.lang.ada>

>The addition of tagged types in Ada 95 is good, but it doesn't change
>implementation techniques that much from Ada 83, because you should
>already be using type-based abstractions where that makes sense.
  There are some things Ada 95 lets you do that were *extremely*
inconvenient, in Ada 83.  Controlled types, which let a task execute
code after an "or terminate" select alternative, and "access T'Class"
which lets you link heterogenous data simply, come to mind.  For a
slightly more complex example, a system I'm working on has a
library-level task and task safety (and the OS, in this case) requires
that certain things only be done by that task.  Occasionaly, we've
needed to add a new operation.  At first we added a new 'entry' and
recompiled the world.  Then we realized that there can be one entry
that takes a classwide parameter and calls a standard "Do_Whatever"
procedure, passing that parameter.  Now a new operation merely
requires a new descendant of the (abstract) root type with appropriate
information in the new extension part, and an overriding "Do_Whatever"
that will be run from the correct task.  No spec changes to the task
and thus no recompilation.  Tagged types are a new tool.  There's no
requirement to limit their use to what the designers, or OO
philosophy, intended.