From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: *
X-Spam-Status: No, score=1.2 required=5.0 tests=BAYES_00,FROM_WORDY,
	INVALID_MSGID autolearn=no autolearn_force=no version=3.4.4
X-Google-Language: ENGLISH,ASCII-7-bit
X-Google-Thread: 103376,47def5aa7b3182bd
X-Google-Attributes: gid103376,public
From: "Nick Roberts" <Nick.Roberts@dial.pipex.com>
Subject: Re: How to write TYPECASE in Ada 95?
Date: 1999/02/06
Message-ID: <79gmmp$eli$4@plug.news.pipex.net>#1/1
X-Deja-AN: 441273288
References: <79fct8$9k3$1@murdoch.acc.Virginia.EDU>
 <Pine.BSF.4.05.9902051123560.3807-100000@shell5.ba.best.com>
 <1103_918264881@DZOG-CHEN>
X-MimeOLE: Produced By Microsoft MimeOLE V4.72.3110.3
Organization: UUNET WorldCom server (post doesn't reflect views of UUNET
 WorldCom)
Newsgroups: comp.lang.ada
Date: 1999-02-06T00:00:00+00:00
List-Id: <comp.lang.ada>

Given

   type Root_Type is abstract tagged ...;

and

   type Variation_1 is new Root_Type with ...;
   type Variation_2 is new Root_Type with ...;

and you have a particular 'something' you might want to do to an object
which could be either of type Variation_1 or Variation_2, you might find it
clearest to create an abstract procedure

   procedure Something (X: Root_Type; ...) is abstract;

and then override this procedure with

   procedure Something (X: Variation_1; ...) ...

whose body does what is appropriate for objects of type Variation_1, and
then

   procedure Something (X: Variation_2; ...) ...

whose body does what is appropriate for objects of type Variation_2. A call
such as

   Something(A,...);

will then automatically call the correct version of Something, based on the
type of object A. This is called 'dispatching'.

Dispatching generally obviates any need for a TYPECASE construct. However,
it is sometimes the case that all those separate procedures would be
overkill. Given

   type Subvariation_1_1 is new Variation_1 with ...;
   type Subvariation_1_2 is new Variation_1 with ...;

you may have a piece of code which operates on objects of type Variation_1,
but has an extra operation, somewhere in the middle, necessary for objects
of type Subvariation_1_2 (say). In these cases, it is more sensible to use
the 'in' operator to factor out this snippet of code, e.g.

   ...
   if X in Subvariation_1_2'Class then
      ...
   end if;
   ...

You should (almost) always test for membership of a class. E.g. test "X in
Subvariation_1_2'Class" rather than "X in Subvariation_1_2", so that further
descendants of Subvariation_1_2 (if you should add any at a later date) will
still be catered for.

Use the 'in' operator, not 'Tag. The need to use 'Tag is very rare (and
always to do with I/O or interfacing).

-------------------------------------------
Nick Roberts
-------------------------------------------