Re: How to write TYPECASE in Ada 95?

["Nick Roberts" <Nick.Roberts@dial.pipex.com>]

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).

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Nick Roberts
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