Re: How to write TYPECASE in Ada 95?

[Matthew Heaney <matthew_heaney@acm.org>]

"David C. Hoos, Sr." <david.c.hoos.sr@ada95.com> writes:

> procedure Test_Points is
> 
>    -- Since an object of Points.Point'Class must be constrained at the time
> of
>    -- declaration to some one of the types derived from Points.Point, and
>    -- therefore cannot be re-used for other types derived from Points.Point,
>    -- we use an access object, instead.  But first, we declare the type of
>    -- that access object, as well as a deallocation procedure for it.
>    type Point_Class_Access is access all Points.Point'Class;
> 
>    procedure Free is new Ada.Unchecked_Deallocation
>      (Object => Points.Point'Class,
>       Name   => Point_Class_Access);
> 
>    -- We do not need to initialize this access object here, but we do so
>    -- just to illustrate that it can be done.
>    P_Access : Point_Class_Access := new Points.Tall_Point'(3.0, 4.0, 5.0);


Note that there's another way to illustrate these concepts, that does
not require heap.  Just use a declare block to declare each new kind of
point, as follows:

with Ada.Text_Io;
with Ada.Unchecked_Deallocation;
with Points; use Points;
procedure Test_Points is
begin

   declare
      P : Tall_Point := (3.0, 4.0, 5.0);
   begin
      Points.Show_Type (P);
      Points.Show_Properties (Point'Class (P));
   end;

   declare
      P : Painted_Point := (3.0, 4.0, Points.Blue);
   begin
      Points.Show_Type (P);
      Points.Show_Properties (Point'Class (P));
   end;

end Test_Points;



Declare blocks are your friend.  Unnecessary use of heap is your enemy.