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From: Maciej Sobczak <no.spam@no.spam.com>
Newsgroups: comp.lang.ada
Subject: Constructing an object
Date: Wed, 21 Sep 2005 10:46:35 +0200
Organization: CERN - European Laboratory for Particle Physics
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Xref: g2news1.google.com comp.lang.ada:4988
Date: 2005-09-21T10:46:35+02:00
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Hi,

I'm learning Ada and I would like to better understand the ways used to 
create objects.

Let's say I have the following:

    type Color is (Black, Red, Green, Blue, White);

    type Shape is tagged record
       C : Color;
    end record;

    type Rectangle is new Shape with record
       A, B : Positive;
    end record;


 From what I've learnt ("Ada 95: The Craft of Object-Oriented 
Programming" by John English), there are two possible ways to implement 
"constructors" for these types:

1. By procedure, which is a primitive operation for each type in the 
hierarchy, for example:

    procedure New_Shape(C : in Color; S : out Shape) is
    begin
       S.C := C;
    end;

    procedure New_Rectangle(A, B : in Positive; C : in Color; R : out 
Rectangle) is
    begin
       New_Shape(C, Shape(R));
       R.A := A;
       R.B := B;
    end;

    -- another version (btw - which version do you prefer?)
    procedure New_Rectangle(A, B : in Positive; C : in Color; R : out 
Rectangle) is
       St : Shape;
    begin
       New_Shape(C, St);
       R := (St with A => A, B => B);
    end;


This does not please me much, because I can have uninitialized objects, 
which may not make sense in the program:

    R : Rectangle;

2. By function:

    function New_Shape(C : in Color) return Shape is
       S : Shape;
    begin
       S.C := C;
       return S;
    end;

    function New_Rectangle(A, B : in Positive; C : in Color) return 
Rectangle is
       R : Rectangle;
    begin
       Shape(R) := New_Shape(C);   -- is this OK?
       R.A := A;
       R.B := B;
       return R;
    end;

This does not please me for the same reason - no enforcement of proper 
initialization, if a given type makes no sense uninitialized.


What approach do you actually use in the real (non-tutorial) code?
Are there other techniques?
(I'm aware of the controlled types, but I need to provide parameters for 
construction, which the special Initialize procedure does not have.)


-- 
Maciej Sobczak : http://www.msobczak.com/
Programming    : http://www.msobczak.com/prog/