From: "Rick Duley" <rickduley@hotmail.com>
Subject: OO in Ada
Date: Fri, 04 Oct 2002 02:14:44 +0000
Date: 2002-10-04T02:14:44+00:00 [thread overview]
Message-ID: <mailman.1033697703.29112.comp.lang.ada@ada.eu.org> (raw)
Hi all
I believe that I am the last surviving academic Ada programmer in Perth,
Western Australia and I am in need of some help. I am having problems
coming to terms with the rationale behind Object Oriented programming in Ada
and there is no-one I know in Perth to talk to. I'm hoping you folk won't
mind giving me a hand.
Perhaps I had better start by explaining my (mis-) understanding of the
general principle of inheritance.
Suppose I have defined an object Pen to have the form:
+-------------------------------------------+
| Pen |
+-------------------------------------------+
| X : Integer; |
| Y : Integer; |
| Color : Color_Type; |
+-------------------------------------------+
| procedure Move_To(This : in out Pen; |
| X : in Natural; |
| Y : in Natural); |
| -- move the pen to the specified location |
| -- without drawing |
| |
| procedure Draw_To(This : in out Pen; |
| X : in Natural; |
| Y : in Natural); |
| -- draw a line from the current location |
| -- to (X,Y) using the pen's |
| -- current color. the pen's location |
| -- moves to (X,Y) |
| |
| function New_Pen(X : Natural; |
| Y : Natural; |
| Color : Color_Type) |
| return Pen_Access; |
| -- returns access to an initialised Pen |
+-------------------------------------------+
This object should be initialised by New_Pen and all the functionality it
should need should be supplied by the other two methods. Of course, if you
want to change Pen Colour in midstream you would have to provide the
functionality to do so but let's keep this simple so I can get a handle on
it ;)
Extension of this class to Thick_Pen should, according to me, result in a
class having the form:
+-------------------------------------------+
| Thick_Pen |
+-------------------------------------------+
| X : Integer; |
| Y : Integer; |
| Color : Color_Type; |
| Thickness : Positive; |
+-------------------------------------------+
| --=== inherited from Pen_Class ===-- |
| procedure Move_To(This : in out Pen; |
| X : in Natural; |
| Y : in Natural); |
| -- move the pen to the specified location |
| -- without drawing |
| |
| procedure Draw_To(This : in out Pen; |
| X : in Natural; |
| Y : in Natural); |
| -- draw a line from the current location |
| -- to (X,Y) using the pen's |
| -- current color. the pen's location |
| -- moves to (X,Y) |
| |
| function New_Pen(X : Natural; |
| Y : Natural; |
| Color : Color_Type) |
| return Pen_Access; |
| -- returns access to an initialised Pen |
| |
| --=== new in Thick_Pen_Class ===-- |
| procedure Move_To(This : in out Thick_Pen;|
| X : in Natural; |
| Y : in Natural); |
| -- move the pen to the specified location |
| -- without drawing |
| |
| procedure Draw_To(This : in out Thick_Pen;|
| X : in Natural; |
| Y : in Natural); |
| -- draw a line from the current location |
| -- to (X,Y) using the pen's |
| -- current color. the pen's location |
| -- moves to (X,Y). The |
| -- vertical thickness of the pen is given |
| -- by Thickness. |
| |
| function New_Thick_Pen |
| (X : Natural; |
| Y : Natural; |
| Color : Color_Type; |
| Thickness : in Positive) |
| return Thick_Pen_Access; |
| -- returns access to initialised Thick_Pen|
|-------------------------------------------+
I would expect all the class attributes, the data variables, to be accessed
by the methods of the derived class with the overloading providing the
polymorphism. My problems start when this doesn't happen.
1. All the texts I can find which deal in any degree at all with OO in Ada
teach that the tagged record in an object declaration should be 'private'.
When you follow this line, a derived class does not have direct access to
the object attributes of the base class, i.e. _there_is_no_inheritace_.
2. To provide the derived class with access to the object attributes of
the base class I have to create user-defined methods in the base class.
This has two effects:
a) the object attributes of the base class are now effectively public,
i.e. accessible (through the user-defined methods) to any client module
through 'with' and 'use' - which effectively negates the act of making them
private in the first place;
b) having to provide accessibility in this manner emphasises the fact
that inheritance did not occur.
3. If the tagged record in the base class is left public and the derived
class is in a child package of the package defining the base class, then the
base class attributes are accessible to the derived class. So far so good
:) However, if in a program 'use'ing the child package of 'Thick_Pen' (and
not mentioning the base package of 'Pen') I make a call to the routine
'Draw' with an actual parameter of the type 'Pen' I get a compiler error
message to the effect that 'Draw' is not visible. In other words, Thick_Pen
has not inherited the operation Draw for 'Pen' Again,
_there_is_no_inheritace_.
4. Further along that line, Thick_Pen does not inherit the type Pen so I
cannot declare a Pen (or Pen_Access) unless I 'with' and 'use' the package
in which Pen is declared. This means that making the package in which
Thick_Pen is declared a child of the package in which Pen is declared
totally useless. _There_is_no_inheritance_!
I have to say that this is the first time in pretty near a decade I have
been writing in and teaching with Ada that Ada hasn't come up with the
goods. Do I labour under some serious misunderstanding, do I have something
terribly wrong?
5. One final thing (for this time anyway ;), why is it that that Ada does
not use the intuitive 'object.method' syntax for making calls to and object.
This would mean that (in the case described in section 3) the call would
read
Pen.Draw(To_X => n, To_Y => n);
and with inheritance this would be accessible through the child package
declaring Thick_Pen. While I'm okay with using the existing syntax, I feel
the 'object.method' syntax is more intuitive and in line with OO thinking.
If there are people working on Ada0x then maybe we should be putting this
forward for consideration.
So, you see, I'm all at sea. Can someone help me out?
Thanks
-------------------------------------------------
Rick Duley
23/209 Walcott St
North Perth, Western Australia 6006
mob: +61 040 910 6049
/-_|\
/ \
perth *_.-._/
v
Experience is the worst of teachers
It gives you the exam
before it gives you the lecture
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next reply other threads:[~2002-10-04 2:14 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2002-10-04 2:14 Rick Duley [this message]
2002-10-04 2:55 ` OO in Ada Jim Rogers
2002-10-04 17:35 ` Hyman Rosen
2002-10-05 0:20 ` Jim Rogers
2002-10-05 23:38 ` Dmitry A.Kazakov
2002-10-05 15:25 ` Jim Rogers
2002-10-06 21:37 ` Dmitry A.Kazakov
2002-10-06 2:18 ` Hyman Rosen
2002-10-06 3:00 ` Jim Rogers
2002-10-08 21:08 ` Gisle Sælensminde
2002-10-04 3:37 ` Chad R. Meiners
2002-10-04 5:32 ` Simon Wright
2002-10-04 6:01 ` tmoran
2002-10-04 15:05 ` Matthew Heaney
2002-10-05 2:14 ` SteveD
2002-10-05 8:54 ` Preben Randhol
2002-10-07 14:10 ` Matthew Heaney
2002-10-07 19:52 ` Jeffrey Carter
2002-10-08 21:18 ` Dmitry A.Kazakov
2002-10-08 9:53 ` John McCabe
2002-10-08 15:37 ` Matthew Heaney
2002-10-08 16:47 ` Georg Bauhaus
2002-10-08 17:48 ` Matthew Heaney
2002-10-08 17:16 ` Warren W. Gay VE3WWG
2002-10-08 17:58 ` Matthew Heaney
2002-10-09 16:59 ` Warren W. Gay VE3WWG
2002-10-08 10:21 ` Preben Randhol
-- strict thread matches above, loose matches on Subject: below --
1998-05-15 0:00 Gisle S{lensminde
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