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=-0.6 required=5.0 tests=BAYES_00,FREEMAIL_FROM, HELO_NO_DOMAIN,MAILING_LIST_MULTI,RDNS_NONE,REPLYTO_WITHOUT_TO_CC, SPOOFED_FREEMAIL_NO_RDNS autolearn=no autolearn_force=no version=3.4.4 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: 103376,8adc09d3ab9fad6e,start X-Google-Attributes: gid103376,public X-Google-ArrivalTime: 2002-10-07 19:18:03 PST Path: archiver1.google.com!news1.google.com!newsfeed.stanford.edu!logbridge.uoregon.edu!fr.usenet-edu.net!usenet-edu.net!enst.fr!not-for-mail From: "Rick Duley" Newsgroups: comp.lang.ada Subject: Ada OO Date: Tue, 08 Oct 2002 02:17:07 +0000 Organization: ENST, France Sender: comp.lang.ada-admin@ada.eu.org Message-ID: Reply-To: comp.lang.ada@ada.eu.org NNTP-Posting-Host: marvin.enst.fr Mime-Version: 1.0 Content-Type: text/plain; format=flowed X-Trace: avanie.enst.fr 1034043482 81217 137.194.161.2 (8 Oct 2002 02:18:02 GMT) X-Complaints-To: usenet@enst.fr NNTP-Posting-Date: Tue, 8 Oct 2002 02:18:02 +0000 (UTC) Return-Path: X-Originating-IP: [202.83.72.120] X-OriginalArrivalTime: 08 Oct 2002 02:17:08.0282 (UTC) FILETIME=[CD7EB1A0:01C26E70] Errors-To: comp.lang.ada-admin@ada.eu.org X-BeenThere: comp.lang.ada@ada.eu.org X-Mailman-Version: 2.0.13 Precedence: bulk List-Unsubscribe: , List-Id: comp.lang.ada mail<->news gateway List-Post: List-Help: List-Subscribe: , Errors-To: comp.lang.ada-admin@ada.eu.org X-BeenThere: comp.lang.ada@ada.eu.org Xref: archiver1.google.com comp.lang.ada:29572 Date: 2002-10-08T02:17:07+00:00 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_Class 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_Class 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 shoo\uld 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 _________________________________________________________________ Send and receive Hotmail on your mobile device: http://mobile.msn.com