OXC@psuvm.psu.edu (10/02/90)
>I'm interested in Virtual and dynamic topics with complete coverage. I'm >new to OOP and would like some help wadeing through the Objects unit supplied >with TP 5.5! Any personal experience and/or knowledge of pitfalls, etc. >would be welcome. ... I have recently spent some time with Object-Oriented Programming Turbo 5.5 style. Here are some of my experiences. In retrospect, my first exposure to OOP concepts which involved how different vehicles stop, is not a bad way explain polymorphism, but at the time, I felt sidetracked by this and similar examples. Currently, I think it best to initially learn OOP with as little fanfare as possible: by treating OBJECTs as just another programming construct, and using more typical examples, or at least ones that are easy to relate to. If you have implemented Turbo Pascal Units (and know records), you will find it easy to implement TP OBJECTS (especially if you avoid virtual methods and dynamic objects at first). You may start with something simple such as an abstract data type that you have already implemented as a UNIT. It's then a matter of placing data fields and method headings together in an object. Try an array implementation of an Integer stack even though there are may be no descendants. Borland's DragIt operation (explained in their OOP guide) was enough motivation to look further into virtual methods but I found abstract objects such as BankAccount and LibraryItem were more appropriate at this point. These are simple objects that have easily identified descendants, common operations and data fields. Thinking in is-a relationships, a Savings account is-a BankAccount with an ApplyInterest operation. I found virtual methods easier to comprehend after I had time to let some simple object implementations and basic OOP concepts to sink in. Again, simple objects were better than more complex ones at first. For example, a Book is-a LibraryItem that may have a Fine operation called in an abstract operation called LibraryItem.Return. Other descendants of LibraryItem may have different Fine and CheckOut operations called when the abstract operation LibraryItem.Checkout is eventually reached. For instance, A VHSTape is-a LibraryItem with a two dollar borrowing charge. This virtual method should display a message like 'Please pay $2.00' whereas Book.Checkout would not. The next thing to tackle is Dynamic OBJECTS. (Thomas: I had trouble wading through UNIT Objects also). Things became less muddy when I implemented a Stack and a Queue by extending OBJECT Node and OBJECT List contained in the Objects unit distributed with Turbo 5.5. Implementing these common abstract data types required overcoming these hurdles: Typecast operations are necessary to override Turbo's one-way object type compatibility. Use of the Self parameter to find the previous node in FUNCTION Node.Prev was a bit awkward to understand although it does save the extra pointer field of a doubly linked circular list. The documentation (OOPDEMOS.DOC) claims that procedure Append will append a node as the first node on the list. Actually, the new node becomes the last (at least I think so). PROCEDURE New and PROCEDURE Dispose have been extended to also act like functions. In order to extend OBJECT List, careful attention must be paid to this dual use of New and Dispose when inserting or appending nodes. All in all, extending Objects List and Node was one way to experience the extension (modification) of a unit containing objects without changing that source code. Rick Mercer Penn State Berks Campus oxc@psuvm
dmo@cs.vu.nl (=Osinga Douwe M) (10/09/90)
Hi,
How do I change the default (8x8) font used by GRAPH? And why does the
hercules card driver not include characters above 128? Anybody with an
answer to any of these questions please email or send to the news.
Douwe M Osinga. (dmo@cs.vu.nl)bobb@vice.ICO.TEK.COM (Bob Beauchaine) (10/10/90)
In article <7877@star.cs.vu.nl> dmo@cs.vu.nl (=Osinga Douwe M) writes: >Hi, > >How do I change the default (8x8) font used by GRAPH? And why does the >hercules card driver not include characters above 128? Anybody with an >answer to any of these questions please email or send to the news. > > Douwe M Osinga. (dmo@cs.vu.nl) Two possibilities come to mind: 1. You haven't read your documentation thoroughly enough. You seem to know that the default font is indeed 8x8 and is used by Graph, but haven't discovered the 'settextstyle' function. 2. You don't have proper documentation, which invites unlimited speculation... Either way, look up 'settextstyle'. As for the Herc question, sorry, I've never owned one.