TweetFollow Us on Twitter

About MacApp
Volume Number:3
Issue Number:8
Column Tag:MacApp Applications

How to Think in MacApp

By Howard Katz, British Columbia, Canada

This is very important. You must think in Russian. You cannot think in English and transpose. Do you think you can do that, Mr. Grant?

With those immortal words, our hero, Clint Eastwood, alias Mr. Grant, steals a Russian thought-controlled steath fighter, out-shoots his way from behind the Iron Curtain, and flies to freedom. Learning to think in MacApp is a little like that thought-controlled Jet Fighter. You must think in MacApp. You cannot think in Pascal and transpose. Do you think you can do that Mr. Grant?

Thinking StarTrek in Object Pascal

If you’ve been at all intrigued by what you’ve been reading about MacApp and object-oriented programming, you’re not alone. Apple’s been promoting MacApp heavily, and a number of developers, myself included, have discovered that object-oriented programming is a new and exciting way of doing and thinking about applications. But if you’re even the slightest bit confused by what you’ve read, don’t feel too bad - again, you’re not alone. I had a lot of trouble when I was first starting out (an understatement!), and I’ve talked to other developers who’ve also experienced similar difficulties. Much of my confusion centered not so much on MacApp itself, but rather on the more fundamental language issues introduced by Object Pascal (aka MPW Pascal). If you don’t have a good, solid understanding of what objects are and how to work with them, you won’t have a hope in a hot place of understanding what MacApp is all about.

This article, then, is an attempt to focus on a few of these new language issues, to hopefully cast them in a new light. I don’t think my treatment here is really all that different from what’s been presented in Apple’s documentation, in Kurt Schmucker’s Object-Oriented Programming for Macintosh, or in earlier issues of MacTutor. In some cases, it’s simply a question of emphasis, or of looking at a particular concept or programming construct in a slightly different way.

To make this exposition as “real” as possible, I’m going to assume that we’re writing a hypothetical Star Trek game and use that as a vehicle for my discussion (I personally need to see lots of concrete code before I can understand new concepts; you might be similar). Anyway, my apologies to Gene Roddenberry and Trekkies everywhere for any mistakes; I’m not trying too hard to be accurate (although I am trying to be objective).

As you’re probably aware, the fundamental new programming structure introduced by Object Pascal is the object (if you knew that, a cigar). Objects are simply packages of data, together with the specific code that acts on that data. Objects present a good way of modeling the behavior of a particular programming entity.

In a Star Trek game, for example, a good candidate for such an entity might be one of the many ships that are manipulated during the game. Let’s consider, for example, creating an object that represents a Klingon warship. Such a Klingon object would represent one ship in our game. It would use its data fields to maintain information on its current weapons status, its position, and so forth. The methods belonging to the Klingon object would manipulate this information to enact the specific behavior we expect of Klingon vessels.

Creating this object in our program is going to involve coding statements in at least three different places in the program. First, in an INTERFACE section of our program we’re going to find something like the following:

 TKlingonVessel = OBJECT
 fNumTorpedoes :  INTEGER;
 ... { other relevant fields }
 PROCEDURE TKlingonVessel.LaunchTorpedoes;
 ... { other relevant methods }
 END;{ TKlingonVessel object type }

Notice, first of all, that this is a TYPE declaration, and that somewhere else in our code we can therefore expect to find a corresponding VAR declaration for a variable of this type. In particular, this is a declaration for an object type. This object-type declaration is our first interesting extension of standard Pascal syntax. It shares some of the characteristics of a RECORD type, except, most notably, that standard Pascal records don’t contain procedures as fields. Strange concept number one. Also note that the procedure name LaunchTorpedoes is prefixed by the object-type name, TKlingonVessel.

The naming conventions in the above piece of code, by the way, are just that - conventions. Object type identifiers start with a “T” and data fields start with an “f”. I’ll point out later why these conventions are useful.

Somewhere else in our program we’ll find an IMPLEMENTATION section that contains the actual code for the procedure (ie, method) TKlingonVessel.LaunchTorpedoes. It might look something like the following:

PROCEDURE TKlingonVessel.LaunchTorpedoes;
 IF fNumTorpedoes > 0 THEN 
 fNumTorpedoes := fNumTorpedoes - 1;

The first interesting question I’d like to address is this: given this declaration of an object type and the IMPLEMENTATION of the single procedure it contains (or at least the single one I’ve shown), how do we invoke, or execute, the code for the procedure TKlingonVessel.LaunchTorpedoes?

If we were working in standard Pascal, the question would be so trivial as to be meaningless: you simply invoke the procedure by naming it at some point in your program. In Object Pascal, it’s not quite that simple. In Object Pascal, you can’t execute the code for this method until the object containing it has been created. And we haven’t created the object yet; we’ve simply declared an object TYPE, a template for the object to be.

This is one of the fundamental differences between standard Pascal and Object Pascal: in standard Pascal, code is fixed and immutable - it simply is. In Object Pascal, code has to be created on the fly at runtime before you can use it. Now, that’s a dramatic, though slightly inaccurate statement. It’s close enough to the way things work, however, to be useful.

How do we create the actual TKlingonVessel object and execute its code? The third piece of our program looks something like this:

 aKlingonVessel  :  TKlingonVessel;
 NEW( aKlingonVessel );
 aKlingonVessel.fNumTorpedoes := 10;

Obviously this piece of code is a wee bit strange - it’s unlikely that we’d create a new Klingon object and then immediately ask it to blindly launch a torpedo. I plead pedagogical considerations. At any rate, here’s the VAR statement for the variable I mentioned. This code fragment says that we’re going to create a new object, and that object will be of type TKlingonVessel as declared earlier. An object of this type will contain the data fields and methods that were declared for that object type. The NEW statement then actually creates the object at runtime and makes its fields and methods available for use.

This use of NEW is an extension of the standard Pascal NEW procedure. The compiler recognizes that we’re creating an object and not a standard data structure by the type of the variable that we’re NEWing, in this case aKlingonVessel.

Once we’ve created our object, its data fields become accessible. The statement

 aKlingonVessel.fNumTorpedoes := 10;

initializes the field fNumTorpedoes; prior to this statement, the value of the field was undefined. Note again the RECORD-like syntax used here. Only this time, we’re working with a variable and not an object type: note that the prefix, or qualifier, is changed accordingly.

Finally, we can execute the code of our launch procedure with the statement:


This creation of a new object is known as instantiation, a wonderful term; we have created an instance of this object type. Its data fields are now stuffable; its code is now executable.

To confuse matters just a bit (just when you thought you were getting things under control): the variable aKlingonVessel is not the object itself. Close, but no cigar. The variable aKlingonVessel is an object reference variable, or simply an object reference. Why?

The relationship between an object reference variable and an object is very similar to that between a handle and the handled block it points to. An object actually is a handled block, but with a few important differences from our standard understanding of the term. It floats on the heap, just like a normal handled block, and is just large enough to contain space for its data fields and code (well, almost). The handle itself, or more properly the object reference variable, is exactly four bytes long, as you’d expect for a handle.

OK, I was bending the truth - our object doesn’t actually contain the code for its methods, as I’ve stated. Rather, it contains a pointer that points to where the code actually resides in memory (and who knows, or cares where that is?). That’s why I said earlier that my statement about creating code on the fly at runtime is somewhat inaccurate - the code is already there; we just create the object that contains the pointer to it. Ken Doyle gave a good description of the method-table mechanism that handles this in the December ’86 issue of MacTutor (saving me from having to explain an implementation issue that I don’t fully understand anyway).

Syntactically, while an object-reference variable such as aKlingonVessel acts much like a handle, notice that we don’t have to use Pascal’s caret symbol to dereference it in order to get at the fields of the object it points to. The period separator is sufficient.

There’s one other interesting thing to look at. When we make the statement:


we might say that we’re invoking this method from outside the object. But once that method begins to execute, we are, in a sense, inside the object. I’m talking here about the subsequent code that gets executed by the above line:

IF fNumTorpedoes > 0 THEN BEGIN
 fNumTorpedoes := fNumTorpedoes - 1;

Notice, since we’re now on the inside looking out, that we needn’t qualify the fieldname fNumTorpedoes with the name of the object, aKlingonVessel, or the typename, TKlingonVessel. Either, in fact, would be an error. And here’s one place where naming conventions are useful: the “f” in “fNumTorpedoes” immediately tells us that this is a field belonging to our object, and not something else such as a global variable (in which case it would probably start with a “g,” again by convention). What’s important is that any of the data fields belonging to this object are accessible from within any of its methods, as long as the object exists. This is an extension of Standard Pascal’s scoping rules and has important consequences which we’ll look at later.

The matter of DoLaunch is slightly more involved. Since we’re inside a Klingon vessel object, DoLaunch might be the name of another method belonging to type KlingonVessel (that I haven’t shown), or it might be the name of a standard Pascal procedure that’s not a method at all. Again, once we’re inside an object and executing one of its methods, any other methods that we invoke that belong to that object are not qualified. Finally, there’s a minor variation on the first possibility that we’ll cover when we look at the subject of inheritance.

OK, we’ve now got Klingon vessel objects. More precisely, we’ve got one Klingon vessel object. This represents one ship. In a real Star Trek game, we would probably expect to find numerous Klingons, and there’s nothing to stop us from creating other objects of the same TKlingonVessel type. For example:

 aKlingon1:  TKlingonVessel;
 aKlingon2:  TKlingonVessel;
 NEW( aKlingon1 );
 aKlingon1.fNumTorpedoes := 10;
 NEW( aKlingon2 );
 aKlingon2.fNumTorpedoes := 30;

Now we’ve got two Klingon vessel objects floating in quadrant four, as well as in the heap. They share the same code (there are two pointers to the single method, LaunchTorpedoes), but it’s important to note that they each exist independently of the other one. In particular, their data fields are unique. This shouldn’t be a big surprise if you think about creating two RECORD variables in Pascal that are both based on the same type definition.

At the end of the above sequence of statements, aKlingon1 has 9 torpedoes left, and aKlingon2 has 29 torpedoes remaining.

OK, we’ve now got Klingon objects galore, one for every Klingon vessel in our game. Let’s back up a bit and put the above piece of code in context. The question is: where are these Klingons being created? In other words, who is creating them? Somebody has that responsibility.

In a typical game, we’ll probably have another object whose job it is to mind the board and keep track of turns and other things like that. We might call this the game object and declare it to be of type TGame. Our TGame object will also be responsible for creating all the vessels that are going to appear during the course of the game. This sequence of events (non-Macintosh usage here) is highly typical of the way most object-oriented programs behave at runtime: we initially instantiate one object; it in turn instantiates another; and so on down the line. (If you’re astute, you might well ask at this point what happens if we just keep on instantiating objects, knowing that every instantiation creates a new block in the heap. A very good question. Don’t ask; I’ll come back to this later).

In any event, if we go back and expand the above piece of code just a bit, it’ll look something like this:


 aKlingon1:  TKlingonVessel;
 aKlingon2:  TKlingonVessel;
 NEW( aKlingon1 );
 aKlingon1.fNumTorpedoes := 10;
 NEW( aKlingon2 );
 aKlingon2.fNumTorpedoes := 30;
 ... { other stuff }
END;  { TGame.NewVessel }

All I’ve really done is bracket the code we saw earlier between the name of the game method and an END statement. Again, we’re being somewhat unrealistic for the sake of pedagogy. It’s much more likely that this NewVessel method of our game object would be used to create one Klingon, and not two, at a time, and that we’d invoke it whenever we wanted to create a new one (as indicated by a menu or dialog selection, or whatever). Since these objects differ only in the number of torpedoes we initialize them with (at least according to the limited context I’m showing here), we’d probably pass in a parameter like NumTorpedoes that immediately gets stuffed into the fNumTorpedoes field. In other words:

TGame.NewVessel( NumTorpedoes : INTEGER );
 aKlingon :  TKlingonVessel;
 NEW( aKlingon );
 aKlingon.fNumTorpedoes := NumTorpedoes;
 ... { other stuff }
END;  { TGame.NewVessel }

To be able to keep track of individual Klingons, the TKlingonVessel type would also probably have a field called fID, and we’d increment this field by one for each new ship we added so that each Klingon had a unique number.

Rather than initializing our objects exactly as I’ve shown above, however, it’s a much more common practise to provide each object with its own initialization method, and pass our parameters to the method to let the object initialize its own fields. This occurs throughout MacApp. To wit:

NEW( aKlingon );
aKlingon.IKlingonVessel( NumTorpedoes );


TKlingonVessel.IKlingonVessel( NumTorpedoes:INTEGER);
 fNumTorpedoes := NumTorpedoes;
 ... { other initialization stuff }

What can we say about the name of the method, IKlingonVessel? Again, simply a matter of convention, in which an “I” (obviously standing for “Init”) is prefixed to the object name. OK, that’s a long digression. The main reason I’ve shown the above code is to pose one further query (I love doing that; can’t you tell?).

The question is this: once the delimiting END statement is reached in the NewVessel method, what happens to the objects that were created there? Well, in a word: nothing. They continue to exist in the heap, but there’s no longer any way to reference their fields or methods from outside them. The only way we had of doing so within the NewVessel block was to use our reference variable, aKlingonVessel (or aKlingon1 or aKlingon2, as appropriate). Pascal’s scoping rules say that these variables are local to the method and cease to be once the block is exited. This is a problem, since our game object is likely to want to communicate with them later on.

The answer is to realize again that an object-reference variable is just that: a variable. And the value of a variable is a perfectly good candidate for sticking into one of the data fields of our game object via a Pascal assignment statement. That way, since the fields of the object continue to exist as long as the object itself exists, we’ll be able to get at any “subordinate” objects that are referenced there at any time we like. First, we’ll have to add the necessary reference field to our TGame TYPE declaration:

 fTheKlingon :  TKlingon;
 ...    { other relevant fields }
 PROCEDURE TGame.NewVessel;
 ...  { other relevant methods }
 END;  { TGame object type }

We can then do the following simple assignment in our TGame.NewVessel method:

 aKlingonVessel :  TKlingonVessel;
 NEW( aKlingonVessel );
 fTheKlingon := aKlingonVessel; { <<-- }
 aKlingonVessel.fNumTorpedoes := 10;
 { or fTheKlingon.fNumTorpedoes := 10 }
 { ... etc. }

That’s it! We’ve now established a communcation link, if you will, between our game object and this particular Klingon vessel. No matter what other method of the game object may be executing later on, the game will be able request this Klingon to launch torpedoes or perform any of its other methods by using the fTheKlingon reference field. The syntax for doing so, by the way, is almost identical to what we’ve already seen. For example, if Klingons have a method that allows them to fire a phaser bank (I don’t even know if Klingons have phaser banks!), the game object can request one to do so simply by saying


We can even extend this usage into stranger realms. If our Klingon type has a method that allows it to scan neighboring quadrants for enemy warships and report their location, the game object can ask it to do so by saying

EnemyPosit := fTheKlingon.ReportEnemyPosit;

This is an example of a method that’s actually a function, rather than a procedure. This construct might seem somewhat strange if you haven’t encountered it before. I remember when I was reading the documentation and seeing constructs like this for the first time; there was a lot of head scratching. Hopefully, you’re not as slow as I was.

I’m going to leave it at that for the moment. There is a lot more. And we haven’t even talked about inheritance, overriding, SELF, or a number of other object-oriented subjects. Stay tuned next issue for Romulans, Vulcans, and the other denizens of deep space. Get objective.


Community Search:
MacTech Search:

Software Updates via MacUpdate

Arq 5.8.5 - Online backup to Google Driv...
Arq is super-easy online backup for Mac and Windows computers. Back up to your own cloud account (Amazon Cloud Drive, Google Drive, Dropbox, OneDrive, Google Cloud Storage, any S3-compatible server... Read more
Backblaze - Online backup servi...
Backblaze is an online backup service designed from the ground-up for the Mac. With unlimited storage available for $5 per month, as well as a free 15-day trial, peace of mind is within reach with... Read more
Instaradio 7.1 - Listen to your favorite...
Instaradio is fast, and it could be the radio player you have been waiting for. Try the app thousands of people rely on for listening to radio. Features Listen to radio from all around the world... Read more
EtreCheck 3.3.3 - For troubleshooting yo...
EtreCheck is an app that displays the important details of your system configuration and allow you to copy that information to the Clipboard. It is meant to be used with Apple Support Communities to... Read more
Hopper Disassembler 4.2.1- - Binary disa...
Hopper Disassembler is a binary disassembler, decompiler, and debugger for 32-bit and 64-bit executables. It will let you disassemble any binary you want, and provide you all the information about... Read more
Slack 2.6.2 - Collaborative communicatio...
Slack is a collaborative communication app that simplifies real-time messaging, archiving, and search for modern working teams. Version 2.6.2: Fixed Inexplicably, context menus and spell-check... Read more
Apple Final Cut Pro X 10.3.4 - Professio...
Apple Final Cut Pro X is a professional video editing solution.Completely redesigned from the ground up, Final Cut Pro adds extraordinary speed, quality, and flexibility to every part of the post-... Read more
Numi 3.15 - Menu-bar calculator supports...
Numi is a calculator that magically combines calculations with text, and allows you to freely share your computations. Numi combines text editor and calculator Support plain English. For example, '5... Read more
TunnelBear 3.0.14 - Subscription-based p...
TunnelBear is a subscription-based virtual private network (VPN) service and companion app, enabling you to browse the internet privately and securely. Features Browse privately - Secure your data... Read more
Apple iMovie 10.1.6 - Edit personal vide...
With an all-new design, Apple iMovie lets you enjoy your videos like never before. Browse your clips more easily, instantly share your favorite moments, and create beautiful HD movies and Hollywood-... Read more

Latest Forum Discussions

See All

Goat Simulator PAYDAY (Games)
Goat Simulator PAYDAY 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: ** IMPORTANT - SUPPORTED DEVICES **iPhone 4S, iPad 2, iPod Touch 5 or better Goat Simulator: Payday is the most... | Read more »
Zombie Gunship Survival Beginner's...
The much anticipated Zombie Gunship Survival is here. In this latest entry in the Zombie Gunship franchise, you're tasked with supporting ground troops and protecting your base from the zombie horde. There's a lot of rich base building fun, and... | Read more »
Mordheim: Warband Skirmish (Games)
Mordheim: Warband Skirmish 1.2.2 Device: iOS Universal Category: Games Price: $3.99, Version: 1.2.2 (iTunes) Description: Explore the ruins of the City of Mordheim, clash with other scavenging warbands and collect Wyrdstone -... | Read more »
Mordheim: Warband Skirmish brings tablet...
Legendary Games has just launched Mordheim: Warband Skirmish, a new turn-based action game for iOS and Android. | Read more »
Magikarp Jump splashes onto Android worl...
If you're tired ofPokémon GObut still want something to satisfy your mobilePokémon fix,Magikarp Jumpmay just do the trick. It's out now on Android devices the world over. While it looks like a simple arcade jumper, there's quite a bit more to it... | Read more »
Purrfectly charming open-world RPG Cat Q...
Cat Quest, an expansive open-world RPG from former Koei-Tecmo developers, got a new gameplay trailer today. The video showcases the combat and exploration features of this feline-themed RPG. Cat puns abound as you travel across a large map in a... | Read more »
Jaipur: A Card Game of Duels (Games)
Jaipur: A Card Game of Duels 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: ** WARNING: iPad 2, iPad Mini 1 & iPhone 4S are NOT compatible. ** *** Special Launch Price for a limited... | Read more »
Subdivision Infinity (Games)
Subdivision Infinity 1.03 Device: iOS Universal Category: Games Price: $2.99, Version: 1.03 (iTunes) Description: Launch sale! 40% Off! Subdivision Infinity is an immersive and pulse pounding sci-fi 3D space shooter. https://www.... | Read more »
Clash of Clans' gets a huge new upd...
Clash of Clans just got a massive new update, and that's not hyperbole. The update easily tacks on a whole new game's worth of content to the hit base building game. In the update, that mysterious boat on the edge of the map has been repaired and... | Read more »
Thimbleweed Park officially headed to iO...
Welp, it's official. Thimbleweed Park will be getting a mobile version. After lots of wondering and speculation, the developers confirmed it today. Thimbleweed Park will be available on both iOS and Android sometime in the near future. There's no... | Read more »

Price Scanner via

Free Tread Wisely Mobile App Endorsed By Fath...
Just in time for the summer driving season, Cooper Tire & Rubber Company has announced the launch of a new Tread Wisely mobile app. Designed to promote tire and vehicle safety among teens and... Read more
Commercial Notebooks And Detachable Tablets W...
Worldwide shipments of personal computing devices (PCDs), comprised of traditional PCs (a combination of desktop, notebook, and workstations) and tablets (slates and detachables), are forecast to... Read more
Best value this Memorial Day weekend: Touch B...
Apple has Certified Refurbished 2016 15″ and 13″ MacBook Pros available for $230 to $420 off original MSRP. An Apple one-year warranty is included with each model, and shipping is free: - 15″ 2.6GHz... Read more
13-inch MacBook Airs on sale for up to $130 o... has 13″ MacBook Airs on sale for up to $130 off MSRP including free shipping: - 13″ 1.6GHz/128GB MacBook Air (sku MMGF2LL/A): $869.99 $130 off MSRP - 13″ 1.6GHz/256GB MacBook Air (sku... Read more
2.8GHz Mac mini available for $973 with free...
Adorama has the 2.8GHz Mac mini available for $973, $16 off MSRP, including a free copy of Apple’s 3-Year AppleCare Protection Plan. Shipping is free, and Adorama charges sales tax in NY & NJ... Read more
15-inch 2.2GHz Retina MacBook Pro on sale for...
Amazon has 15″ 2.2GHz Retina MacBook Pros (MJLQ2LL/A) available for $1749.99 including free shipping. Apple charges $1999 for this model, so Amazon’s price is represents a $250 savings. Note that... Read more
Huawei Unveils New ‘Business-Styled’ MateBook...
Huawei has introduced a trio of new MateBook laptops, expanding its mobile portfolio and building on its success in delivering attractive and powerful high-end devices. The company claims the HUAWEI... Read more
Deal! Gold 12-inch 1.2GHz Retina MacBook for...
Amazon has the 2016 Gold 12″ 1.2GHz Retina MacBook (MLHF2LL/A) on sale for $350 off MSRP for a limited time. Shipping is free: - 12″ 1.2GHz Gold Retina MacBook: $1249.99 $350 off MSRP We expect this... Read more
13-inch 2.0GHz MacBook Pros on sale for $100...
B&H has the non-Touch Bar 13″ 2.0GHz MacBook Pros in stock today and on sale for $100 off MSRP. Shipping is free, and B&H charges NY & NJ sales tax only: - 13″ 2.0GHz MacBook Pro Space... Read more
15-inch 2.2GHz Retina MacBook Pro, Apple refu...
Apple has Certified Refurbished 2015 15″ 2.2GHz Retina MacBook Pros available for $1699. That’s $300 off MSRP, and it’s the lowest price available for a 15″ MacBook Pro. An Apple one-year warranty is... Read more

Jobs Board

*Apple* Mac and Mobility Engineer - Infogrou...
Title: Apple Mac and Mobility Engineer Location: Portland, OR Area Type: 12 month contract Job: 17412 Here's a chance to take your skills to the limit, learn new Read more
*Apple* Retail - Multiple Positions, White P...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
Best Buy *Apple* Computing Master - Best Bu...
**509110BR** **Job Title:** Best Buy Apple Computing Master **Location Number:** 000048-Topeka-Store **Job Description:** **What does a Best Buy Apple Computing Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description:SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.