TweetFollow Us on Twitter

June 91 - The Soup Kitchen - C++ing with MacApp

The Soup Kitchen - C++ing with MacApp

Eric M. Berdahl

ETO #4 will soon arrive on our doorsteps, if you believe the current APDA release schedule. An early version of MacApp 3.0 is promised on that CD-the so-called "C++ MacApp." This means, among other things, that many developers will need to be able to read C++ code, and perhaps even need to patch it. This issue, I'll look at some common MacApp constructs and show how C++ handles them.

ALLOCATION AND DEALLOCATION

One of the basic concepts of object programming is creating objects and disposing of them. These topics, in turn, break down into two separate issues: allocation and deallocation, and construction and destruction. MacApp supports all these areas, sometimes by convention, and other times by collaboration with the development environment.

Object Pascal extends Pascal's allocation and deallocation routines, NEW and DISPOSE, to work on object variables. Thus, the following may be written in Pascal:

VAR
anObject: TObject;
BEGIN
    NEW(anObject);
    DISPOSE(anObject);
END;

The Pascal compiler translates calls to NEW with an object argument into calls to a "magic" subroutine %_OBNEW, which then calls routines to allocate storage. In C++, classes that inherit from PascalObject also get this behavior. Thus, the equivalent C++ code looks like this:

{  TObject*    anObject;
    anObject = new TObject;
    delete anObject;
}

So, judging from this section of code, the new and delete functions are C++ analogs of NEW and DISPOSE. However, you have to tell new to create a TObject, whereas NEW simply knows what to create. When the Pascal compiler sees a call to the NEW procedure, it looks at the argument and decides what type it is. If the variable is an object, it calls the aforementioned %_OBNEW procedure, passing it information about the class of the NEW argument.

C++ takes a slightly different approach. The C++ new is told what type to create- TObject in this example-and calls %_OBNEW with the information about the class indicated by the programmer. The new function then returns a reference to the indicated class. This means that you can assign the result of new to a variable of that class or, alternatively, to a variable of a parent class. Thus, the code in the DoMenuCommand in C++ listing is perfectly legal. The equivalent Pascal code is shown in the DoMenuCommand in Pascal listing. And, of course, the C++ code may also be written using explicit variables for TFooCommand and TBarCommand, if you prefer.

In contrast, delete works on Pascal objects just as the DISPOSE routine, although in the MacApp world the Free method is always used instead.

CONSTRUCTION AND DESTRUCTION

Walking hand-in-hand with allocation and deallocation are the ideas of construction and destruction. Construction is the concept that an object should be placed into a known state as soon as it's created. Destruction is the concept of disposing of "owned" objects or performing other clean up necessary when an object ceases to exist. C++ provides a language mechanism that guarantees that these things occur when and where appropriate. Thus, there is a syntax for defining a constructor for a class that is called when an instance begins its existence, and a destructor that is called when an instance ceases to exist.

The implementation of these features in the language is very robust; they ensure that the parent class is completely constructed before the child class and that the child is destructed before the parent.

Constructors and destructors are common in pure C++ code; however, you should never write constructors for classes inheriting from PascalObject (i.e. classes meant to be link-compatible with Object Pascal). In the MacApp world, initialization methods are always used instead. MacApp 2.0.x "IMyObject" initialization methods have the general form:

BEGIN
    SetMyInstanceVariablesToSafeValues;
    SELF.IMyParentObject;
    InitializeMyInstanceVariablesToRealValues;
END;

The SetMyInstanceVariablesToSafeValues part acts like a constructor. The purpose is to place the object into a state such that Free is safe to call, if necessary. This means that pointers are set to nil, and so on. Some time ago, there was some discussion on MacApp.Tech$ contending that the TObject method Initialize should automagically be called when an object is allocated; this would add a more automatic construction behavior to MacApp.

A sort of automatic construction behavior is present in the MacApp 3.0 world. MacApp 3.0 IMyObject methods simply move the constructor portion shown above into an Initialize method. Initialize is invoked from IObject, and is implemented for all the standard MacApp classes. All subclasses define overrides of Initialize which first call the inherited version, then initialize local instance variables to safe values. Thus, the only time it is dangerous to Free an object is between allocation (i.e. new) and initialization (i.e. IMyObject). In practice this is not a problem if you follow the convention that Initialize must not fail. Following this convention should not be difficult since Initialize should only set instance variables to default values, and nothing more.

Destruction is handled by Free methods. By convention, a class' Free method does all necessary clean-up before invoking the parent class' version of Free. This does by convention what C++ destructors do automagically. So, if C++ constructors and destructors are so great, why not use them in MacApp? Because that would result in code that isn't link-compatible with Object Pascal. Pascal won't call C++'s constructors or destructors, so relying on them could lead to disastrous consequences.

A SIMPLE ROUTINE

The GetQDExtent in C++ listing shows an actual method taken from the MacApp 3.0 source code. I'll refer to it several times to denote various constructs used in C++ coding. The GetQDExtent in Pascal listing shows the equivalent Pascal code.

One of the first things to notice is that the C++ version of GetQDExtent uses the pascal void construct. Remember from the introduction to C++ interfaces in the last issue that pascal <Something> denotes a Pascal FUNCTION with return type <Something>, and that pascal void denotes a Pascal PROCEDURE.

Next, notice that "TView::GetQDExtent" correlates with "TView.GetQDExtent" in the Pascal code. The "::" is called the scope resolution operator. It casts a fair amount of magic in purist C++ code, but only has two common uses in the MacApp world. Method declaration as seen here is one place where "::" is used; the other will be revealed shortly.

Further comparison shows that the C++ keyword this is equivalent to the Pascal keyword SELF. Just as SELF is a Pascal meta-variable that indicates the particular instance a method is manipulating, this is the C++ meta-variable. All magic provided by Pascal in regards to SELF carries over to this in C++.

ACCESSING CLASS FEATURES

Method invocations and instance variable access are produced with the arrow operator, "->". Like its Pascal cousin, the dot operator, ".", the C++ arrow operator works on an object to call a method or access an instance variable. So, this->GetExtent(vr) is the C++ equivalent of the Pascal SELF.GetExtent(vr).

Similarly, you use anObject->fAnInstanceVar in a C++ method to do something with the instance variable fAnInstanceVar or the anObject object. Although this- >fAnInstanceVar is syntactically correct, you can just write fAnInstanceVar. The this-> is implied in C++ methods just as SELF. is implied in Pascal methods, and the convention of beginning field names with lowercase f makes it clear that fAnInstanceVar is an instance variable. For clarity, however, most style guides recommend explicitly using this->MethodCall() in C++ just as one would use SELF.MethodCall in Pascal.

There is another form of method invocation common to MacApp programming: calling the parent class' version of a method. Object Pascal provides the INHERITED keyword for this purpose. For a discussion of this topic, see James Plamondon's article "TAspectPicture-A problem to sleep on" in the April '91 issue of FrameWorks.

In C++, you can call the parent class' version of a method in two different ways. Traditional C++ programmers use the construct TParentClass::MethodCall(arg). In this form, MethodCall names the method you want to invoke, and TParentClass is the class that implements the version of MethodCall you want to use (passing arg as an argument). This construct bypasses the method dispatcher and explicitly calls the indicated implementation of MethodCall. This construct allows you to skip up the inheritance chain directly to any class that implements the method you name-parent class, grandparent class, etc-without executing implementations of that method that are made by intervening classes in the inheritance chain.

Usually you don't want to bypass the method dispatcher in this fashion. Instead, you want to dispatch your method starting with the parent class. Due to what I feel is a deficit in C++, no shorthand exists for calling an inherited method in this manner; however, MPW C++ provides an extension to do just that based on Pascal's INHERITED keyword. In MPW C++ you can write inherited::Draw(aRect) just as you might write INHERITED Draw(aRect) in Pascal.

VARIABLE DECLARATIONS

One major difference between C++ and Pascal methods is their local variable declarations. Pascal provides an explicit VAR area for all variable declarations. In C++, a variable declaration (e.g. "char aChar;") is a full-fledged statement; thus, it can appear anywhere a statement may appear in code.

Arbitrary placement of local variable declaractions is another feature of the language that may be important if you do pure, non-MacApp C++ coding; however, as a matter of style, many Mac C++ programmers declare all local variables in a cluster at the beginning of a method. A common variant on the simple declaration is the addition of an initial value to the declaration. Thus, "char aChar = 'a';" not only declares a variable named aChar of type char, it also immediately sets it to be the character 'a'.

REFERENCE VARIABLES

I'm going to take a break from dissecting this method to discuss C++ reference variables. These are possibly the most difficult concept of C++ to grasp, because they have no correlation in Pascal. A reference variable looks like this:
short       anInteger;
short&      someInteger = anInteger;

Here, anInteger is an integer, and someInteger is a reference to an integer variable (anInteger in this case).

References can be thought of as pointers that must always point to something. Another popular analogy is that references are aliases to another variable. Since references must always refer to something, when a reference is declared, it must be initialized with a valid variable, as above. Using the declarations above, someInteger may be substituted for anInteger everywhere. Literally, if you do something to someInteger, you're really doing it to anInteger. You won't be using references in MacApp programming, except for…

PARAMETER PASSING

The rules for Pascal parameter passing are something C++ programmers recite in their sleep. What the Pascal compiler does for you, the C++ interface must be designed to emulate. The rules are very simple:
  • All VAR parameters are passed by pushing a pointer to the variable on the stack.
  • Non-VAR parameters that are 4 bytes or smaller are passed by pushing a copy of the variable directly on the stack.
  • Non-VAR parameters that are larger than 4 bytes are passed by pushing a pointer to the variable on the stack.

So, if you have a routine like FrameRect, "PROCEDURE FrameRect(aRect: Rect)", you could declare it in C++ as "pascal void FrameRect(Rect* aRect)". Further, since aRect is a value parameter, you can denote that it won't be altered by changing the declaration to "pascal void FrameRect(const Rect* aRect)". This is perfectly legal, but has a minor pitfall. Since all C++ knows that FrameRect wants a pointer passed, it is syntactically legitimate to call "FrameRect(nil)". Guess what happens when you do that? What you really want the compiler to do is pass FrameRect a pointer to a Rect and ensure that the pointer in not nil-you want a reference to a Rect. To do this, the declaration then becomes "pascal void FrameRect(const Rect& aRect)", which is just the way it's defined in the C++ toolbox interfaces distributed with MacApp 3.0.

A BIT OF HISTORICAL IRONY

If you've been reading C++ interface files provided with MacApp 2.0.x and MPW, you may be more than a bit confused. These products don't work with reference variables as I've described above. Instead, they use the intermediate "pascal void FrameRect(const Rect* aRect)" form. However, MacApp 3.0 ships with C++ interface files that use reference variables. The hope is that the toolbox interfaces will be merged with the MPW product at some time in the near future. In any case, MacApp 3.0 C++ coders will use them.

By the way, this change absolutely guarantees that any existing C++ MacApp 2.0 code will fail to compile under MacApp 3.0. Because C++ programmers have been passing pointers to routines that now expect the real McCoy, all that code will need to be revamped.

A side effect of using this convention of passing parameters is that the MacApp C++ sources have a distinctly Pascal-like flavor to them, as does MacApp 3.0 C++ code in general. That is, I don't need to know whether a method can change a variable or not (i.e. is it VAR?). I write my code the same way in either case; just write the name of the variable and let the compiler worry about whether to push a pointer or a copy of the variable. Pascal programmers should feel very comfortable with this situation since this is exactly the convention used by the Pascal language.

NEXT TIME…

…I'll be looking at some real magic of Pascal and C++, and some of MacApp 3.0's new features. Each language provides interesting and useful constructs, especially if you happen to be programming in that language. I'm looking for those wonderful "How do you do <feature of one language> in <the other language>?" and "Isn't there a better way?" questions. As always, questions, comments, and other feedback are encouraged at AppleLink: BERDAHL.
 

Community Search:
MacTech Search:

Software Updates via MacUpdate

MacUpdate Desktop 6.0.8 - Search and ins...
MacUpdate Desktop 6 brings seamless 1-click installs and version updates to your Mac. With a free MacUpdate account and MacUpdate Desktop 6, Mac users can now install almost any Mac app on macupdate.... Read more
BitTorrent Sync 2.1.1 - Sync files secur...
BitTorrent Sync allows you to sync unlimited files between your own devices, or share a folder with friends and family to automatically sync anything. File transfers are encrypted. Your information... Read more
Quicksilver 1.3.0 - Application launcher...
Quicksilver is a light, fast and free Mac application that gives you the power to control your Mac with keystrokes alone. Quicksilver allows you to find what you need quickly and easily, then act... Read more
iWatermark Pro 1.72 - Easily add waterma...
iWatermark Pro is the essential watermarking app for professional, business, and personal use. iWatermark, is the number 1 and only watermarking tool available for all 4 platforms iPhone/iPad, Mac,... Read more
RapidWeaver 6.3 - Create template-based...
RapidWeaver is a next-generation Web design application to help you easily create professional-looking Web sites in minutes. No knowledge of complex code is required, RapidWeaver will take care of... Read more
Adobe Photoshop CC 2015 16.0.1 - Profess...
Photoshop CC 2015 is available as part of Adobe Creative Cloud for as little as $19.99/month (or $9.99/month if you're a previous Photoshop customer). Photoshop CS6 is still available for purchase (... Read more
Together 3.4.6 - Store and organize all...
Together helps you organize your Mac, giving you the ability to store, edit and preview your files in a single clean, uncluttered interface. Smart storage. With simple drag-and-drop functionality,... Read more
Monosnap 3.1.0 - Versatile screenshot ut...
Monosnap lets you capture screenshots, share files, and record video and .gifs! Capture: Capture full screen, just part of the screen, or a selected window Make your crop area pixel perfect with... Read more
Cocktail 8.5.1 - General maintenance and...
Cocktail is a general purpose utility for OS X that lets you clean, repair and optimize your Mac. It is a powerful digital toolset that helps hundreds of thousands of Mac users around the world get... Read more
Vienna 3.0.6 :5eaf312: - RSS and Atom ne...
Vienna is a freeware and Open-Source RSS/Atom newsreader with article storage and management via a SQLite database, written in Objective-C and Cocoa, for the OS X operating system. It provides... Read more

Five Nights at Freddy's 4 has Been...
In keeping with tradition, Five Nights at Freddy's 4 has made its way to the App Store with zero warning fanfare. Honestly I'd be disappointed it if happened any other way at this point. [Read more] | Read more »
Doom & Destiny Advanced (Games)
Doom & Destiny Advanced 2.3.4.0 Device: iOS Universal Category: Games Price: $.99, Version: 2.3.4.0 (iTunes) Description: ---SUPER GIGA LAUNCH DISCOUNT! Get it now for cheap or pay more later!---An additional 143 MB download is... | Read more »
Five Nights at Freddys 4 (Games)
Five Nights at Freddys 4 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: This time, the terror has followed you home. | Read more »
This Week at 148Apps: July 27-31, 2015
Winding Down July With 148Apps How do you know what apps are worth your time and money? Just look to the review team at 148Apps. We sort through the chaos and find the apps you're looking for. The ones we love become Editor’s Choice, standing out... | Read more »
You'll Want to Keep an Eye Out for...
If you're the kind of person who had fun hunting down and completing all the codex puzzles in Assassin's Creed 2, then are you ever in for a treat. The Guides looks like it's going to be a very robust collection of similarily odd, seemingly... | Read more »
Vivid Games has Announced Real Boxing 2...
The original Real Boxing was a pretty impressive bit of fisticuffs, but if the trailer Vivid Games is showing off for GamesCom is any indication Real Boxing 2 is going to be even better. [Read more] | Read more »
PAC-MAN Championship Edition DX - Tips,...
[Read more] | Read more »
Card King: Dragon Wars - Tips, Tricks an...
[Read more] | Read more »
Pac-Man Championship Edition DX has brou...
Bandai Namco has released Pac-Man Championship Edition DX on iOS and Android, which features the classic arcade gameplay that we've all grown to love. Pac-Man Championship Edition DX can be enjoyed in much shorter bursts than the arcade versions... | Read more »
Cosmonautica (Games)
Cosmonautica 1.1 Device: iOS Universal Category: Games Price: $6.99, Version: 1.1 (iTunes) Description: Cast off! Are you ready for some hilarious adventures in outer space? | Read more »

Price Scanner via MacPrices.net

MacBook 12 Retina Pundit’s New Favorite Mac
If you’re eyeing the purchase of a new 12-inch Retina MacBook, but wondering if you can live with a single USB-C port, 2011-level CPU performance, and the unorthodox keyboard, you should find... Read more
PDF Element Tool Kit For PDF For Windows 10,...
South Surrey, British Columbia based software developer Wondershare has posted an interesting infographic tracking the development of Microsoft’s flagship Windows operating system over the years,... Read more
27-inch 3.5GHz 5K iMac on sale for $81 off MS...
Adorama has the 27″ 3.5GHz 5K iMac on sale for $2218, $81 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
Back-to-School with Tablet and Smartphone Acc...
Belkin helps you prepare for the coming school year with a wide variety of the latest mobile and tablet accessories to outfit both grade school and college students. The line-up includes charging... Read more
11-inch MacBook Airs on sale for $100 off MSR...
Best Buy has 11-inch MacBook Airs on sale for $100 off MSRP. Choose free shipping or free local store pickup (if available). Sale prices for online orders only, in-store prices may vary: - 11″ 1.6GHz... Read more
iPad Air 2 on sale for up to $100 off MSRP
Best Buy has iPad Air 2s on sale for up to $100 off MSRP on their online store for a limited time. Choose free shipping or free local store pickup (if available). Sale prices available for online... Read more
Sale! 13-inch MacBook Pros on sale for $100 o...
B&H Photo has 13″ MacBook Pros on sale for $100 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 2.5GHz/500GB MacBook Pro: $999.99 save $100 - 13″ 2.7GHz/128GB Retina... Read more
Sale! Save $100 on 13-inch MacBook Airs this...
B&H Photo has the 13″ 1.6GHz/128GB MacBook Air on sale for $899.99 including free shipping plus NY tax only. Their price is $100 off MSRP, and it’s the lowest price available for this model.... Read more
Worldwide Tablet Market Decline Continues, Ap...
The worldwide tablet market declined -7.0% year-over-year in the second quarter of 2015 (2Q15) with shipments totaling 44.7 million units according to preliminary data from the International Data... Read more
TP-LINK TL-PA8030P KIT Powerline Featuring Ho...
Consumer and business networking products provider TP-LINK is now shipping its TL-PA8030P KIT AV1200 3-Port Gigabit Passthrough Powerline Starter Kit that expands your home’s network over its... Read more

Jobs Board

*Apple* Retail - Multiple Positions (US) - A...
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
Infrastructure Engineer - *Apple* /Mac - Hil...
Infrastructure Engineer - Apple /Mac Job Code: 1608 # of openings: 1 Description Our fortune 500 client is looking to hire an experienced Infrastructure Engineer to join Read more
Executive Administrative Assistant, *Apple*...
…supporting presentation development for senior leadership. * User experience with Apple hardware and software is preferred. Additional Requirements The following list Read more
*Apple* Bus Company is now hirin - Apple Bus...
Apple Bus Company is now hiring school bus drivers in the Pettis County area. Class B CDL preferred. Free training provided. No nights or weekends required. Flexible Read more
*Apple* Certified Mac Technician - Updated 6...
…and friendly, hands-on technical support to customers troubleshooting and repairing Apple /Mac products with courtesy, speed and skill. Use your problem-solving skills Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.