TweetFollow Us on Twitter

Polymorphic Code Resources In C++

Polymorphic Code Resources In C++


The C++ programming language supports data abstraction and object programming. Until now, using C++ to its full capacity in stand-alone code has not been possible. This article demonstrates how you can take advantage of two important features of C++, inheritance and polymorphism, in stand-alone code. An example shows how to write a window definition function using polymorphism.

In object programming, polymorphism gives programmers a way to solve problems by beginning with the general and proceeding to the specific. This process is similar to top-down programming, in which the programmer writes the skeleton of a program to establish the overall structure and then fills in the details later. Polymorphism differs from top-down programming, however, in that it produces designs that are reusable outside the context of the original structure. The attractiveness of reusable code is one of the reasons object programming is catching on.

The shape hierarchy shown in Figure 1 is one of the most frequently cited examples of polymorphism.

[IMAGE polymorphic_html1.GIF]

Figure 1Shape Hierarchy

The most general concept is the shape; all objects in the hierarchy inherit attributes from the shape. Area, perimeter, centroid, and color are attributes common to all shapes. Notice that the hierarchy proceeds from the general to the specific:

  • Polygons are shapes with a discrete number of sides.
  • Rectangles are polygons that have four sides and all right angles; squares are rectangles having all equal sides.
  • Ellipses are shapes that have a certain mathematical description; circles are ellipses whose widths equal their heights.

In C++, concepts are represented as classes. The more abstract the concept, the higher in the inheritance hierarchy the concept resides. Two key C++ features support polymorphism: inheritance and virtual member functions. We can use these to develop more concretely specified shapes and ask questions of any shape about its area, perimeter, or centroid.

The virtual functions provide a protocol for working with shapes. Here is an example of the shape hierarchy as it could be represented in C++:

class Shape {
    virtual float area();       // Area of the shape.
    virtual float perimeter();  // Its perimeter.
    virtual Point centroid();   // Its centroid.

class Ellipse : public Shape {
    virtual float area();       // Area of the shape.
    virtual float perimeter();  // Its perimeter.
    virtual Point centroid();   // Its centroid.
    Point center;           // Center of ellipse.
    float height;           // How high.
    float width;            // How wide.

class Circle : public Ellipse {
    virtual float area();       // Area of the shape.
    virtual float perimeter();  // Its perimeter.
    virtual Point centroid();   // Its centroid.

In this implementation, a circle is an ellipse with the additional constraint that its width and height must be equal.

Once an object of a type derived from Shape has been instantiated, it can be manipulated with general code that knows only about shapes. The benefit is that, having written and debugged this general code, you can add more kinds of shapes without having to alter the general code. This eliminates many potential errors.


MPW C++ is a language translator that translates C++ to C. Programs are compiled by first being translated to C, after which the MPW C compiler takes over and compiles the C to object code.


As noted, polymorphism is accomplished by using inheritance and virtual member functions. How does the C++ compiler decide which function should be called when an instance of unknown type is used? In the current release of MPW C++, every instance of an object in an inheritance hierarchy has a hidden data member, which is a pointer to a virtual function table. Each member function is known to be at a particular offset in the table. The member functions for the different classes in an inheritance chain are stored in different tables. The table pointed to is determined at the time of object creation. (See the sidebar called "Layout of Objects and Their Virtual Functions in Memory.")So far, nothing in the implementation of virtual functions seems to preclude their use in nonapplication contexts. Once an object is instantiated, the code needed to call a virtual function can be executed from any context, including stand-alone code resources. However, MPW C++ does not currently support a mechanism to allocate storage for, or to initialize, the virtual function tables in nonapplication contexts.


As noted above, virtual function tables are required for polymorphism in C++. To support virtual function tables in stand-alone code, two issues must be resolved:
  • How to allocate the virtual function tables.
  • How to initialize the virtual function tables.

In MPW C++, virtual function tables live in C global variable space. Unfortunately, the MPW languages do not support the use of global variables in stand-alone code. However, Technical Note #256, Stand- Alone Code,ad nauseam , shows how to add support for global variables in standalone code resources. In simple terms, this involves allocating storage for the globals, initializing the globals, and arranging for the proper value to be placed in machine register A5. These functions can be neatly expressed as a class in C++. The following class, called A5World, provides these services.

class A5World {
    A5World();      // Constructor sets up world.
    ~A5World();     // Destructor destroys it.

    // Main functions:  Enter(), Leave().
    void Enter();   // Go into our world.
    void Leave();   // Restore old A5 context.

    // Error reporting.
    OSErr Error()   { return error; }

    OSErr error;    // The last error that occurred.
    long worldSize; // How big our globals are.
    Ptr ourA5;      // The storage for the globals.
    Ptr oldA5;      // Old A5.

To use globals, a code resource written in C++ merely creates an instance of an A5World object. Here is an example:

// Hello_A5World.cp
// Simple code resource that uses global variables.

#include "A5World.h"

// Array of characters in a global.
char global_string[256];

void main()
    // Temporarily create global space.
    A5World ourWorld;

    // Check for errors.
    if(ourWorld.Error() != noErr)

    // We got it; let's go inside our global space.

    // Use our global variable.
    strcpy(global_string, "Hi there!");

    // Time to go home now.

    // The destructor automatically deallocates 
    // our global space.

The full implementation of class A5World appears on the Developer Essentials disc (Poly. in Code Resources folder). By itself, this is a useful piece of code.

As noted, MPW C++ is implemented as a language translator (called CFront) that translates C++ to C. As you might guess, classes are implemented as structs in C, and member functions are just ordinary C functions. As also noted, the virtual function tables are implemented as global variables. We have solved the problem of having globals in stand-alone code, so the remaining issue is how to initialize these tables with the proper pointers to the member functions.

The initialization of a global variable with a pointer to a function is not supported in stand-alone code written in MPW languages. This initialization is normally done by the linker, which creates a jump table, and the current version of the MPW Linker will not generate jump tables for stand- alone code. Therefore, the only way to initialize global variables with pointers to code is manually at run time.

To understand the solution to this problem, let's take a look at what CFront does when it sees a hierarchy of classes with virtual functions. Here is a simple hierarchy of two classes, Base andDerived:

class Base {
    virtual void Method();

class Derived : public Base {
    virtual void Method();

When MPW C++ sees these class definitions, it emits the following C to allocate and initialize the virtual function tables:

struct __mptr __vtbl__7Derived[]={0,0,0,
struct __mptr *__ptbl__7Derived=__vtbl__7Derived;
struct __mptr __vtbl__4Base[]={0,0,0,
struct __mptr *__ptbl__4Base=__vtbl__4Base;

The variables __vtbl__4Base[] and __vtbl__7Derived[] are the virtual function tables for the classes Base and Derived. To support polymorphism in stand-alone code, this code must be split into two parts: a declaration part and an initialization part. The initialization part is simply a C function that initializes the tables. The following code shows how the tables might be transformed for use in stand-alone code:

struct __mptr __vtbl__7Derived[3];
struct __mptr *__ptbl__7Derived;
struct __mptr __vtbl__4Base[3];
struct __mptr *__ptbl__4Base;

void init_vtbls(void)
    __vtbl__7Derived[1].d = 0;
    __vtbl__7Derived[1].i = 0;
    __vtbl__7Derived[1].f = (__vptp)Method__7DerivedFv;
    __vtbl__4Base[1].d = 0;
    __vtbl__4Base[1].i = 0;
    __vtbl__4Base[1].f = (__vptp)Method__4BaseFv;

What we end up with is a declaration of global variables and a simple C function that must be called before the virtual functions are called. The code transformation shown is easy to implement. TheDeveloper Essentials disc contains a simple MPW Shell script and two MPW tools that perform this function. The script is called ProcessVTables, and the tools are called FixTables andFilterTables.

What remains is to combine the concepts of allocating global variables and initializing virtual function tables into a single construct. The following code, class VirtualWorld, based on classA5World, provides these two services.

class VirtualWorld : public Relocatable {
    // Constructor sets up world.
    VirtualWorld(Boolean worldFloats);
    // Destructor destroys it.
    // Main functions; Enter sets A5 to point to 
    // our world.
    // Go into our world.
    void Enter();
    // Restore old A5 context.
    void Leave();
    // Error reporting.
    OSErr Result()  { return error; }

    // The last error that occurred.
    OSErr error;
    // Whether we have to call the vtable init.
    Boolean codeFloats;
    // How big our globals are.
    long worldSize;                     

    // The storage for the virtual world.
    Ptr ourA5;                          
    // Old A5.
    Ptr oldA5;                          

The constructor for class VirtualWorld requires one parameter, worldFloats, a Boolean value that tells whether or not the code resource floats between calls. This flag is used to decide whether or not the virtual function tables need reinitializing on every call to the code resource. Code resources such as WDEFs do float, and can even be purged, so this flag is essential. If worldFloatsis false, the virtual function tables are initialized once in the constructor. This initialization is performed by calling the function init_vtables(), shown earlier.

The Enter() and Leave() member functions set up and restore the A5 global space, respectively. If the member variable codeFloats is true, Enter() calls theinit_vtables() each time.

As in the A5World class, the Error() member function reports error conditions, which should be checked before assuming the world is set up correctly.


To show off this really cool technique, I have written one of everybody's favorite code resources, a window definition function, or WDEF, that uses polymorphism. The example demonstrates how to define a base class for windows that is easy to inherit from--so you can add a feature to a window while leaving the original window code untouched.

Class WindowDefinition forms the template for all other window definitions. Here is its interface:

class WindowDefinition : public Relocatable {
    // Initialize window.
    virtual void New(WindowPeek theWindow)
        { itsWindow = theWindow; }
    // Destroy window.
    virtual void Dispose() {}                               
    // Compute all relevant regions.
    virtual void CalcRgns() {}
    // Draw the frame of the window.
    virtual void DrawFrame() {}
    // Draw the goaway box (toggle state).
    virtual void DrawGoAwayBox() {}
    // Draw window's grow icon.
    virtual void DrawGIcon() {}
    // Draw grow image of window.
    virtual void DrawGrowImage(Rect& growRect) {}
    // Do hit testing.
    virtual long Hit(Point& whereHit)
        { return wNoHit; }
    // Window we are keeping track of.
    WindowPeek itsWindow;

WindowDefinition uses methods to respond to all the messages to which a WDEF is expected to respond. All the methods are just placeholders here, as WindowDefinition is an abstract base class.

Class WindowDefinition's superclass, Relocatable, provides services to all handle-based classes, such as locking this, moving it high, and unlocking it. This class makes the casts to type Handle that are normally necessary and makes dealing with handle-based classes pleasant--and safer.

The next class to look at isWindowFrame. WindowFrameimplements a basic window that can be resized, moved, and shown in highlighted or unhighlighted state.

class WindowFrame : public WindowDefinition {
    virtual void New(WindowPeek theWindow);
    virtual void Dispose();
    virtual void CalcRgns();
    virtual void DrawFrame();
    virtual void DrawGrowImage(Rect& growRect);
    virtual long Hit(Point& whereHit);

    // Border between content and structure
    // boundaries.
    RgnHandle itsBorderRgn;

The code that makes this window work is included in the full source example on the Developer Essentials disc.

[IMAGE polymorphic_html2.GIF]

Figure 3 Class WindowFrame's Window on the Desktop

To implement a window that is iconifiable, we can derive from the class WindowFrame, and modify its behavior, without having to rewrite the code that implements the window. All an icon has to do is respond to clicks and be dragged around. So, the class IconWDef just has to worry about keeping track of whether the window is iconified or not, and lets the WindowFrame part take care of being a window. Here is the interface to class IconWDef.

class IconWindowDef : public WindowFrame {
    // Window methods.
    virtual void New(WindowPeek theWindow);
    // We have different regions when iconified.
    virtual void CalcRgns();
    // We draw an icon if in the iconified state.
    virtual void DrawFrame();
    virtual long Hit(Point& whereHit);
    // State of our window.
    Boolean iconified;
    // If we've ever been iconified.
    Boolean everIconified;
    // Flag that says we want to change our state.
    Boolean requestingStateChange;
    // How many times CalcRgns has been called.
    short calcRgnsCount;
    // Place to hit to iconify window.
    Rect iconifyRect;   
    // Where to put when iconified.
    Point iconifiedLocation;

The decision about when to iconify the window is made in theIconWDef's Hit()method. In the current implementation, if the window's zoom box is hit with the Option key held down, the window toggles between being an icon and being a window.


This article has shown how to use polymorphism--the combination of inheritance and run-time binding of functions to objects--in the context of stand-alone code resources. Issues that had to be resolved were how to provide support for globals in stand-alone code and how to arrange for the initialization of virtual function tables.

Although the code for the example shows how to use polymorphism for window definition functions, you can use the same technique to write any type of code resource: menu definitions, list definitions, control definitions, and even drivers.

The code could be improved in two ways:

  • By using handles to allocate the A5 globals and passing in a parameter to tell VirtualWorld that the data can float.
  • By removing the QuickDraw-specific code and placing it in a subclass of VirtualWorld.

A couple of words of warning are in order. The tools that process the virtual function tables depend on the way CFront generates the tables. If AT&T or Apple ever decides to change the way these tables are generated (probably unlikely), the tools described in the example will probably break. However, it would not be difficult to modify the tools if changes were made.

Classes inherited from class PascalObject are not supported by the techniques described in this article. This is because these classes do not implement run-time binding using virtual function tables. This is not a problem since PascalObjects were intended only for use with MacApp, and (for now) MacApp can be used only for applications.

Look at the code on Developer Essentials for more information, and good luck!


For a typical class such as class foo, how does the compiler generate code to call the proper virtual function at run time? The following class and diagram show how this is accomplished.

class foo {
    virtual void method1 ();
    virtual void method2 ();
    int member1;
    int member2;

As shown by the figure, an instance of class foo has three data members. Two of the members, member1 and member2, are part of the class definition, while a third member we'll call pVTable is a hidden member automatically created by the compiler. pVTable is a pointer to a table of function pointers (also automatically generated by the compiler) that holds pointers to all the functions in the class that are declared virtual. The code that is generated to call a virtual function is therefore something like this:

// Code written in C++:
/* Becomes this code in C: */

This is the memory layout for a virtual function table used in single inheritance. For multiple inheritance, the structures used are more complicated.

[IMAGE polymorphic_html3.GIF]

Figure 2 Calling Virtual Functions at Run Time


The C++ default storage strategy is to create objects as pointers. As we all know, using pointers to allocate storage on the Macintosh makes memory management a lot less efficient. The ability to store data in relocatable blocks allows the Macintosh to use more of its memory since relocatable blocks can be shuffled around to make space.

Luckily, Apple has extended C++ in a way that allows us to take advantage of the Macintosh Memory Manager by adding the built-in classHandleObject. The only restrictions placed on handle-based objects is that they can be used only for single-inheritance hierarchies. Most object programming tasks, however, can be handled using single inheritance. To make handle-based objects easier to work with, here is class Relocatable, a class derived fromHandleObject. Class Relocatable provides functions for manipulating handle-based objects without the hassle of all those casts.

class Relocatable : HandleObject {
    void Lock()             {HLock((Handle)this);}
    void Unlock()           {HUnlock((Handle)this);}
    void MoveHigh()         {MoveHHi((Handle)this);}
    SignedByte GetState()   {return HGetState((Handle)this);}
    void SetState(SignedByte flags)
        {HSetState((Handle)this, flags);}

PATRICK BEARD of Berkeley Systems, Inc., is a totally rad dude, living in a world somewhere between hard-core physics and fantasy. He prepared for this lifestyle by getting a B.S.M.E. at the University of California, Berkeley. He claims native Californian status, although he's from Illinois. A programming addict who relishes treading the very edge of what's possible,Pat dreams of writing his own programming language so he can really express himself. Meanwhile, he has written screen- savers and sundry compiler hacks, and has helped develop a Macintosh talking interface for the blind. He's a jazz musician (looking for a rhythm section--any takers?), a snow skier, snowboarder, and skateboarder, whose motto in life is " Stop and breathe from time to time." He never puts anything away, fearing an inability to find stuff when he needs it; the piles are growing at an alarming rate. However, he swears his brain is organized and that he knows where everything is, except Tech Note #31. *

Thanks to Our Technical Reviewers Herman Camarena, Jack Palevich *


Community Search:
MacTech Search:

Software Updates via MacUpdate

Bookends 12.6.0 - Reference management a...
Bookends is a full-featured bibliography/reference and information-management system for students and professionals. Access the power of Bookends directly from Mellel, Nisus Writer Pro, or MS Word (... Read more
Apple iBooks Author 2.4 - Create and pub...
Apple iBooks Author helps you create and publish amazing Multi-Touch books for iPad. Now anyone can create stunning iBooks textbooks, cookbooks, history books, picture books, and more for iPad. All... Read more
Web Snapper 3.3.9 - Capture entire Web p...
Web Snapper lets you capture Web pages exactly as they appear in your browser. You can send them to a file as images or vector-based, multi-page PDFs. It captures the whole Web page - eliminating the... Read more
Tunnelblick 3.6beta10 - GUI for OpenVPN...
Tunnelblick is a free, open source graphic user interface for OpenVPN on OS X. It provides easy control of OpenVPN client and/or server connections. It comes as a ready-to-use application with all... Read more
EtreCheck 2.5.1 - For troubleshooting yo...
EtreCheck is a simple little app to display 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... Read more
Paragraphs 1.0.4 - Writing tool just for...
Paragraphs is an app just for writers. It was built for one thing and one thing only: writing. It gives you everything you need to create brilliant prose and does away with the rest. Everything in... Read more
Things 2.8 - Elegant personal task manag...
Things is a task management solution that helps to organize your tasks in an elegant and intuitive way. Things combines powerful features with simplicity through the use of tags and its intelligent... Read more
Coda 2.5.12 - One-window Web development...
Coda is a powerful Web editor that puts everything in one place. An editor. Terminal. CSS. Files. With Coda 2, we went beyond expectations. With loads of new, much-requested features, a few surprises... Read more
PCalc 4.3 - Full-featured scientific cal...
PCalc is a full-featured, scriptable scientific calculator with support for hexadecimal, octal, and binary calculations, as well as an RPN mode, programmable functions, and an extensive set of unit... Read more
DiskMaker X 5.0 - Make a bootable OS X i...
DiskMaker X (was Lion DiskMaker) helps you to build a bootable drive from the official OS X installer app (the one you download from the Mac App Store). It detects the OS X Install program with... Read more

Camel Up (Games)
Camel Up 1.0.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0.0 (iTunes) Description: | Read more »
The Martian: Bring Him Home (Games)
The Martian: Bring Him Home 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: Based on the best selling novel and critically acclaimed film, THE MARTIAN tells the story of Astronaut Mark... | Read more »
This Week at 148Apps: September 21-30, 2...
Leap Into Fall 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 above... | Read more »
Tweetbot 4 for Twitter (Social Networki...
Tweetbot 4 for Twitter 4.0 Device: iOS Universal Category: Social Networking Price: $4.99, Version: 4.0 (iTunes) Description: *** 50% off for a limited time. *** | Read more »
Mori (Games)
Mori 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: Stop, rewind and unwind with Mori. Time is always running, take a moment to take control. Mori is an action puzzle game about infinitely... | Read more »
100 Years' War (Games)
100 Years' War 1.0 Device: iOS Universal Category: Games Price: $3.99, Version: 1.0 (iTunes) Description: | Read more »
Tower in the Sky (Games)
Tower in the Sky 0.0.60 Device: iOS Universal Category: Games Price: $1.99, Version: 0.0.60 (iTunes) Description: | Read more »
hocus. (Games)
hocus. 1.0.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0.0 (iTunes) Description: New, polished, mind-bending, minimal puzzle game with dozens of levels and extra-ordinary design Features:- Beautifully crafted... | Read more »
Mos Speedrun 2 (Games)
Mos Speedrun 2 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: Mos is back, in her biggest and most exciting adventure ever! Wall-jump to victory through 30 mysterious, action packed levels... | Read more »
3D Touch could be a game-changer, but it...
Were you one of the lucky/financially secure enough ones to buy a new iPhone 6s or iPhone 6s Plus over the weekend? Yup, me too (I’m not convinced I was either of those two things, but let’s go with lucky for now), so I thought I’d delve into just... | Read more »

Price Scanner via

12-inch MacBooks in stock for up to $120 off,...
Adorama has 12″ Retina MacBooks in stock for up to $120 off MSRP including free shipping plus NY & NJ sales tax only. For a limited time, Adorama will include a free Apple USB-C to USB Adapter,... Read more
15-inch 2.2GHz Retina MacBook Pro on sale for... has the 15″ 2.2GHz Retina MacBook Pro on sale for $1799 including free shipping. Their price is $200 off MSRP, and it’s the lowest price available for this model (except for Apple’s $1699... Read more
iPhone 6s and 6s Plus Feature Improved Durabi...
Upgraded components in the new iPhone 6s Plus cost $16 more than the components in the earlier iPhone 6 Plus according to a preliminary estimate from IHS Inc. The bill of materials (BOM) for an... Read more
13-inch Retina MacBook Pros on sale for up to...
Adorama has 13″ Retina MacBook Pros on sale for up to $130 off MSRP. Shipping is free, and Adorama charges sales tax for NY & NJ residents only: - 13″ 2.7GHz/128GB Retina MacBook Pro: $1199.99 $... Read more
Apple refurbished 2014 13-inch Retina MacBook...
Apple has Certified Refurbished 2014 13″ 2.6GHz/128GB SSD Retina MacBook Pros available $979, $320 off original MSRP. An Apple one-year warranty is included, and shipping is free: - 13″ 2.6GHz/128GB... Read more
iOS 9 Reflections Ten Days In – The ‘Book Mys...
I’ve never been much of an early adopter by philosophy or temperament, although I did buy the iPad Air 2 I’m typing this column on last fall only about a month after Apple unveiled it. However, my... Read more
Apple refurbished Time Capsules available for...
Apple has certified refurbished Time Capsules available for $120 off MSRP. Apple’s one-year warranty is included with each Time Capsule, and shipping is free: - 2TB Time Capsule: $179, $120 off - 3TB... Read more
OS X El Capitan Available as a Free Update To...
OS X El Capitan, the latest major release of Apple’s desktop operating system, is available today, September 30 as a free update for Mac users. “People love using their Macs, and one of the biggest... Read more
15-inch Retina MacBook Pros on sale for $150-...
B&H Photo has 2015 15″ Retina MacBook Pros on sale for up to $200 off MSRP including free shipping plus NY sales tax only: - 15″ 2.2GHz Retina MacBook Pro: $1815 $184 off - 15″ 2.5GHz Retina... Read more
Updated For iOS 9, InterConneX Lets You Store...
InterConneX version 1.2 is now a 64-bit app that’s completely compatible with iOS 9. InterConneX is a file storage, file management, and file sharing app for the iPhone or iPad that now takes... Read more

Jobs Board

*Apple* Systems Engineer (Mclean, VA and NYC...
Title: Apple Systems Engineer (Mclean, VA and NYC)Location: United States-New York-New York-NYC 200 Park Avenue (22005)Other Locations: United Read more
*Apple* Retail - Multiple Positions (US) - A...
Job Description:SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, you're Read more
*Apple* Retail - Multiple Customer Support P...
Job Description:Customer SupportSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the Read more
*Apple* Solutions Consultant - Retail Sales...
**Job Summary** As an Apple Solutions Consultant (ASC) you are the link between our customers and our products. Your role is to drive the Apple business in a retail Read more
SW QA Engineer - *Apple* TV - Apple (United...
**Job Summary** The Apple TV team is looking for experienced Quality Assurance Engineers with a passion for delivering first in class home entertainment solutions. **Key Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.