TweetFollow Us on Twitter

TCL OOPs Intro
Volume Number:6
Issue Number:9
Column Tag:C Forum

TCL OOPs Introduction

By Mark B. Kauffman, Tucson, AZ

Note: Source code files accompanying article are located on MacTech CD-ROM or source code disks.

An Introduction to Object-Oriented Programming With THINK C

[Mark Kauffman is currently employed at Burr-Brown Research Corp. in Tucson, Arizona, designing and programming test equipment. He has been programming micro-computers for 14 years. He has owned and programmed a Macintosh since its introduction in 1984.]

Introduction

This article is written for C programmers who are interested in learning the object-oriented programming concepts available in THINK C. It includes example code that defines a class of shapes and then draws some shapes in a window on the Macintosh screen. This example is designed to be used with the Starter project that comes with the THINK C 4.0 compiler. By reading this article and trying out the example code, you will gain a clear understanding of the object-oriented concepts of class, inheritance, objects, polymorphism and messages.

Create a New Project and Add the Example Code

Create a working copy of the Starter Folder included in the THINK C compiler as follows: Copy the Starter folder, change the new folder name from “Copy of Starter Folder” to “Object1 Folder”. Open the Object1 Folder and change the word “Starter” to “Object1” in all of the file names that contain the word “Starter”. Now open the Object1 project (Object1.Π) and use the THINK C Find... command in the Search menu to find and replace all occurrences of “Starter” with “Object1” in all of the files used by the project. This procedure allows you to preserve the original Starter Folder for later use in creating new projects.

Use the THINK C file editor to create and type in the three source file listings. Name the files CShapes.c, CShapes.h and DrawMyStuff.c. Add CShapes.c and DrawMyStuff.c to the Object1 project using the Add... command under the Source menu. Open the source code file CObject1Pane.c and find the Draw method...

void CObject1Pane::Draw(Rect *area)

Underneath the line,

 {  /* draw your stuff */

add the following line...

   DrawMyStuff();

When you run the project you should see some circles, rectangles and a line drawn in a window. One of the rectangles should be missing a line across the top and one of the “circles” will be oval shaped. See figure 1.

Figure 1.

OOP Concepts

CShapes.c and CShapes.h are the files that define the shape classes that DrawMyStuff.c uses to create shapes. I’ve followed Symantec’s convention of separating class definitions into two files, a header file (the dot-h file) and a code file (the dot-c file); the header file is always included at the beginning of the code file. CShapes.h, like all dot-h files used to define a class, contains class declarations. A class declaration declares the existence of a class of objects by naming the class (by convention all of the class names begin with a C), by identifying the parent class (In Think C, unlike in some other object-oriented languages, classes undergo asexual reproduction and therefore each class has only one parent.), by stating the method prototypes (i.e. stating the names and parameters of all methods available to the class), and by declaring the instance variables of the class. CShapes.c, like all dot-c files used to define a class, consists of all the methods whose prototypes are given in its dot-h file.

Each method is an algorithm that describes an action. Messages are requests for the action that are sent to an object from the user of the program or from other objects. For example, in response to the message “Draw”, an object of class “CLine” uses the Macintosh QuickDraw routines “MoveTo” and “LineTo” to draw a line at the location given in its instance variable “Location”. Each instance of a class (i.e. each individual object in the class) may respond to the same message differently, depending on the values of its instance variables.

Instance variables are the attributes of objects. Different instances (objects) of the same class can have different values of their instance variables. For example, in the class of objects CPeople, individual objects in the class can have different values for the instance variable “leg strength”. As a result, each object of the class may respond to the same message differently. You may be able to jump (a method) higher than I can because your leg strength (an instance variable) is larger (the value of the instance variable) than mine.

The Sample Program’s Classes

In the example program there are four classes. Class CShape is the root or parent class of the other three classes. CShape is also referred to as an abstract class because there will never be any objects created of class CShape. All of the objects used in the example are of classes that are descendants of CShape. CShapes purpose is to define the methods and instance variables that its descendants will inherit. The inherited methods are: SetShapeLoc, Draw, and Erase. The inherited instance variable is Location.

The descendants of CShape are COval, CRectangle and CLine. These descendants do not need to define the method SetShapeLoc or the instance variable Location because that method and instance variable are inherited from the parent class CShape. One of the advantages of object-oriented programming is this property of new classes inheriting the methods and instance variables of their parent class.

I’ve defined three methods for the CShape class: SetShapeLoc, Draw and Erase. Objects that belong to a class that is a descendant of CShape will all be able to respond to the messages SetShapeLoc, Draw and Erase. Objects in a descendant class of CShape will respond to SetShapeLoc by setting their Location instance variable to the values passed with the SetShapeLoc message. The response inherited by classes that are descendants of CShape to the Draw or Erase message is to do nothing. However, descendants of class CShape can respond to Draw and Erase messages with actions other than the inherited response by overriding the Draw and Erase methods. A descendant class overrides an inherited method by declaring the method prototype in its class declaration, using the same method name, and redefining the method. This ability of objects of different classes to respond differently to the same message is called polymorphism.

Draw!

The sample program’s pane object gets a Draw message whenever it needs to be updated due to some action by the user such as starting the application or resizing the window. In the example the pane’s Draw method has been modified to call DrawMyStuff, a function that draws various lines, rectangles and ovals on the screen. The code in DrawMyStuff could have been in the CObject1Pane.c file as part of its Draw method but by placing the code as a function in a file separate from CObject1Pane.c, compile times are cut tremendously.

DrawMyStuff draws by creating shape objects using the Think C new() function, and then sending SetShapeLoc and Draw messages to the objects. Rectangle and oval objects are created in arrays of their respective class type. Each shape in the arrays is then sent a SetShapeLoc message and a Draw message.

There is only one instance of a line object. The line object is used twice. First, it is sent a SetShapeLoc message with parameters that are the end points of the top of one of the rectangles, and an Erase message so that the top of the rectangle is erased. Second, it is sent another SetShapeLoc message with end points that are above the rectangles and ovals, and a Draw message so that a line is drawn above the rectangles and ovals.

What Next?

This shape library has many possibilities. Obviously, one could add more shapes. There could be an instance variable, “pattern”, so that shapes could be drawn filled with a pattern. It should be possible to animate the shapes using the Dawdle message sent by the application object to the pane by adding a Dawdle method to the pane.

The concepts presented in this article are the basics of object-oriented programming with THINK C. Once you understand these concepts, you will be able to use the THINK Class Library to create complete Macintosh applications with less time and effort than was possible with previous development systems. Have fun!

Listing:  CShape.h

/*
 * Source - CShape.h
 * Author - Mark Bykerk Kauffman
 * Purpose- To define a small class of shapes
 * using THINK C, and to illustrate several
 * key concepts of object oriented programming.
 * All of these classes use the convention of
 * having a C as the first letter their class
 * name.
 */
      
struct CShape : indirect  
{
 Rect Location;
 
 void SetShapeLoc(int A, int B,int C,int D);
 void Draw(void);
 void Erase(void);
};

/*
 * All of the following classes are descendants
 * of CShape.  They inherit all of CShapes 
 * methods and instance variables.
 */ 
    
struct COval : CShape
{
 void Draw(void);
 void Erase(void);
};

struct CRectangle : CShape
{
 void Draw(void);
 void Erase(void);
};

struct CLine : CShape
{
 void Draw(void);
 void Erase(void);
};
Listing:  CShape.c

/*
 * Source - CShape.c
 * Author - Mark Bykerk Kauffman
 * Purpose- This file contains the
 * implementaion of the methods
 * prototyped by shape classes in CShape.h
 */
#include “CShape.h”
#include “Quickdraw.h”

void CShape :: Draw(void)
{
}

void CShape :: Erase(void)
{
}

void CShape :: SetShapeLoc(Left,Top,Right,Bottom)
int Left;
int Top;
int Right;
intBottom;
{
 Rect Location;  
/* LOCAL location variable */
 
 SetRect(&Location,Left,Top,Right,Bottom);
 this->Location = Location;

/* 
 * Set the instance variable Location to
 * the local Location variable value. The 
 * reason for doing this is that
 * Semantec states in the THINK C manual
 * “Your program should not rely on the
 * addresses of instance variables...”.
 * Semantec describes the above technique as
 * “shadowing”.  
 */
}

void COval :: Draw(void)
{
 Rect Location;
 
 Location = this->Location; 
 FrameOval(&Location);    
}

void COval :: Erase(void)
{
 Rect Location;
 
 Location = this->Location;
 EraseOval(&Location);  
}

void CRectangle :: Draw(void)
{
 Rect Location;
 
 Location = this->Location;
 FrameRect(&Location);  
}

void CRectangle :: Erase(void)
{
 Rect Location;
 
 Location = this->Location;
 EraseRect(&Location);  
}

void CLine :: Draw(void)
{
 MoveTo(Location.left,Location.top);
 LineTo(Location.right,Location.bottom);
}

void CLine :: Erase(void)
{
 int    oldPenMode;

 oldPenMode = thePort->pnMode;
 PenMode(notPatCopy);
 MoveTo(Location.left,Location.top);
 LineTo(Location.right,Location.bottom);
 PenMode(oldPenMode);
}
Listing:  DrawMyStuff.c

/*
 * Source - DrawMyStuff.c
 * Author - Mark Bykerk Kauffman
 * Purpose- To demonstrate how to create
 * instances of objects and send them
 * messages using Think C 4.0.  Call this 
 * function from the Draw method of the Pane 
 * object for your application. 
 */
 
#include “oops.h”
#include “CShape.h”

/* 
 * oops.h is included so the compiler knows how 
 * to deal with the object-oriented programming 
 * in this section of code.  CShape.h is
 * included so that all of the shapes defined 
 * in CShape.h are available. 
 */
 
void DrawMyStuff()
{
 int    i;
 CLine  *ALine;
 COval  *AnOval[4];
 CRectangle *ARectangle[4];
 
 for (i=0;i<4;i++) 
 ARectangle[i] = new(CRectangle);
 ARectangle[0]->SetShapeLoc(200,100,300,200);
 ARectangle[1]->SetShapeLoc(210,110,290,190);
 ARectangle[2]->SetShapeLoc(220,120,280,180);
 ARectangle[3]->SetShapeLoc(230,130,270,170);
 for (i=0;i<4;i++) 
 ARectangle[i]->Draw();
 
 ALine = new(CLine); 
 ALine->SetShapeLoc(210,110,290,110);
 ALine->Erase();
 ALine->SetShapeLoc(100,50,300,50);
 ALine->Draw();
 
 for (i=0;i<4;i++) 
 AnOval[i] = new(COval);
 AnOval[0]->SetShapeLoc(100,100,200,200);
 AnOval[1]->SetShapeLoc(110,110,190,190);
 AnOval[2]->SetShapeLoc(120,120,180,185); 
 AnOval[3]->SetShapeLoc(130,130,170,170);
 for (i=0;i<4;i++) 
 AnOval[i]->Draw();
 
 for (i=0;i<4;i++)
 { 
 delete(ARectangle[i]);   
 delete(AnOval[i]);
 }
 delete(ALine);
}

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

TechTool Pro 9.5.1 - Hard drive and syst...
TechTool Pro has long been one of the foremost utilities for keeping your Mac running smoothly and efficiently. With the release of version 9, it has become more proficient than ever. TechTool... Read more
Jamf Pro 9.99.0 - Powerful sysadmin/ente...
Jamf Pro (formerly Casper Suite) is the EMM tool that delights IT pros and the users they support by delivering on the promise of unified endpoint management for Apple devices. At Jamf, connecting... Read more
VueScan 9.5.78 - Scanner software with a...
VueScan is a scanning program that works with most high-quality flatbed and film scanners to produce scans that have excellent color fidelity and color balance. VueScan is easy to use, and has... Read more
Adobe Lightroom 6.10.1 - Import, develop...
Adobe Lightroom is available as part of Adobe Creative Cloud for as little as $9.99/month bundled with Photoshop CC as part of the photography package. Lightroom 6 is also available for purchase as a... Read more
iPhoto Library Manager 4.2.7 - Manage mu...
iPhoto Library Manager allows you to organize your photos among multiple iPhoto libraries, rather than having to store all of your photos in one giant library. You can browse the photos in all your... Read more
Smultron 9.4 - Easy-to-use, powerful tex...
Smultron 9 is an elegant and powerful text editor that is easy to use. Use it to create or edit any text document. Everything from a web page, a note or a script to any single piece of text or code.... Read more
TextSoap 8.4 - Automate tedious text doc...
TextSoap can automatically remove unwanted characters, fix up messed up carriage returns, and do pretty much anything else that we can think of to text. Save time and effort. Be more productive. Stop... Read more
Merlin Project 4.2.3 - $349.00
Merlin Project is the leading professional project management software for OS X. If you plan complex projects on your Mac, you won’t get far with a simple list of tasks. Good planning raises... Read more
QuarkXPress 13.0.0.0 - Desktop publishin...
QuarkXPress 2017 is the new version that raises the bar for design and productivity. With non-destructive graphics and image editing directly within your layout, you no longer have to choose between... Read more
Path Finder 7.5 - Powerful, award-winnin...
Path Finder makes you a master of file management. Take full control over your file system. Save your time: compare and synchronize folders, view hidden files, use Dual Pane and full keyboard... Read more

Latest Forum Discussions

See All

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 »
Pokémon GO might be getting legendaries...
The long-awaited legendary Pokémon may soon be coming to Pokémon GO at long last. Data miners have already discovered that the legendary birds, Articuno, Moltres, and Zapdos are already in the game, it’s just a matter of time. [Read more] | Read more »

Price Scanner via MacPrices.net

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
Apple refurbished 9-inch and 12-inch iPad Pro...
Apple has Certified Refurbished 9″ and 12″ Apple iPad Pros available for up to $160 off the cost of new iPads. An Apple one-year warranty is included with each model, and shipping is free: - 32GB 9″... Read more
Apple Certified Refurbished iMacs available f...
Apple has Certified Refurbished 2015 21″ & 27″ iMacs available for up to $350 off MSRP. Apple’s one-year warranty is standard, and shipping is free. The following models are available: - 21″ 3.... Read more
Sale! 15-inch 2.6GHz Silver Touch Bar MacBook...
DataVision has the 15″ 2.6GHz Silver Touch Bar MacBook Pro (MLW72LL/A) on sale for $2199 including free shipping. Their price is $200 off MSRP, and it’s the lowest price available for this model (... Read more
A Kaby Lake Processor Upgrade For The MacBook...
Now they tell me! Well, actually Apple hasn’t said anything official on the subject, but last week Bloomberg News’s Mark Gurman and Alex Webb cited unnamed “people familiar with the matter”... Read more
Kodak’s Camera-First Smartphone EKTRA Launche...
The Eastman Kodak Company and Bullitt Group have announced the availability of a U.S. GSM version of the KODAK EKTRA Smartphone. The U.S. launch coincides with a software update addressing requests... Read more
Apple Launches App Development Curriculum for...
Apple today launched a new app development curriculum designed for students who want to pursue careers in the fast-growing app economy. The curriculum is available as a free download today from Apple... Read more

Jobs Board

Data Engineer - *Apple* Media Products - Ap...
Changing the world is all in a day's work at Apple . If you love innovation, here's your chance to make a career of it. You'll work hard. But the job comes with more Read more
*Apple* Store Leader Program - Apple, Inc (U...
…Summary Learn and grow as you explore the art of leadership at the Apple Store. You'll master our retail business inside and out through training, hands-on Read more
*Apple* Integration Specialist - A3 Solution...
Apple Integration Specialist Contract-To-HireWe are searching for dedicated, well-experienced and energetic individuals for an information technology corporation Read more
Sr. Software Engineer, *Apple* Online Store...
Changing the world is all in a day's work at Apple . If you love innovation, here's your chance to make a career of it. You'll work hard. But the job comes with more Read more
Senior Engineering Project Manager, *Apple*...
Changing the world is all in a day's work at Apple . If you love innovation, here's your chance to make a career of it. You'll work hard. But the job comes with more Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.