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);
}

 
AAPL
$93.94
Apple Inc.
-0.49
MSFT
$44.84
Microsoft Corpora
+0.15
GOOG
$589.47
Google Inc.
-5.61

MacTech Search:
Community Search:

Software Updates via MacUpdate

OS X Yosemite 10.10 DP4 - Developer Prev...
Note: This is a Developer Preview. You must be a registered Apple Mac Developer to download this update. OS X Yosemite is Apple's newest operating system for Mac. An elegant design that feels... Read more
FinderPop 2.5.6 - Classic Mac utility, n...
FinderPop is a Universal preference pane that extends OS X's contextual menus using a FinderPop Items folder much as the Apple Menu Items folder used to do for the Apple menu. It has other features... Read more
SpiderOak 5.1.7 - Secure cloud backup, s...
SpiderOak is a multi-platform secure online backup, storage, access, and sharing solution engineered for the consumer and small businesses. You must first sign up to use SpiderOak. Running natively... Read more
Espionage 3.6 - Simple, state of the art...
Espionage offers state-of-the-art encryption and plausible deniability for your confidential data. Sometimes, encrypting your data isn't enough to protect it. That's why Espionage 3 goes beyond data... Read more
calibre 1.45.0 - Complete e-library mana...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital... Read more
iFFmpeg 4.3.1 - Convert multimedia files...
iFFmpeg is a graphical front-end for FFmpeg, a command-line tool used to convert multimedia files between formats. The command line instructions can be very hard to master/understand, so iFFmpeg does... Read more
Chromium 36.0.1985.125 - Fast and stable...
Chromium is an open-source browser project that aims to build a safer, faster, and more stable way for all Internet users to experience the web. FreeSMUG-Free OpenSource Mac User Group build is... Read more
pwSafe 3.0 - Secure password management...
pwSafe provides simple and secure password management across devices and computers. pwSafe uses iCloud to keep your password databases backed-up and synced between Macs and iOS devices. It is... Read more
Day One 1.9.6 - Maintain a daily journal...
Day One is the easiest and best-looking way to use a journal / diary / text-logging application for the Mac. Day One is well designed and extremely focused to encourage you to write more through... Read more
Google Chrome 36.0.1985.125 - Modern and...
Google Chrome is a Web browser by Google, created to be a modern platform for Web pages and applications. It utilizes very fast loading of Web pages and has a V8 engine, which is a custom built... Read more

Latest Forum Discussions

See All

HELMUT Review
HELMUT Review By Andrew Fisher on July 21st, 2014 Our Rating: :: TRUNDLE SIMULATOR 2014Universal App - Designed for iPhone and iPad HELMUT is a fun, fleeting time-sink that offers a momentary distraction and nothing else.   | Read more »
Walkr Review
Walkr Review By Jennifer Allen on July 21st, 2014 Our Rating: :: ORIGINAL WALKINGiPhone App - Designed for the iPhone, compatible with the iPad Walking is a bit more exciting thanks to this planet building/discovering sim reliant... | Read more »
Zombie Commando Review
Zombie Commando Review By Jennifer Allen on July 21st, 2014 Our Rating: :: MINDLESS SLAUGHTERUniversal App - Designed for iPhone and iPad Briefly fun but ultimately forgettable, Zombie Commando will scratch an itch then be... | Read more »
Swords & Poker Adventures Review
Swords & Poker Adventures Review By Jennifer Allen on July 21st, 2014 Our Rating: :: SOULLESS POKER PLAYUniversal App - Designed for iPhone and iPad Swords & Poker Adventures is a mishmash of Poker and RPGing, but it lacks... | Read more »
Warhammer 40,000: The Horus Heresy: Drop...
Warhammer 40,000: The Horus Heresy: Drop Assault Coming Soon to iOS Posted by Jennifer Allen on July 21st, 2014 [ permalink ] Coming soon to iOS will be an all-new Warhammer 40,000 tactical strategy game by the name of The Horus Heresy: Drop As | Read more »
A Life Worth Dying For Review
A Life Worth Dying For Review By Jordan Minor on July 21st, 2014 Our Rating: :: A BEAUTIFUL MINDUniversal App - Designed for iPhone and iPad A Life Worth Dying For is a fascinating portrait of a serious subject.   | Read more »
Zombie Puzzle Panic Review
Zombie Puzzle Panic Review By Jordan Minor on July 21st, 2014 Our Rating: :: THE MATCHING DEADUniversal App - Designed for iPhone and iPad Zombie Puzzle Panic puts some pretty neat undead twists on Match-3 puzzling.   | Read more »
This Week at 148Apps: July 14-18, 2014
Expert App Reviewers   So little time and so very many apps. What’s a poor iPhone/iPad lover to do? Fortunately, 148Apps is here to give you the rundown on the latest and greatest releases. And we even have a tremendous back catalog of reviews; just... | Read more »
Fallen Lords Review
Fallen Lords Review By Andrew Fisher on July 18th, 2014 Our Rating: :: FALLS SHORTiPad Only App - Designed for the iPad Fallen Lords is a decent game, but its similarity and inferiority compared to Ghost Stories makes it ultimately... | Read more »
Real Boxing’s New Combo Update is a Knoc...
Real Boxing’s New Combo Update is a Knockout Posted by Blake Grundman on July 18th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »

Price Scanner via MacPrices.net

Twelve South HiRise For MacBook – Height-Adju...
If you use your MacBook as a workhorse desktop substitute, as many of us do, a laptop stand combined with an external keyboard and pointing device are pretty much obligatory if you want to avoid... Read more
Why The Mac Was Not Included In The Apple/IBM...
TUAW’s Yoni Heisler cites Fredrick Paul of Network World whoi blogged last week that the Mac’s conspicuous absence from Apple and IBM’s landmark partnership agreement represents a huge squandered... Read more
Save $100 on 13-inch Retina MacBook Pros, plu...
Adorama has 13″ Retina MacBook Pros on sale for $100 off MSRP. Shipping is free, and Adorama charges sales tax in NY & NJ only: - 13″ 2.4GHz/128GB MacBook Pro with Retina Display: $1199 - 13″ 2.... Read more
Blurr it 2.3 for iOS – Quickly Blurs Selected...
Hyderabad, India based TouchLabs has announced a new update of Blurr it 2.3, their photography app for iOS users. Blurr it allows you to blur part of the image to hide potentially sensitive or... Read more
MacBook Airs on sale for $100 off MSRP, start...
Best Buy has the new 2014 MacBook Airs on sale for up to $100 off MSRP on their online store. Choose free home shipping or free local store pickup (if available). Prices valid for online orders only... Read more
Amazon Announces Kindle Unlimited: Unlimited...
Amazon.com has introduced Kindle Unlimited — a new subscription service which allows customers to freely read as much as they want from over 600,000 Kindle books, and listen as much as they want to... Read more
New Linksys Wireless Range Extenders Boost Wi...
Linksys has announced its new lineup of Linksys Wi-Fi Range Extenders. Consumers often experience a weak wireless signal in some parts of their house or apartment caused by blocking elements such as... Read more
MacBook Airs available starting at $719
The Apple Store has Apple Certified Refurbished 2013 & 2012 MacBook Airs in stock today starting at $719. An Apple one-year warranty is included with each MacBook, and shipping is free: 2013... Read more
Get the best deals on iPad minis with Apple r...
The Apple Store has Certified Refurbished 2nd generation iPad minis with Retina Displays available for up to $130 off the cost of new models, starting at $339. Apple’s one-year warranty is included... Read more
Best Buy’s College Student Deals: $100 off Ma...
Take an additional $100 off all MacBooks and iMacs, $50 off iPad Airs and iPad minis, at Best Buy Online with their College Students Deals Savings, valid through July 25th. Anyone with a valid .EDU... Read more

Jobs Board

*Apple* Computer Technician - Fairfield Coun...
Company DescriptionWe are an Apple Authorized Sales and Service Provider. We have been selling and servicing Apple computers in the Fairfield County area for over 20 Read more
*Apple* Computer Technician - Fairfield Coun...
Company DescriptionWe are an Apple Authorized Sales and Service Provider. We have been selling and servicing Apple computers in the Fairfield County area for over 20 Read more
Mac Expert - *Apple* Online Store Mexico -...
…MUST be fluent in English and Spanish to be considered for this position At Apple , we believe that hard work, a fun environment, creativity and innovation fuel the Read more
*Apple* Computer Technician - Fairfield Coun...
Company DescriptionWe are an Apple Authorized Sales and Service Provider. We have been selling and servicing Apple computers in the Fairfield County area for over 20 Read more
Mac Expert - *Apple* Online Store - Apple (...
**Job Summary** At Apple , we believe that hard work, a fun environment, creativity and innovation fuel the ultimate customer experience. We believe each customer Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.