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

Dropbox 193.4.5594 - Cloud backup and sy...
Dropbox is a file hosting service that provides cloud storage, file synchronization, personal cloud, and client software. It is a modern workspace that allows you to get to all of your files, manage... Read more
Google Chrome 122.0.6261.57 - 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
Skype 8.113.0.210 - Voice-over-internet...
Skype is a telecommunications app that provides HD video calls, instant messaging, calling to any phone number or landline, and Skype for Business for productive cooperation on the projects. This... Read more
Tor Browser 13.0.10 - Anonymize Web brow...
Using Tor Browser you can protect yourself against tracking, surveillance, and censorship. Tor was originally designed, implemented, and deployed as a third-generation onion-routing project of the U.... Read more
Deeper 3.0.4 - Enable hidden features in...
Deeper is a personalization utility for macOS which allows you to enable and disable the hidden functions of the Finder, Dock, QuickTime, Safari, iTunes, login window, Spotlight, and many of Apple's... Read more
OnyX 4.5.5 - Maintenance and optimizatio...
OnyX is a multifunction utility that you can use to verify the startup disk and the structure of its system files, to run miscellaneous maintenance and cleaning tasks, to configure parameters in the... Read more
Hopper Disassembler 5.14.1 - Binary disa...
Hopper Disassembler is a binary disassembler, decompiler, and debugger for 32- and 64-bit executables. It will let you disassemble any binary you want, and provide you all the information about its... Read more
WhatsApp 24.3.78 - Desktop client for Wh...
WhatsApp is the desktop client for WhatsApp Messenger, a cross-platform mobile messaging app which allows you to exchange messages without having to pay for SMS. WhatsApp Messenger is available for... Read more
War Thunder 2.33.0.135 - Multiplayer war...
In War Thunder, aircraft, attack helicopters, ground forces and naval ships collaborate in realistic competitive battles. You can choose from over 1,500 vehicles and an extensive variety of combat... Read more
Iridient Developer 4.2 - Powerful image-...
Iridient Developer (was RAW Developer) is a powerful image-conversion application designed specifically for OS X. Iridient Developer gives advanced photographers total control over every aspect of... Read more

Latest Forum Discussions

See All

Gorgeous Tactical Puzzle Game ‘Howl’ is...
Following its release on PC and Nintendo Switch this past November, and it’s arrival on Xbox and PlayStation back in January, publisher Astragon Entertainment and developer Mi’pu’mi Games are now bringing their super stylish tactical puzzler Howl to... | Read more »
Best iPhone Game Updates: ‘Shoot the Moo...
Hello everyone, and welcome to the week! It’s time once again for our look back at the noteworthy updates of the last seven days. It feels like a bit of a dry spell this week, at least in terms of really interesting updates. I mean, I found some... | Read more »
Celebrate Phobies spooky second annivers...
Get ready to have that classic song stuck in your head, as Phobies celebrates its second anniversary with the release of its latest update; Birthday Bash, Monster Mash. Starting March 5th and lasting for four weeks, it will be a month of... | Read more »
‘Dissidia Final Fantasy Opera Omnia’ Sto...
Square Enix finally shut down Dissidia Final Fantasy Opera Omnia (Free) on iOS and Android last week following the end of service announcement back in November last year. Following the game shutting down, Square Enix | Read more »
‘Monster Hunter Now’ Is Celebrating the...
Niantic and Capcom have begun celebrating the 20th anniversary of Capcom’s best franchise from today inside Monster Hunter Now (Free) on iOS and Android for a limited time. | Read more »
New ‘Warframe Mobile’ Update Adds 60fps...
Warframe Mobile (Free) launched worldwide on iOS just under two weeks ago. I’ve been playing it for review across multiple iOS devices, but have also been picking it up on Steam Deck and Switch to compare. Right from launch, I was impressed with... | Read more »
Passionate About Fidget Toys – The Touch...
In this week’s episode of The TouchArcade Show we kick things off with some passionate discussion about… fidget toys? For some reason? We quickly change gears to talk about the card-based rogulike Balatro, which we’ve both been playing and enjoying... | Read more »
TouchArcade Game of the Week: ‘Flying Ta...
For me Hexage is one of those developers that harkens back to the early days of the App Store and really the beginnings of iPhone gaming. I have spent many collective hours playing the likes of Totemo, Radiant, Radiant Defense, EVAC, Reaper… the... | Read more »
SwitchArcade Round-Up: ‘Ufouria 2: The S...
Hello gentle readers, and welcome to the SwitchArcade Round-Up for March 1st, 2024. In today’s article, we’re looking at the remaining releases of the week. There are a few really good ones today, but the bin bunch certainly isn’t going home hungry... | Read more »
Steam Deck Weekly: Reviews of PowerWash...
Welcome to the first Steam Deck Weekly of March and this week’s edition is bigger than usual. I was a bit unwell last week and had to push some reviews to this week. Alongside that, there have been many notable announcements, releases, and new Steam... | Read more »

Price Scanner via MacPrices.net

Deal Alert! B&H is now selling 13-inch M2...
B&H Photo has 13″ MacBook Airs with M2 CPUs and 256GB of storage in stock and on sale for $100 off Apple’s new MSRP, now only $899. Free 1-2 day delivery is available to most US addresses. Their... Read more
At $999, Apple’s 13-inch M2 MacBook Air is th...
With today’s introduction of the new 13-inch M3 MacBook Air for $1099, Apple dropped prices on the previous-generation 13-inch M2 MacBook Air to $999. At the same time, Apple discontinued the 13-inch... Read more
Apple discontinues 15-inch M2 MacBook Airs, d...
With today’s introduction of new M3-powered 15″ MacBook Airs, Apple has dropped prices on clearance, Certified Refurbished, 15″ M2 MacBook Airs to a new low of $1019. These are the cheapest 15″... Read more
Price Drop! 13-inch M2 MacBook Airs at Apple...
Apple has dropped prices on Certified Refurbished 13″ M2 MacBook Airs to a new low of $849. These are the cheapest M2-powered MacBooks for sale at Apple. Apple’s one-year warranty is included,... Read more
Apple finally discontinues the 13-inch M1 Mac...
With the introduction of M3-powered 13″ MacBook Airs today, Apple has dropped prices on clearance 13″ M1 MacBook Airs, Certified Refurbished, to $759 for 8-Core CPU/7-Core GPU/256GB models and $929... Read more
Updated Apple iPad Price Trackers
Our Apple award-winning iPad Price Trackers are the best place to find the latest information on iPad sales and deals. We track prices from 20+ Apple retailers, including Apple, Amazon, Best Buy,... Read more
Updated Apple MacBook Price Trackers
Our Apple award-winning MacBook Price Trackers are continually updated with the latest information on prices, bundles, and availability for 16″, 14″, and (recently-discontinued) 13″ MacBook Pros... Read more
Mac Studios with Apple M2 Max and M2 Ultra CP...
B&H Photo has the standard-configuration Mac Studio model with Apple’s M2 Ultra CPU in stock today and on sale for $300 off MSRP, now $3699 (24-Core CPU and 64GB RAM/1TB SSD). B&H Photo has... Read more
Extended: Switch to Verizon and get the Apple...
Verizon has the iPhone 15 on sale for $0 per month when you add a new line if service. Discount is applied to your account monthly over a 36 month term and is valid for the 128GB model. For the first... Read more
Select 16-inch M3 Pro and M3 Max MacBook Pros...
B&H Photo has select 16-inch M3 Pro and M3 Max MacBook Pros on sale for $250 off MSRP. Their prices are the lowest currently available for these configurations. Free 1-2 day shipping is available... Read more

Jobs Board

Teller Part Time *Apple* Valley MN *Apple*...
…is not eligible for Visa sponsorship **Posting Location:** + 15574 Pilot Knod Road Apple Valley, MN 55124 @RWF22 **Posting End Date:** Job posting may come down Read more
*Apple* End User Support Specialist - North...
…that they are performed. + Responsible for support of all College owned Apple computers, mobile ios devices, and peripherals, and for diagnosing and resolving Read more
Omnichannel Associate - *Apple* Blossom Mal...
Omnichannel Associate - Apple Blossom Mall Location:Winchester, VA, United States (https://jobs.jcp.com/jobs/location/191170/winchester-va-united-states) - Apple Read more
Teller Part Time *Apple* Valley MN *Apple*...
…is not eligible for Visa sponsorship **Posting Location:** + 15574 Pilot Knod Road Apple Valley, MN 55124 @RWF22 **Posting End Date:** Job posting may come down Read more
Nurse Anesthetist - *Apple* Hill Surgery Ce...
Nurse Anesthetist - Apple Hill Surgery Center WellSpan Medical Group, York, PA | Advanced Practice Providers | Certified Registered Nurse Anesthetists | FTE: 1 | Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.