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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.]


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...


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.


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;
 Rect Location;  
/* LOCAL location variable */
 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; 

void COval :: Erase(void)
 Rect Location;
 Location = this->Location;

void CRectangle :: Draw(void)
 Rect Location;
 Location = this->Location;

void CRectangle :: Erase(void)
 Rect Location;
 Location = this->Location;

void CLine :: Draw(void)

void CLine :: Erase(void)
 int    oldPenMode;

 oldPenMode = thePort->pnMode;
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);
 for (i=0;i<4;i++) 
 ALine = new(CLine); 
 for (i=0;i<4;i++) 
 AnOval[i] = new(COval);
 for (i=0;i<4;i++) 
 for (i=0;i<4;i++)


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