TweetFollow Us on Twitter

May 97 Getting Started

Volume Number: 13 (1997)
Issue Number: 5
Column Tag: Getting Started

More Objective-C

by Dave Mark, ©1997, All Rights Reserved. http://www.spiderworks.com

Last month, we took our first taste of Objective-C. We learned that Objective-C sources use the extension ".m" for main source files and ".h" for header files. The type "id" is used to declare a generic pointer which allows us to delay type binding decisions. We learned that most objects are derived from the Root class Object. The Object class features variables and methods inherited by all other classes. One of these variables is "isa" which specifies the class to which an object belongs.

We saw the form for a class interface:

 MySuperClass
{
 instance variable declarations
}
method declarations

We also learned about Objective-C's funky form of method declaration:

- (int)getX:(int)x andY:(int)y;

Where the leading minus sign marks the function as an instance method - as opposed to a "+", which marks a method as a class method, like a C++ static method. The first "int" is the return type, and the other "int"s are the parameter types. The name of the method above is "getX:andY:" The default type is id, so if you leave out any of the types (including the return type) the type is set to id.

We also learned about the #import compiler directive, used to avoid multiple inclusion of a .h file:

#import "Object.h"

Alternatively, you can avoid all the extra junk you get when you import a classes' header file by using the @class directive to let the compiler know that a reference is to a legal class:


Finally, we saw the form used for the actual implementation of a class:

#import "MyClass.h"

Message Receivers and Message Syntax

Now it's on to new material. Once you declare your class and use that declaration to define an object, it's time to bring that object to life by sending it a message. In C++, you call an object's member function using the object itself to make the call:

myObjectPtr->MemberFunction();

In Objective-C, a message is said to be sent to a receiver using the following syntax:

[receiver message];

The receiver is an object and the message is a method, along with its associated parameters. The message is sent (and the appropriate method selected) at runtime. For example, suppose the class named MyClass included the following method:

- (int)getX:(int)x andY:(int)y;

Just as a reminder, this method is called getX:andY:, takes two parameters (both of them ints), and returns an int. Here's a line of code that sends a message asking a MyClass object to perform the getX:andY: method:

myNum = [myObj getX:27:50];

In this case, the object myObj (we'll see how to allocate an object in a second), presumably of class MyClass, gets sent the getX:andY: message, along with the parameters 27 and 50.

Allocating an Object

To allocate a new object, you'll send an alloc message to the class whose object you want to create. Here's an example:

id myObj = [[MyClass alloc] init];

You'll use this line of code again and again, with a few alterations of course. We start off by defining an id to hold the reference to the allocated object. The right side of the "=" operator is a message expression embedded in another method expression, much as you might embed a function call inside a second function call (printf( "%d", GetMyValue()); for example).

In this case, we are first sending the alloc message to MyClass. The alloc method is a factory method (it is declared with a leading "+" sign instead of a leading "-"), similar to a static member function in C++. It is inherited from the Object class. The alloc method allocates the memory for a single object of the specified class and returns a pointer to the object.

That object is then sent the init message, which causes that object's init method to be performed. A classes' init method should return a pointer to the object (otherwise this code wouldn't work). The object pointer returned by this nested pair of message expressions is assigned to the object pointer myObj.

As you would expect, Objective-C has an analogy to the C++ keyword "this". To refer to the current object, use the term "self". When you write your init method (to initialize your newly alloc'ed object), you'll end it by returning self:

return self;

Our First Objective-C Program

Let's bring all these concepts together with a simple example. We'll create a Number class that features a single variable, an int to hold the Number's current value. We'll add methods for initialization, one to square the Number's current value, and one to print out the current value. While this example may seem trivial, it will act as a nice syntax reference as you build your own classes.

By the time you read this, Metrowerks should have delivered their first Objective-C tools (scheduled for release as part of May's CW12). Since I don't have those tools in hand yet, I had to look at alternative Objective-C environments. As I mentioned last month, Tenon makes a Mac environment called CodeBuilder which supports the gcc Objective-C compiler. CodeBuilder supports an X environment called AfterStep, along with a more conventional unix terminal environment running either C Shell, T-C Shell (an improved C Shell), the traditional Bourne Shell (less powerful, but uses less memory), and the Bourne Again Shell (bash, Bourne Shell with improvements). All this rides on top of a Berkeley 4.4BSD unix system.

If you are into unix, CodeBuilder is definitely a lot of fun to play with. On the other hand, I find myself aching for the CodeWarrior IDE and editor to tie my projects together. Though I must admit that I did enjoy using vi to edit all my source code. Amazingly, I remembered all those cryptic vi commands that made vi such a pleasure/pain to use. Ah, well, enough reminiscing - on to the code...

The first thing I did was create a new folder to hold the source code (if you are using CodeBuilder, the unix command "mkdir dirName" creates a new directory, "cd new dir" changes directories, "mv oldname newname" changes file or directory names, "rmdir dir" deletes an empty directory, "vi filename" invokes the vi editor and "man command" brings up an online manual for the specified command. If you are new to unix or to vi, I would definitely try "man vi" to get a sense of the editor before you get into it).

Inside the new folder, I created three source files: Number.m, Number.h, and main.m. Here's the source for Number.m:

#import "Number.h"
//#include "Number.h"

(int)startValue
{
  [super init];

  value = startValue;

  return self;
}

- squareSelf
{
  value *= value;

  return self;
}

- print
{
  printf( "Number value: %d\n", value );

  return self;
}

The release of CodeBuilder I had did not support #import (at least not in the usual fashion). If your environment does not support #import, comment out that line and uncomment the #include. Remember, if you use #include, you'll need to also make a corresponding change to the header file (we'll get to it next).

Number.m contains the implementation of the Number class. All three methods are instance methods and are declared using the leading "-". Notice that the init method takes a single parameter, startValue, an int used to set the value of the instance variable named value. Value is declared in Number.h.

The init method starts off by sending an init method to its superclass (in this case, the Object class). Doing this gives your superclass a chance to initialize its superclasses and itself, a real good idea. After setting value to startValue, init returns a pointer to the newly initialized object (remember, self is like this in C++).

squareSelf and print are fairly straightforward. Note that both return self, though they don't necessarily need to, since their return value is ignored by main. Also, you can see that the C standard library function printf() is available in Objective-C. As Rhapsody starts to kick in, I'll start making use of Mac interface calls instead of using console i/o but, for now, we need to make do with what we have.

Here's the source for Number.h:

//#ifndef _Number_h_
//#define _Number_h_

#import <objc/Object.h>
//#include <objc/Object.h>


 Object
{
  int value;
}

- init:(int)startValue;
- squareSelf;
- print;

//#endif

Once again, if your environment doesn't support #import, uncomment all the commented code above and comment out the #import line. Note the declaration of the instance variable named value along with the declaration of the three methods init, squareSelf, and print. Note that all three return an id, since no return value is declared. That's why the:

return self;

at the end of each method makes sense.

Here's the source for main.m:

#include "Number.h"

void main()
{
  id number = [[Number alloc] init:27];

  [number print];

  [number squareSelf];
  [number print];

  [number free];
}

main() is just a sequence of method expressions. First, we declare number to be an object pointer, then we allocate and initialize a new Number object and assign the object's pointer (returned by init) to number. Note that the value 27 was passed in as a parameter to init. Got it?

The rest of the program is cake. Send number the print message. Here's the line of output that appears in the console window:

Number value: 27

Next, send number the squareSelf message. This causes the number object to square the value in value. Before you reach for your calculators, 27 * 27 = 729. Really.

To verify this, we pass another print message on to number. Here's the line that appears in the console window:

Number value: 729

Finally, we send the free message to number. Since the Number class does not override the Object classes' free method, the Object classes' free method is the one that ends up getting called free. The Object version of free deallocates all memory allocated for the object by alloc. Note that the Object version of free does not follow any pointers in your object to free any secondary memory. If you do allocate additional memory in your init method (don't override alloc), you'll want to override free and deallocate the additional memory.

If you do override free, be sure to send the free message to super:

[super free];

at the end of your free method to give super classes a chance to clean up after themselves.

By the way, if you are using CodeBuilder, you might want to take advantage of the unix tool called Make. Create a file called Makefile in the same directory as your sources, and type in these two lines:

Number: main.m Number.m Number.h
 gcc -o Number main.m Number.m -lobjc

Now you can recompile your program by just typing the command Make.

Till Next Month...

I am really enjoying the opportunity to mess with Objective-C. Though the syntax threw me at first, it didn't take me long before I was thinking in it. This is not a hard language. Just a bit weird! I'm definitely looking forward to getting Rhapsody up and running on my machine so I can experiment with true dynamic binding. Excellent!

Just a heads up - I have gotten a lot of email asking me to update my How to Get Started With Mac Programming article. So much has changed in the Mac universe (Rhapsody, the Internet, Objective-C, Java, etc.) that the old recommendations just don't hold water anymore. Within the next couple of months, I'm going to interrupt the Objective-C series (just for one month!) to run a new version of that article. For those of you who are already way Mac programmers, please bear with me. Till then, it's back to unix, vi, and Objective-C...

 
AAPL
$108.00
Apple Inc.
+1.02
MSFT
$46.95
Microsoft Corpora
+0.90
GOOG
$559.08
Google Inc.
+8.77

MacTech Search:
Community Search:

Software Updates via MacUpdate

Vitamin-R 2.20b1 - Personal productivity...
Vitamin-R creates the optimal conditions for your brain to work at its best by structuring your work into short bursts of distraction-free, highly focused activity alternating with opportunities for... Read more
Dropbox 2.10.44 - Cloud synchronization...
Dropbox is an application that creates a special Finder folder that automatically syncs online and between your computers. It allows you to both backup files and keep them up-to-date between systems... Read more
Sandvox 2.9.2 - Easily build eye-catchin...
Sandvox is for Mac users who want to create a professional looking website quickly and easily. With Sandvox, you don't need to be a Web genius to build a stylish, feature-rich, standards-compliant... Read more
Cocktail 8.0.1 - General maintenance and...
Cocktail is a general purpose utility for OS X that lets you clean, repair and optimize your Mac. It is a powerful digital toolset that helps hundreds of thousands of Mac users around the world get... Read more
LibreOffice 4.3.3.2 - Free Open Source o...
LibreOffice is an office suite (word processor, spreadsheet, presentations, drawing tool) compatible with other major office suites. The Document Foundation is coordinating development and... Read more
VMware Fusion 7.0.1 - Run Windows apps a...
VMware Fusion allows you to create a Virtual Machine on your Mac and run Windows (including Windows 8.1) and Windows software on your Mac. Run your favorite Windows applications alongside Mac... Read more
OneNote 15.3.2 - Free digital notebook f...
OneNote is your very own digital notebook. With OneNote, you can capture that flash of genius, that moment of inspiration, or that list of errands that's too important to forget. Whether you're at... Read more
Audio Hijack Pro 2.11.4 - Record and enh...
Audio Hijack Pro drastically changes the way you use audio on your computer, giving you the freedom to listen to audio when you want and how you want. Record and enhance any audio with Audio Hijack... Read more
Iridient Developer 3.0.0 beta 3 - Powerf...
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
TextWrangler 4.5.11 - Free general purpo...
TextWrangler is the powerful general purpose text editor, and Unix and server administrator's tool. Oh, and also, like the best things in life, it's free. TextWrangler is the "little brother" to... Read more

Latest Forum Discussions

See All

Monster Flash Review
Monster Flash Review By Jordan Minor on October 31st, 2014 Our Rating: :: ALONE IN THE DARKUniversal App - Designed for iPhone and iPad Solid shooting and a surprising amount of spooky tension make Monster Flash a great portable... | Read more »
Retry Review
Retry Review By Rob Thomas on October 31st, 2014 Our Rating: :: SOARING HIGHUniversal App - Designed for iPhone and iPad Flappy who? Let Retry wash all those bad bird-related memories away on a cool retro-flavored flight… right... | Read more »
Dementia: Book of the Dead Review
Dementia: Book of the Dead Review By Lee Hamlet on October 31st, 2014 Our Rating: :: A TOUGH READUniversal App - Designed for iPhone and iPad A witch hunter is sent after a demonic book in the spooky but short-lived Dementia: Book... | Read more »
Card Dungeon, the Semi-Board Game Roguel...
Card Dungeon, the Semi-Board Game Roguelike, Has Been Renovated Posted by Jessica Fisher on October 31st, 2014 [ permalink ] | Read more »
Logitech Protection + Power iPhone5/5S C...
Made by: Logitech Price: $99.99 Hardware/iOS Integration Rating: 3 out of 5 stars Usability Rating: 0.5 out of 5 stars Reuse Value Rating: 0.75 out of 5 stars Build Quality Rating: 0.75 out of 5 stars Overall Rating: 1.25 out of 5 stars | Read more »
This Is Not a Test Goes Free, Permanentl...
This Is Not a Test Goes Free, Permanently Posted by Jessica Fisher on October 31st, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Swap Heroes Review
Swap Heroes Review By Campbell Bird on October 31st, 2014 Our Rating: :: STRATEGIC SWAPPINGUniversal App - Designed for iPhone and iPad Rotate a cast of heroes to fend of waves of monsters in this difficult, puzzle rpg.   | Read more »
Night Sky Pro™ (Reference)
Night Sky Pro™ 3.0.1 Device: iOS Universal Category: Reference Price: $2.99, Version: 3.0.1 (iTunes) Description: Night Sky Pro™Wonder No More™ Night Sky Pro™ is the ultimate stargazing experience. From the creators of the original... | Read more »
Audio Defence : Zombie Arena (Games)
Audio Defence : Zombie Arena 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: A zombie shooter audio game. Made from gut-wrenching 3D binaural sound, for a new kind of weird immersion. You... | Read more »
RPG Asdivine Hearts (Games)
RPG Asdivine Hearts 1.1.0 Device: iOS Universal Category: Games Price: $3.99, Version: 1.1.0 (iTunes) Description: SPECIAL PRICE50% OFF (USD 7.99 -> USD 3.99)!!! Travel alongside four companions and a cat in the adventure of a... | Read more »

Price Scanner via MacPrices.net

Apple now offering refurbished 2014 13-inch R...
The Apple Store is now offering Apple Certified Refurbished 2014 13″ Retina MacBook Pros for up to $270 off the cost of new models. An Apple one-year warranty is included with each model, and... Read more
Apple Regains Momentum As Windows Stutters An...
The latest smartphone sales data from Kantar Worldpanel ComTech, for the three months to March 2014, shows Apple performing strongly in the first quarter of the year, with sales bouncing back in... Read more
Worldwide Smartphone Shipments Increase 25.2%...
New smartphone releases and an increased emphasis on emerging markets drove global smartphone shipments above 300 million units for the second consecutive quarter, according to preliminary data from... Read more
Apple now offering refurbished 2014 15-inch M...
The Apple Store is now offering Apple Certified Refurbished 2014 15″ Retina MacBook Pros for up to $400 off the cost of new models. An Apple one-year warranty is included with each model, and... Read more
Apple drops prices on refurbished 2013 Retina...
The Apple Store has dropped prices on 2013 Apple Certified Refurbished 13″ and 15″ Retina MacBook Pros, with Retina models now available starting at $999. Apple’s one-year warranty is standard, and... Read more
New 2.8GHz Mac mini on sale for $949, save $5...
Abt Electronics has the new 2.8GHz Mac mini in stock and on sale for $949.05 including free shipping. Their price is $50 off MSRP, and it’s the lowest price available for this model from any reseller... Read more
Sale! 3.7GHz Quad Core Mac Pro available for...
 B&H Photo has the 3.7GHz Quad Core Mac Pro on sale for $2649 including free shipping plus NY sales tax only. Their price is $350 off MSRP, and it’s the lowest price for this model from any... Read more
Mujjo Steps Up The Game With Refined Touchscr...
Netherlands based Mujjo have just launched their Refined Touchscreen Gloves, stepping up their game. The gloves feature a updated elegant design that takes these knitted gloves to the next level. A... Read more
Sale! Preorder the new 27-inch 5K iMac for $2...
 Abt Electronics has the new 27″ 3.5GHz 5K iMac on sale and available for preorder for $2374.05 including free shipping. Their price is $125 off MSRP, and it’s the lowest price available for this... Read more
Simplex Solutions Inc. Brings Secure Web Surf...
New York based Simplex Solutions Inc. has announced the release and immediate availability of Private Browser 1.0, its revolutionary new secure web browser developed for iPhone, iPad and iPod touch... Read more

Jobs Board

Solutions Specialist with *Apple* Knowledge...
Company Description: We are an Apple Authorized Sales and Service Provider. We have been selling and servicing Apple computers in the Fairfield County area for over Read more
Position Opening at *Apple* - Apple (United...
**Job Summary** Every day, business customers come to the Apple Store to discover what powerful, easy-to-use Apple products can do for them. As a Business Leader, Read more
Sr. Manager, *Apple* Deployment Programs fo...
**Job Summary** Apple is seeking candidates for a new position on the Education Content and Technology team. iPad and Mac is in the hands of millions of teachers and Read more
*Apple* Solutions Consultant (ASC) - Apple I...
…important role that the ASC serves is that of providing an excellent Apple Customer Experience. Responsibilities include: * Promoting Apple products and solutions Read more
*Apple* Solutions Consultant (ASC) - Apple I...
…important role that the ASC serves is that of providing an excellent Apple Customer Experience. Responsibilities include: * Promoting Apple products and solutions Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.