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
$102.99
Apple Inc.
+0.52
MSFT
$44.38
Microsoft Corpora
-0.50
GOOG
$532.71
Google Inc.
+6.17

MacTech Search:
Community Search:

Software Updates via MacUpdate

OmniOutliner 4.1.3 - Organize your ideas...
OmniOutliner is a flexible program for creating, collecting, and organizing information. Give your creativity a kick start by using an application that's actually designed to help you think. It's... Read more
BBEdit 11.0 - Powerful text and HTML edi...
BBEdit is the leading professional HTML and text editor for the Mac. Specifically crafted in response to the needs of Web authors and software developers, this award-winning product provides a... Read more
Apple Security Update 2014-005 - For OS...
Apple Security Update is recommended for all users and improves the security of Mac OS X. For information on the security content of this update, please visit this website: http://support.apple.com/... Read more
EyeTV 3.6.6 - Watch and record TV on you...
EyeTV brings a rich TV experience to your Mac. Watch live TV on your Mac. Pause, rewind, and record whenever you want. EyeTV gives you powerful control over what you watch and how you watch it. Put... Read more
RapidWeaver 6.0 - Create template-based...
RapidWeaver is a next-generation Web design application to help you easily create professional-looking Web sites in minutes. No knowledge of complex code is required, RapidWeaver will take care of... Read more
NTFS 12.0.39 - Provides full read and wr...
Paragon NTFS breaks down the barriers between Windows and OS X. Paragon NTFS effectively solves the communication problems between the Mac system and NTFS, providing full read and write access to... Read more
RestoreMeNot 2.0.3 - Disable window rest...
RestoreMeNot provides a simple way to disable the window restoration for individual applications so that you can fine-tune this behavior to suit your needs. Please note that RestoreMeNot is designed... Read more
Command-C 1.1.5 - Clipboard sharing tool...
Command-C is a revolutionary app which makes easy to share your clipboard between iOS and OS X using your local WiFi network, even if the app is not currently opened. Copy anything (text, pictures,... Read more
Macgo Blu-ray Player 2.10.9.1750 - Blu-r...
Macgo Mac Blu-ray Player can bring you the most unforgettable Blu-ray experience on your Mac. Overview Macgo Mac Blu-ray Player can satisfy just about every need you could possibly have in a Blu-ray... Read more
Apple iOS 8.1 - The latest version of Ap...
The latest version of iOS can be downloaded through iTunes. Apple iOS 8 comes with big updates to apps you use every day, like Messages and Photos. A whole new way to share content with your family.... Read more

Latest Forum Discussions

See All

TinType by Hipstamatic (Photography)
TinType by Hipstamatic 1.0 Device: iOS iPhone Category: Photography Price: $.99, Version: 1.0 (iTunes) Description: Create hauntingly beautiful, soul capturing portraits with TinType by Hipstamatic. Inspired by daguerreotypes,... | Read more »
The Latest Update for Heroes of Dragon A...
The Latest Update for Heroes of Dragon Age Introduces Daily PvE Challenges Posted by Ellis Spice on October 22nd, 2014 [ permalink ] | Read more »
A New Trailer has Been Revealed for Epic...
A New Trailer has Been Revealed for Epic of Kings Posted by Jessica Fisher on October 22nd, 2014 [ permalink ] Dead Mage Inc. has released a new, action-packed trailer for the upcoming Epic of Kings. | Read more »
Find the Line Review
Find the Line Review By Campbell Bird on October 22nd, 2014 Our Rating: :: INSLIDE THE LINESUniversal App - Designed for iPhone and iPad Tease out beautiful line drawings in this unique, free-to-play puzzle game.   | Read more »
The Silent Age Episode 2 Review
The Silent Age Episode 2 Review By Jennifer Allen on October 22nd, 2014 Our Rating: :: ROUNDING THINGS OFF NICELYUniversal App - Designed for iPhone and iPad Rounding off a great point and click adventure comes The Silent Age... | Read more »
Craft Your Own Mini-Games with Papercade
Craft Your Own Mini-Games with Papercade Posted by Jessica Fisher on October 22nd, 2014 [ permalink ] iPad Only App - Designed for the iPad Scrapbookers move over, Scrapgaming is the new thing. | Read more »
Reshape Review
Reshape Review By Jennifer Allen on October 22nd, 2014 Our Rating: :: SIMPLE SHAPESUniversal App - Designed for iPhone and iPad Match triangles together to form cubes in this fast-paced and twitchy game.   | Read more »
Miika (Games)
Miika 1.0.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0.0 (iTunes) Description: Miika is a beautiful 3D puzzle game based on camera perspectives combined with the use of optical illusions. Miika challenges... | Read more »
Infuse Pro (Photography)
Infuse Pro 3.0 Device: iOS Universal Category: Photography Price: $9.99, Version: 3.0 (iTunes) Description: ** All-new version 3 includes fully licensed and certified DTS® and DTS-HD® audio! ** | Read more »
Swap Heroes (Games)
Swap Heroes 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: **Half price for a limited time only** Swap Heroes is a casual turn-based strategy adventure. Form a group of heroes and guide them... | Read more »

Price Scanner via MacPrices.net

Save with Best Buy’s College Student Deals
Take an additional $50 off all MacBooks and iMacs at Best Buy Online with their College Students Deals Savings, valid through November 1st. Anyone with a valid .EDU email address can take advantage... Read more
iPad Air 2 & iPad mini 3 Best Tablets Yet...
The new iPads turned out to be pretty much everything I’d been hoping for and more than I’d expected.”More” particularly in terms of a drinking-from-a-firehose choice of models and configurations,... Read more
Drafts 4 Reinvents iOS Productivity App
N Richland Hills, Texas based Agile Tortoise has announced the release of Drafts 4 for iPhone and iPad. Drafts is a quick capture note taking app with flexible output actions. Drafts 4 scales from... Read more
AT&T accepting preorders for new iPads fo...
AT&T Wireless is accepting preorders for the new iPad Air 2 and iPad mini 3, cellular models, for $100 off MSRP with a 2-year service agreement: - 16GB iPad Air 2 WiFi + Cellular: $529.99 - 64GB... Read more
Apple offering refurbished Mac Pros for up to...
The Apple Store is offering Apple Certified Refurbished 2013 Mac Pros for up to $600 off the cost of new models. An Apple one-year warranty is included with each Mac Pro, and shipping is free. The... Read more
Select MacBook Airs $100 off MSRP, free shipp...
B&H Photo has 2014 a couple of MacBook Airs on sale for $100 off MSRP. Shipping is free, and B&H charges NY sales tax only. They also include free copies of Parallels Desktop and LoJack for... Read more
13-inch 2.5GHz MacBook Pro on sale for $100 o...
B&H Photo has the 13″ 2.5GHz MacBook Pro on sale for $999.99 including free shipping plus NY sales tax only. Their price is $100 off MSRP. Read more
Strong iPhone, Mac And App Store Sales Drive...
Apple on Monday announced financial results for its fiscal 2014 fourth quarter ended September 27, 2014. The Company posted quarterly revenue of $42.1 billion and quarterly net profit of $8.5 billion... Read more
Apple Posts How-To For OS X Recovery
OS X 10.7 Lion and later include OS X Recovery. This feature includes all of the tools you need to reinstall OS X, repair your disk, and even restore from a Time Machine backup. OS X Recovery... Read more
Mac OS X Versions (Builds) Supported By Vario...
Apple Support has posted a handy resource explaining which Mac OS X versions (builds) originally shipped with or are available for your computer via retail discs, downloads, or Software Update. Apple... Read more

Jobs Board

Senior Event Manager, *Apple* Retail Market...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global event strategy. Delivering an overarching brand story; in-store, Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
Project Manager / Business Analyst, WW *Appl...
…a senior project manager / business analyst to work within our Worldwide Apple Fulfillment Operations and the Business Process Re-engineering team. This role will work Read more
*Apple* Retail - Multiple Positions (US) - A...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Position Opening at *Apple* - Apple (United...
…customers purchase our products, you're the one who helps them get more out of their new Apple technology. Your day in the Apple Store is filled with a range of Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.