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.

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:

 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:


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"

  [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>

  int value;

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


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...


Community Search:
MacTech Search:

Software Updates via MacUpdate

Google Chrome 54.0.2840.71 - 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
HoudahSpot 4.2.6 - Advanced file-search...
HoudahSpot is a powerful file search tool. Use HoudahSpot to locate hard-to-find files and keep frequently used files within reach. HoudahSpot will immediately feel familiar. It works just the way... Read more
Yummy FTP Pro 1.11.11 - $29.99
Yummy FTP Pro is an advanced Mac file transfer app which provides a full-featured professional toolkit combined with blazing speeds and impeccable reliability, so whether you want to transfer a few... Read more
Shimo - VPN client – for everyon...
Shimo is the most versatile VPN client for OS X and it enables really everybody to master secure network. It supports more protocols than any other VPN application out there! CiscoVPN, AnyConnect,... Read more
Dash 3.4.0 - Instant search and offline...
Dash is an API documentation browser and code snippet manager. Dash helps you store snippets of code, as well as instantly search and browse documentation for almost any API you might use (for a full... Read more
AirRadar 3.1.9 - $9.95
With AirRadar, scanning for wireless networks is now easier and more personalized! It allows you to scan for open networks and tag them as favourites or filter them out. View detailed network... Read more
Printopia 2.1.22 - Share Mac printers wi...
Run Printopia on your Mac to share its printers to any capable iPhone, iPad or iPod Touch. Printopia will also add virtual printers, allowing you to save print-outs to your Mac and send to apps.... Read more
SteerMouse 5.0 - Powerful third-party mo...
SteerMouse is an advanced driver for USB and Bluetooth mice. It also supports Apple Mighty Mouse very well. SteerMouse can assign various functions to buttons that Apple's software does not allow,... Read more
Alarm Clock Pro 10.2.5 - $19.95
Alarm Clock Pro isn't just an ordinary alarm clock. Use it to wake you up in the morning, send and compose e-mails, remind you of appointments, randomize the iTunes selection, control an internet... Read more
Cocktail 10.1 - General maintenance and...
Cocktail is a general purpose utility for macOS 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

Latest Forum Discussions

See All

Game of Dice is now available on Faceboo...
After celebrating its anniversary in style with a brand new update, there’s even more excitement in store for Game of Dice has after just being launched on Facebook Gameroom. A relatively new platform, Facebook Gameroom has been designed for PC... | Read more »
4 addictive clicker games like Best Fien...
Clickers are passive games that take advantage of basic human psychology to suck you in, and they're totally unashamed of that. As long as you're aware that this game has been created to take hold of your brain and leave you perfectly content to... | Read more »
Smile Inc. Guide: How not to die on the...
As if Mondays weren't bad enough, at Smile Inc. you have to deal with giant killer donuts, massive hungry staplers, and blasting zones. It's not exactly a happy, thriving work environment. In fact, you'll be lucky to survive the nine to five.... | Read more »
Oh...Sir! The Insult Simulator (Games)
Oh...Sir! The Insult Simulator 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: | Read more »
WitchSpring2 (Games)
WitchSpring2 1.27 Device: iOS Universal Category: Games Price: $3.99, Version: 1.27 (iTunes) Description: This is the story of Luna, the Moonlight Witch as she sets out into the world. This is a sequel to Witch Spring. Witch Spring 2... | Read more »
4 popular apps getting a Halloween makeo...
'Tis the season for all things spooky. So much, so, in fact, that even apps are getting into the spirt of things, dressing up in costume and spreading jack o' lanterns all about the place. These updates bring frightening new character skins, scary... | Read more »
Pokémon GO celebrates Halloween with can...
The folks behind Pokémon GO have some exciting things planned for their Halloween celebration, the first in-game event since it launched back in July. Starting October 26 and ending on November 1, trainers will be running into large numbers of... | Read more »
Best Fiends Forever Guide: How to collec...
The fiendship in Seriously's hit Best Fiends has been upgraded this time around in Best Fiends Forever. It’s a fast-paced clicker with lots of color and style--kind of reminiscent of a ‘90s animal mascot game like Crash Bandicoot. The game... | Read more »
5 apps for the budding mixologist
Creating your own cocktails is something of an art form, requiring a knack for unique tastes and devising interesting combinations. It's easy to get started right in your own kitchen, though, even if you're a complete beginner. Try using one of... | Read more »
5 mobile strategy games to try when you...
Strategy enthusiasts everywhere are celebrating the release of Civilization VI this week, and so far everyone seems pretty satisfied with the first full release in the series since 2010. The series has always been about ultra-addictive gameplay... | Read more »

Price Scanner via

Macs’ Superior Enterprise Deployment Cost Eco...
IBM’s debunking of conventional wisdom and popular mythology about the relative cost of using Apple Mac computers as opposed to PCs running Microsoft Windows at the sixth annual Jamf Nation User... Read more
12-inch WiFi Apple iPad Pros on sale for $50-...
B&H Photo has 12″ WiFi Apple iPad Pros on sale for $50-$70 off MSRP, each including free shipping. B&H charges sales tax in NY only: - 12″ Space Gray 32GB WiFi iPad Pro: $749 $50 off MSRP -... Read more
Apple refurbished 12-inch 128GB iPad Pros ava...
Apple has Certified Refurbished 12″ Apple iPad Pros available for up to $160 off the cost of new iPads. An Apple one-year warranty is included with each model, and shipping is free: - 32GB 12″ iPad... Read more
Apple refurbished iPad minis and iPad Air 2s...
Apple recently dropped prices on several Certified Refurbished iPad mini 4s and 2s as well as iPad Air 2s. An Apple one-year warranty is included with each model, and shipping is free: - 16GB iPad... Read more
MacHTTP-js Preview Full-featured Web Server f...
MacHTTP.Org has released MacHTTP-js Preview for macOS, a full-featured Web server for 21st Century desktops and servers. MacHTTP-js is a modern take on the classic stand-alone, desktop computer Web... Read more
Samsung Galaxy Tab A 10.1 with S Pen Makes US...
Samsung Electronics America, Inc. has announced the release of the Galaxy Tab A 10.1 with S Pen in a highly mobile, lightweight tablet. “With an embedded S Pen, consumers can discover more ways to... Read more
13-inch 2.5GHz MacBook Pro (Apple refurbished...
Apple has Certified Refurbished 13″ 2.5GHz MacBook Pros available for $829, or $270 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free: - 13″ 2.5GHz MacBook Pros... Read more
27-inch iMacs on sale for up to $220 off MSRP
B&H Photo has 27″ Apple iMacs on sale for up to $200 off MSRP including free shipping plus NY sales tax only: - 27″ 3.3GHz iMac 5K: $2099 $200 off MSRP - 27″ 3.2GHz/1TB Fusion iMac 5K: $1899.99 $... Read more
13-inch 2.5GHz MacBook Pro available for $927...
Overstock has the 13″ 2.5GHz MacBook Pro available for $926.99 including free shipping. Their price is $172 off MSRP. Read more
Apple refurbished 2015 13-inch MacBook Airs a...
Apple has Certified Refurbished 2015 13″ MacBook Airs available starting at $759. An Apple one-year warranty is included with each MacBook, and shipping is free: - 2015 13″ 1.6GHz/4GB/128GB MacBook... Read more

Jobs Board

Security Data Analyst - *Apple* Information...
…data sources need to be collected to allow Information Security to better protect Apple employees and customers from a wide range of threats.Act as the subject Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Solutions Consultant - Apple (United...
# Apple Solutions Consultant Job Number: 52812872 Houston, Texas, United States Posted: Oct. 18, 2016 Weekly Hours: 40.00 **Job Summary** As an Apple Solutions Read more
Lead *Apple* Solutions Consultant - Apple (...
# Lead Apple Solutions Consultant Job Number: 52812906 Houston, Texas, United States Posted: Oct. 18, 2016 Weekly Hours: 40.00 **Job Summary** The Lead ASC is an Read more
*Apple* Retail - Multiple Positions- Towson,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.