TweetFollow Us on Twitter

Jun 97 - Getting Started

Volume Number: 13 (1997)
Issue Number: 6
Column Tag: Getting Started

Filling in Some of the Objective-C Pieces

by Dave Mark

Last month, we learned most of the syntax of the Objective-C language. Since then, I have been scorching the e-mail, newsgroups, and phone lines trying to learn more. Much thanks to Michael Rutman, David Klingler, Bob McBeth, and Eric Gundrum for their time and energies in trying to get me on the straight and narrow. As always, the good stuff is theirs, the mistakes, mine.

@private, @public, and @protected

For me, much of the Objective-C learning process involves learning the differences between C++ and Objective-C. For example, C++ allows you to use the access specifiers public:, private:, and protected: to define the scope of a classes' data members and member functions. Objective-C offers a similar mechanism you can use to specify the scope of a classes' instance variables (methods are always public): the compiler directives @private, @public, and @protected.

Here's the official description of each of these compiler directives:

@private
the instance variable is accessible only within the class that declares it.
@protected
The instance variable is accessible within the class that declares it and within classes that inherit it.
@public
The instance variable is accessible everywhere.

Instance variables default to @protected, which makes sense. After all, if you mark an instance variable as @public, that would defeat the whole point of data encapsulation. The point is, you want to force access to your instance variables to occur via one of your classes' methods. So just forget about the @public compiler directive. The default setting of @protected will serve you in the vast majority of cases.

The @private directive does have its place, though. You would use @private if you don't want the instance variable inherited by subclasses. Perhaps you don't want a subclass monkeying with a variable that is key to the architecture of the base class. Or perhaps you want to minimize the dependencies between the base and sub classes. Though @private does have its place, don't use it unless you absolutely have a reason to. The general opinion seems to hold that you should never use @private at all -- that all classes should have all functionality overridable. Just wanted to make sure you heard both sides...

Here's an example that uses all three directives:

@interface Employee : Object
{
	char		*name;

@private
	int		yearsWithCompany;
	int		hoursVacation;

@protected
	char		*title;

@public
	id			supervisor;
	id			officeMate;
}

The @public, @private, and @protected compiler directives hold true for all instance variables that follow until either the end of the class or another directive is encountered. In the example above, the name variable is @protected, since it is not marked otherwise. yearsWithCompany and hoursVacation are @private, title is protected, and supervisor and officeMate are public. Of course, this sample was just to show you how this works and is not intended as realistic code.

Bottom line, your best bet is to leave these directives out of your code and just use the default setting of @protected. On the other hand, it is worth knowing how this works so you can read sample code that uses it and so you can use @private if you find a case where it makes sense.

Init vs. Init:

In last month's column, we looked at a sample program that included a simple class named Number. Here's the Number implementation:

#import "Number.h"

@implementation Number 

- init:(int)startValue /* This is BAD FORM - see below */
{
 [super init];

 value = startValue;

 return self;
}

- squareSelf
{
 value *= value;

 return self;
}

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

 return self;
}

@end

The Number class includes a method named init: which takes a single parameter. As it turns out, calling an initialization method init when it takes a parameter is a bad thing. The name init should be reserved for initialization methods with no parameters. Imagine if you had two different classes, each of which declared an init: method, one of which took a float, and one of which took an int as a parameter. Now imagine you had two object pointers, each declared as an id, one pointing to an object of one class, the second pointing to an object of the second class. If you send an init: method to one of these objects, the fact that both init: methods have the same name and yet take different parameter types will cause confusion and potential bad behavior. The name of your initialization method is what sets it apart from others. You'll see examples of this throughout the remainder of this column.

In the simplest case, an initialization method with no parameters, you'll definitely want to use the name init. The name init implies no parameters. The parameterless init starts off by sending the init method to its superclass, then initializing any instance variables that don't depend on parameters, and finally returning self.

Here's an example:

- init
{
	[super init];

	blockSize = 512;

	return self;
}

In this hypothetical example, blockSize is an instance variable whose initial value does not depend on a parameter. (In real life, we'd likely use a #define or, in C++ a const, but bear with the example.) Note that we sent the init message to our superclass before we do anything else. It is important that you send the initialization message to your superclass before you mess with your instance variables or call any of your other methods. Reason being, when you call your superclasses' initialization method, you give your superclass a chance to initialize its variables and a chance to initialize its superclass, etc.

If your class requires an initialization method that takes a parameter, give it a name that starts with init, then add text that reflects the parameters. For example, suppose you had a sequence of classes, Shape, Circle, and Cylinder, where Circle was derived from Shape, and Cylinder derived from Circle. Asssuming it took no parameters, the Shape initialization method would be called init. The Circle initialization method would require a radius, and might be called initRadius:, and the Cylinder's initialization method might be called initRadius:height:. You get the idea.

A Multi-Class Example

Designing your initialization methods can get a little more complex when you are working with subclasses. In the example above, the Shape class has an init method, while the Circle class, derived from Shape, adds a radius parameter in a method named initRadius:. So far, no problem. To initialize its superclass, initRadius: just sends an init message to its superclass:

[super init]

But what about the initRadius:height: method of the Cylinder class? Should it send an init message to its superclass? That doesn't make sense, since its superclasses' initialization method is initRadius: and takes a parameter. The correct approach is for each class to include all initialization methods of its superclass, adding in any additional methods for extra/differing parameters that it brings to the table. In our example, Shape would feature an init method, Circle would feature init and initRadius: methods, and Cylinder would feature init, initRadius:, and initRadius:height: methods.

Each init method will send an init message to its superclass, and set any instance variables unique to its class to a default value. For example, the Circle init method would set radius to 0 (or whatever) and the Cylinder init method would set height to 0.

Additional methods that are overriding existing super class methods send an initialization message to the superclass, passing parameters as appropriate. For example, the Cylinder initRadius: method sends an initRadius: message to Circle.

Finally, methods that don't have a matching method in the superclass send an initialization message to self using the method that most closely matches itself. For example, the Circle classes' initRadius: method sends an init message to self (no parameters), while the Cylinder classes' initRadius:height: method sends an initRadius: message to itself but includes the radius parameter. Once the called initialization method returns, the calling method continues by setting its unique instance variables to the parameter passed in to it. For example, once initRadius:height: calls [self initRadius:r] (which will set the radius instance variable to r), it then sets the height instance variable to h (the passed in height parameter).

If the last few paragraphs have left you a bit dazed and confused, not to worry. Here's a program that brings this all to life. As you go through the code, try to follow the chain of initialization. Where does each instance variable get initialized? Can you predict the sequence of initializations when initRadius:height: gets called? Try to work this out before you get to the project run at the end of the column.

The source code that follows is a ".m" and ".h" file for each of the three classes Shape, Circle, and Cylinder. In addition, you'll see a listing for main.m, the main() function that starts the ball rolling.

Shape.m

#import "Shape.h"

@implementation Shape

- init
{
 [super init];

 printf( "\n[Shape init]\n" );

 return self;
}

@end

Shape.h

#import <Object.h>

@interface Shape : Object
{
}

- init;

@end

Circle.m

#import "Circle.h"

@implementation Circle

- init
{
 [super init];

 printf( "[Circle init] - Set radius to 0...\n" );

 radius = 0;

 return self;
}

- initRadius:(int)r
{
 [self init];
 radius = r;

 printf( "[Circle initRadius] - Set radius to %d...\n",
									r );

 return self;
}

@end

Circle.h

#import "Shape.h"

@interface Circle : Shape
{
 int radius;
}

- init;
- initRadius:(int)r;

@end

Cylinder.m

#import "Cylinder.h"

@implementation Cylinder


- init
{
 [super init];

 printf( "[Cylinder init] - Set height to 0...\n" );

 height = 0;

 return self;
}

- initRadius:(int)r
{
 [super initRadius:r];

 printf( "[Cylinder initRadius]\n" );

 return self;
}

- initRadius:(int)r height:(int)h
{
 [self initRadius:r];

 height = h;

 printf
 ( "[Cylinder initRadius:height:] - Set height to %d...\n", h );

 return self;
}

@end

Cylinder.h

#import "Circle.h"

@interface Cylinder : Circle
{
 int height;
}

- init;
- initRadius:(int)r;
- initRadius:(int)r height:(int)h;

@end

main.m

#include "Cylinder.h"

void main()
{
 id shape = [[Shape alloc] init];
 id circle = [[Circle alloc] initRadius:33];
 id cylinder = [[Cylinder alloc] initRadius:27 height:10];

 [shape free];
 [circle free];
 [cylinder free];
}

Running the Program

When you run the program above, here's what you see:

[Shape init]

[Shape init]
[Circle init] - Set radius to 0...
[Circle initRadius] - Set radius to 33...

[Shape init]
[Circle init] - Set radius to 0...
[Cylinder init] - Set height to 0...
[Circle initRadius] - Set radius to 27...
[Cylinder initRadius]
[Cylinder initRadius:height:] - Set height to 10...

As you can see, the listing is broken into three parts, each produced by the initialization of a Shape, Circle, and Cylinder, respectively. Note that when (inside main.m) we created a Shape and sent it an init message, this produced a call of the Shape classes' init method. Simple. Of course, we really should have left the init method out of the Shape class, since it doesn't do anything but add overhead. If we left it out, the right thing would have happened (the init message would have found its way to the Object class).

When we created a Circle and sent it the initRadius: message, we spawn a chain of init messages to Shape and then Circle. Finally, the initRadius: message gets sent to Circle.

The Cylinder object produces a similar chain of initialization. First, we see the chain of init messages from Shape to Circle to Cylinder, then the chain of initRadius: messages from Circle to Cylinder, followed finally by the initRadius:height: message to Cylinder.

Till Next Month...

Spend some time looking over this output till you get the pattern. Once you understand this initialization technique, think about what would happen if you added an Oval class as a subclass to Circle, with an added width instance variable. How would this affect the initialization chain? If you have access to an Objective-C environment, take the time to enter this code and take it for a spin. Add some methods of your own (an area method for Circle, perhaps?) and experiment! See you next month...

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Luminar 1.0.2 - Powerful, adaptive, conf...
Luminar is the new full-featured image editor that adapts to the way you edit photos. Over 300 essential tools to fix, edit, and enhance your photos with comfort. The future of photo editing is here... Read more
WhatRoute 2.0.10 - Geographically trace...
WhatRoute is designed to find the names of all the routers an IP packet passes through on its way from your Mac to a destination host. It also measures the round-trip time from your Mac to the router... Read more
Slack 2.3.3 - Collaborative communicatio...
Slack is a collaborative communication app that simplifies real-time messaging, archiving, and search for modern working teams. Version 2.3.3: Fixed window zoom jumping back-and-forth OS X 10.9... Read more
Lens Blur 1.4.3 - True out-of-focus boke...
Lens Blur transforms your existing photo into true SLR-quality out-of-focus bokeh effect! Everyone needs a gorgeous personalized background for a social profile, blog, Web/UI design, presentation, or... Read more
CleanMyMac 3.6.0 - $39.95
CleanMyMac makes space for the things you love. Sporting a range of ingenious new features, CleanMyMac lets you safely and intelligently scan and clean your entire system, delete large, unused files... Read more
DEVONthink Pro 2.9.8 - Knowledge base, i...
DEVONthink Pro is your essential assistant for today's world, where almost everything is digital. From shopping receipts to important research papers, your life often fills your hard drive in the... Read more
MacFamilyTree 8.1 - Create and explore y...
MacFamilyTree gives genealogy a facelift: modern, interactive, convenient and fast. Explore your family tree and your family history in a way generations of chroniclers before you would have loved.... Read more
HoudahSpot 4.2.7 - 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
TunnelBear 3.0.7 - Subscription-based pr...
TunnelBear is a subscription-based virtual private network (VPN) service and companion app, enabling you to browse the internet privately and securely. Features Browse privately - Secure your data... Read more
Garmin Express 4.5.0.0 - Manage your Gar...
Garmin Express is your essential tool for managing your Garmin devices. Update maps, golf courses and device software. You can even register your device. Update maps Update software Register your... Read more

Latest Forum Discussions

See All

Amateur Surgeon 4 Guide: Become the worl...
It's time to wield your trusty pizza cutter again, as Amateur Surgeon has returned with a whole fresh set of challenges (and some old, familiar ones, too). Starting anew isn't easy, especially when all you have at your disposal is a lighter, the... | Read more »
Le Parker: Sous Chef Extraordinaire (Ga...
Le Parker: Sous Chef Extraordinaire 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: | Read more »
Telltale Games really is working on a Gu...
Telltale Games' next episodic adventure is indeed Guardians of the Galaxy. A document tied to the voice actors strike suggested that the project was in the work, but now we have direct confirmation following an announcement at the Game Awards that... | Read more »
Amateur Surgeon returns to iOS and Andro...
Amateur Surgeon and its two sequels disappeared from the App Store some time and it was sad days for all. But now, just in time for the holidays, the Adult Swim favorite makes its joyous return in the shape of Amateur Surgeon 4, a remake with... | Read more »
The best board games on mobile
Sometimes you need to ditch all of the high speed, high action games in favor of something a little more traditional. If you don't feel like parting ways from your mobile device, though, there are still plenty of ways to get that old-school fix.... | Read more »
The best Facebook Messenger Instant Game...
Facebook's new Instant Games is now here, meaning you can play games with your friends directly via Facebook. It's a fun new way to connect with friends, of course, but it's also proving to be a solid gaming experience in its own right, with a... | Read more »
You can now play game's on Facebook...
Facebook launched its new Instant Games platform in an exciting new attempt to engage its user base. As a result, you can now play a number of different games directly through Facebook Messenger. All of these games run with HTML5, meaning you play... | Read more »
Apollo Justice Ace Attorney (Games)
Apollo Justice Ace Attorney 1.00.00 Device: iOS Universal Category: Games Price: $.99, Version: 1.00.00 (iTunes) Description: Court Is Back In Session Star as rookie defense attorney, Apollo Justice, as he visits crime scenes,... | Read more »
KORG iWAVESTATION (Music)
KORG iWAVESTATION 1.0 Device: iOS Universal Category: Music Price: $19.99, Version: 1.0 (iTunes) Description: A revolutionary new world of sound.The Wave Sequence Synthesizer for iPad - KORG iWAVESTATION | Read more »
Don't Grind Guide: Tips for becomin...
Don’t Grind is a surprising, derpy little one touch game with fun hand-drawn graphics. The goal is simple -- get the high score without being chopped to bits. That can be tough when you’re not used to the game, and that’s compounded by the fact... | Read more »

Price Scanner via MacPrices.net

Parallels Toolbox 1.3 for Mac Offers 25 Singl...
Parallels has launched Parallels Toolbox 1.3 for Mac, an upgrade that adds five new utilities to the stand-alone application which was released in August and is available exclusively online at http... Read more
OWC Mercury Elite Pro Dual mini Ultra-Portabl...
OWC has introduced the new OWC Mercury Elite Pro Dual mini, a powerful yet ultra-portable dual-drive RAID solution. The new Mercury Elite Pro Dual mini packs phenomenal performance into a small... Read more
Clearance 13-inch Retina MacBook Pros availab...
B&H Photo has clearance 2015 13″ Retina Apple MacBook Pros available for up to $200 off original MSRP. Shipping is free, and B&H charges NY tax only: - 13″ 2.7GHz/128GB Retina MacBook Pro: $... Read more
Roundup of 2016 13-inch 2.0GHz MacBook Pro sa...
B&H has the non-Touch Bar 13″ MacBook Pros in stock today for $50-$100 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 2.0GHz MacBook Pro Space Gray (MLL42LL/A): $1449 $... Read more
New 13-inch 2.0GHz Space Gray MacBook Pro in...
Adorama has the new 13″ 2.0GHz Space Gray MacBook Pro (non-Touch Bar, MLL42LL/A) in stock for $1499 including a free 3-year AppleCare Protection Plan. Shipping is free, and Adorama charges sales tax... Read more
Finnair Adopts iOS Enterprise iPad Apps from...
Finnair and IBM have announced a first-of-its-kind agreement to utilize iOS enterprise apps from IBM to support the airline’s overall digital transformation. Finnair is focused on Asia-Europe traffic... Read more
Tech21 Launches Evo Go iPhone 7 Case Availabl...
Tech21 has announced the launch of the Evo Go case for Apple iPhone 7 and iPhone 7 Plus, exclusively at T-Mobile. Available online and at participating T-Mobile stores nationwide, Evo Go cases start... Read more
Apple Turns (RED) with More Ways to Join the...
In recognition of World AIDS Day, Apple is offering more ways than ever for customers to join (RED) in its mission to create an AIDS-free generation. Apple is the worlds largest corporate contributor... Read more
Deals on new 15-inch Touch Bar MacBook Pros,...
B&H Photo has new 2016 Apple 15″ Touch Bar MacBook Pro models in stock today with some available for $50 off MSRP, each including free shipping plus NY sales tax only: - 15″ 2.7GHz Touch Bar... Read more
12-inch Retina MacBooks on sale for up to $10...
12-inch Retina MacBooks remain on sale at B&H Photo with models available for up to $100 off MSRP. Shipping is free, and B&H charges NY sales tax only. B&H will also include a free copy... Read more

Jobs Board

*Apple* Retail - Multiple Positions- White P...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
Automotive Detailer - *Apple* Used Autos -...
We are currently conductinginterviews and will be accepting applications for a part-time detailer. Apple Used Autos is a great place to work andstart a career. We 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
US- *Apple* Store Leader Program - Apple (Un...
…Summary Learn and grow as you explore the art of leadership at the Apple Store. You'll master our retail business inside and out through training, hands-on Read more
*Apple* & PC Desktop Support Technician...
Apple & PC Desktop Support Technician job in Dallas TX Introduction: We have immediate job openings for several Desktop Support Technicians with one of our most Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.