TweetFollow Us on Twitter

Digging in to Objective-C, Part 2

Volume Number: 19 (2003)
Issue Number: 3
Column Tag: Getting Started

Digging in to Objective-C, Part 2

by Dave Mark

In this month's column, we're going to dig through last month's Objective-C source code. Before we do that, I'd like to take a little detour and share a cool Mac OS X experience I had this weekend.

FTP From the Finder

The more time I spend in Mac OS X, the more I love it. There are so many nooks and crannies to explore. I've installed PHP and mySQL servers on my machine and fiddled with the default Apache installation as well. I've re-honed my rusty Unix skills, spending untold amounts of time in the Terminal app. Before there were any native FTP apps in OS X, I had to do all my FTPing using the Terminal. Finally, apps like Interarchy, Fetch, and the like made the move to OS X and I got used to those drag and drop interfaces. Then I ran into a problem which turned into a really cool discovery.

I own a licensed copy of Interarchy. When they introduced a beta version of their latest and greatest FTP client, someone from Stairways sent out a call for beta-testers. To make a long story short, I deleted the original, did all my FTPing using the beta client. No problem. Except that one day, I came into the office to discover that the beta had expired and I needed an FTP client to retrieve the previous version from the company's site.

So I found myself needing an alternative FTP client. On a whim, I decided to experiment with Safari before turning back to the primitive Unix FTP client. In Safari's address field, I typed in this URL:

ftp.interarchy.com

I hit the return, the screen flashed a bit, but nothing seemed to happen. It was as if Safari ignored my request. But then, out of the corner of my eye, I noticed a new icon on my desktop (see Figure 1.) Very cool! In effect, the FTP site had been mounted as a volume on my desktop. When I opened a new file browser, I was able to navigate through the site, just as I would my hard drive. I used the Finder to copy the new FTP client onto my hard drive.


Figure 1. The ftp.interarchy.com icon on my desktop

Somewhere in the back of my brain, I remembered that I could use the Finder to connect to an AppleShare server. Wonder if that would work with an FTP server as well. Hmmm. I selected Connect to Server... from the Finder's Go menu (Figure 2). When the dialog appeared, I typed in fto.interarchy.com. The Finder reported an error connecting to afp://ftp.interarchy.com.


Figure 2. Using the Finder to connect to an FTP server.

Interesting. Either the Finder uses the AppleShare protocol as its default or I somehow (by accessing an AppleShare server, perhaps) set AppleShare as my default protocol. Hmmm. So I gave this a try:

ftp://ftp.interarchy.com

Guess what? It worked!!! I experimented with some other ftp servers and they all worked the same way. Cool! I love Mac OS X.

To be fair, the Finder doesn't do all the nifty tricks that Interarchy does. For example, the mounted disk is read-only, so you couldn't use this trick to manage your web site. I asked around, and the folks who really know about this stuff all said the same thing. An amazing bit of coding to add this feature to the OS. Nicely done, Apple.

Walking Through the Employee Code

OK, let's get back to the code. Last month, I introduced two versions of the same program, one in C++ and one in Objective-C. The goal was for you to see the similarities between the two languages and to get a handle on some of the basics of Objective-C. If you don't have a copy of last month's MacTech, download the source from mactech.com and take a quick read-through of the C++ code. Our walk-through will focus on the Objective-C code.

    In general, MacTech source code is stored in:

    ftp://ftp.mactech.com/src/

    Once you make your way over there, you'll see a bunch of directories, one corresponding to each issue. This year's issues all start with 19 (the 19th year of publication). 19.02 corresponds to the February issue. You'll want to download cpp_employee.sit and objc_employee.sit. The former is the C++ version and the latter the Objective-C version.

    For convenience, there's a web page that points to this at:

    http://www.mactech.com/editorial/filearchives.html

Employee.h

Employee.h starts off by importing Foundation.h, which contains the definitions we'll need to work with the few elements of the Foundation framework we'll take advantage of.

#import <Foundation/Foundation.h>

The #import directive replaces the #ifdef trick most of us use to avoid cyclical #includes. For example, suppose you have two classes, each of which reference the other. So ClassA.h #includes ClassB.h and ClassB.h #includes ClassA.h. Without some kind of protection, the #includes would form an infinite #include loop.

A typical C++ solution puts code like this in the header:

#ifndef _H_Foo
#define _H_Foo
#pragma once // if your compiler supports it
// put all your header stuff here
#endif // _H_Foo

This ensures that the header file is included only once. In Objective-C, this cleverness is not necessary. Just use #import for all your #includes and no worries.

A Mr. Richard Feder of Ft. Lee, New Jersey writes, "How come we need the Foundation.h include file? Where does Objective-C stop and Foundation begin?" Good question!

As I never get tired of saying, I am a purist when it comes to learning programming languages. Learn C, then learn Objective-C. Learn Objective-C, then learn Cocoa and the Foundation framework. In general, I'll try to keep the Foundation elements out of my Objective-C code. When I do include some Foundation code, I'll do my best to mark it clearly so you understand what is part of the Objective-C language and what is not.

If you refer back to the C++ version of Employee.h, you'll see that our Employee class declaration includes both data members and member functions within its curly braces. In Objective-C, the syntax is a little different. For starters, the class is declared using an @interface statement that follows this format:

@interface Employee : NSObject {
    @private
        char      employeeName[20];
        long      employeeID;
        float      employeeSalary;
}
- (id)initWithName:(char *)name andID:(long)id
    andSalary:(float)salary;
- (void)PrintEmployee;
- (void)dealloc;
@end

Note that the data members, known to Objective-C as fields, are inside the curly braces, while the member functions (methods) follow the close curly brace. The whole shebang ends with @end.

Note that we derived our class from the NSObjecct class. Any time you see the letters NS at the start of a class, object, or method name, think Foundation Framework. NS stands for NextStep and is used throughout the Foundation classes. To keep things pure, I could have declared Employee as an Object subclass or as a root class, but since most of the code you write will inherit from some Cocoa class, I figure let's go with this minimal usage. When you derive from NSObject, you inherit a number of methods and fields. We'll get to know NSObject in detail in future columns.

Notice the use of the @private access modifier. Like C++, Objective-C offers 3 modifiers, @private, @protected, and @public. The default is @protected.

Objective-C method declarations start with either a "-" or a "+". The "-" indicates an instance method, a method tied to an instance of that class. The "+" indicates a class method, a method tied to the entire class. We'll get into class methods in a future column.

Objective-C Parameters

Perhaps the strangest feature of Objective-C is the way parameters are declared. The best way to explain this is by example. Here's a method with no parameters:

- (void)dealloc;

Pretty straightforward. The function is named "dealloc", it has no parameters, and it does not return a value.

- (void)setHeight:(int)h;

This function is named "height:". It does not return a value but does take a single parameter, an int with the name h. The ":" is used to separate parameters from other parameters and from the function name.

    A word on naming your accessor functions: Though this is not mandatory, most folks name their getter functions to match the name of the field being retrieved, then put the word "set" in front of that to name the setter function.

    As an example, consider these two functions:

    - (int)height;        // Returns the value of the height field
    - (void)setHeight:(int)h; // Sets the height field to the value in h

    As I said, this naming convention is not mandatory, but it is widely used. There is a book out there that tells you to use the same exact name for your setter and getter (both would be named height in the example above), but everyone I've discussed this with poo-poos this approach in favor of the standard we just discussed. In addition, as you make your way through the Developer doc on your hard drive, you'll see the accessor naming convention described above used throughout.

The last important change here is when you have more than one parameter. You add a space before each ":(type)name" series. For example:

- (id)initWithName:(char *)name andID:(long)id
    andSalary:(float)salary;

This function is named "initWithName:andID:andSalary:", returns the type "id", and takes 3 parameters (a (char *), a long, and a float). This is a bit tricky, but you'll get used to it.

    The type "id" is Objective-C's generic object pointer type. We'll learn more about it in future columns.

Employee.m

Employee.m starts off by importing the Employee.h header file.

#import "Employee.h"

The rest off the file makes up the implementation of the Employee class. The implementation compares to the C++ concept of definition. It's where the actual code lives. The first function is initWithName:andID:andSalary:. The function takes three parameters and initializes the fields of the Employee object, then prints a message to the console using printf. Note that the function returns self, a field inherited from NSObject, which is a pointer to itself.

@implementation Employee
    - (id)initWithName:(char *)name andID:(long)id
                            andSalary:(float)salary {
        strcpy( employeeName, name );
        employeeID = id;
        employeeSalary = salary;
        
        printf( "Creating employee #%ld\n", employeeID );
        
        return self;
    }
    

PrintEmployee uses printf to print info about the Employee fields to the console.

    - (void)PrintEmployee {
        printf( "----\n" );
        printf( "Name:   %s\n", employeeName );
        printf( "ID:     %ld\n", employeeID );
        printf( "Salary: %5.2f\n", employeeSalary );
        printf( "----\n" );
    }

dealloc is an NSObject method that we are overriding simply so we can print our "Deleting employee" message. The line after the printf is huge. For starters, it introduces the concept of "sending a message to a receiver". This is mildly analogous to the C++ concept of dereferencing an object pointer to call one of its methods. For now, think of the square brackets as surrounding a receiver (on the left) and a message (on the right). In this case, we are sending the dealloc message to the object pointed to by super.

As it turns out, "super" is another field inherited from NSObject. It points to the next object up in the hierarchy, the NSObject the Employee was subclassed from. Basically, the super we are sending the message to is the object we overrode to get our version of dealloc called. By overriding super's dealloc, we've prevented it from being called. By sending it the dealloc method ourselves, we've set things right.

    - (void)dealloc {
        
        printf( "Deleting employee #%ld\n", employeeID );
        
        [super dealloc];
    }
@end

main.m

Finally, here's main.m. We start off by importing Foundation.h and Employee.h. Notice that Employee.h already imports Foundation.h. That's the beauty of #import. It keeps us from including the same file twice.

#import <Foundation/Foundation.h>
#import "Employee.h"

The function main starts off like its C counterpart, with the parameters argc and argv that you've known since childhood.

int main (int argc, const char * argv[]) {

NSAutoreleasePool is a memory management object from the Foundation Framework. We're basically declaring an object named pool that will serve memory to the other Foundation objects that need it. We will discuss the ins and outs of memory management in a future column. For the moment, notice that we first send an alloc message to NSAutoreleasePool, and then send an init message to the object returned by alloc. The key here is that you can nest messages. The alloc message creates the object, the init message initializes the object. This combo is pretty typical.

    NSAutoreleasePool * pool =
                        [[NSAutoreleasePool alloc] init];

Now we create our two employees. Instead of alloc and init, we're going to alloc and initWithName:andID:andSalary: which suits our needs much better than the inherited init would. Still nested though. Note that we are taking advantage of the self returned by initWithName:andID:andSalary:. That's the object pointer whose type is id. Think about the compiler issues here. The compiler did not complain that you were assigning an id to an Employee pointer. This is important and will come up again in other programs we create.

    Employee*   employee1 = [[Employee alloc] initWithName:"Frank Zappa" andID:1 
andSalary:200];
    Employee*   employee2 = [[Employee alloc] initWithName:"Donald Fagen" andID:2 andSalary:300];

Once our objects are created, let's let them do something useful. We'll send them each a PrintEmployee message.

    [employee1 PrintEmployee];
    [employee2 PrintEmployee];
    

Next, we'll send them each a release message. This tells the NSAutoreleasePool that we have no more need of these objects and their memory can be released. Finally, we release the memory pool itself.

    [employee1 release];
    [employee2 release];
    
    [pool release];
    return 0;
}

Till Next Month...

The goal of this program was not to give you a detailed understanding of Objective-C, but more to give you a sense of the basics. Hopefully, you have a sense of the structure of a class interface and implementation, function declarations, parameter syntax, and the basics of sending messages to receivers.

One concept we paid short shrift to was memory management. I realize we did a bit of hand-waving, rather than dig into the details of NSAutoreleasePool and the like, but I really want to make that a column unto itself.

Before I go, I want to say a quick thanks to Dan Wood and John Daub for their help in dissecting some of this stuff. If you see these guys at a conference, please take a moment to say thanks and maybe treat them to a beverage of choice.

And if you experts out there have suggestions on improvements I might make to my code, I am all ears. Please don't hesitate to drop an email to feedback@mactech.com.

See you next month!


Dave Mark is a long-time Mac developer and author and has written a number of books on Macintosh development, including Learn C on the Macintosh, Learn C++ on the Macintosh, and The Macintosh Programming Primer series. Be sure to check out Dave's web site at http://www.spiderworks.com, where you'll find pictures from Dave's visit to MacWorld Expo in San Francisco (yes, there are a bunch of pictures of Woz on there!)

 
AAPL
$116.47
Apple Inc.
+0.16
MSFT
$47.98
Microsoft Corpora
-0.72
GOOG
$537.50
Google Inc.
+2.67

MacTech Search:
Community Search:

Software Updates via MacUpdate

Cobook 3.0.7 - Intelligent address book....
Cobook Contacts is an intuitive, engaging address book. Solve the problem of contact management with Cobook Contacts and its simple interface and powerful syncing and integration possibilities.... Read more
StatsBar 1.9 - Monitor system processes...
StatsBar gives you a comprehensive and detailed analysis of the following areas of your Mac: CPU usage Memory usage Disk usage Network and bandwidth usage Battery power and health (MacBooks only)... Read more
Cyberduck 4.6 - FTP and SFTP browser. (F...
Cyberduck is a robust FTP/FTP-TLS/SFTP browser for the Mac whose lack of visual clutter and cleverly intuitive features make it easy to use. Support for external editors and system technologies such... Read more
Maya 2015 - Professional 3D modeling and...
Maya is an award-winning software and powerful, integrated 3D modeling, animation, visual effects, and rendering solution. Because Maya is based on an open architecture, all your work can be scripted... Read more
Evernote 6.0.1 - Create searchable notes...
Evernote allows you to easily capture information in any environment using whatever device or platform you find most convenient, and makes this information accessible and searchable at anytime, from... Read more
calibre 2.11 - Complete e-library manage...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital... Read more
Herald 5.0.1 - Notification plugin for M...
Note: Versions 2.1.3 (for OS X 10.7), 3.0.6 (for OS X 10.8), and 4.0.8 (for OS X 10.9) are no longer supported by the developer. Herald is a notification plugin for Mail.app, Apple's Mac OS X email... Read more
Firetask 3.7 - Innovative task managemen...
Firetask uniquely combines the advantages of classical priority-and-due-date-based task management with GTD. Stay focused and on top of your commitments - Firetask's "Today" view shows all relevant... Read more
TechTool Pro 7.0.6 - Hard drive and syst...
TechTool Pro is now 7, and this is the most advanced version of the acclaimed Macintosh troubleshooting utility created in its 20-year history. Micromat has redeveloped TechTool Pro 7 to be fully 64... Read more
PhotoDesk 3.0.1 - Instagram client for p...
PhotoDesk lets you view, like, comment, and download Instagram pictures/videos! (NO Uploads! / Image Posting! Instagram forbids that! AND you *need* an *existing* Instagram account). But you can do... Read more

Latest Forum Discussions

See All

Ubisoft Gives Everyone Two New Ways to E...
Ubisoft Gives Everyone Two New Ways to Earn In-Game Stuff for Far Cry 4 Posted by Jessica Fisher on November 21st, 2014 [ permalink ] | Read more »
Golfinity – Tips, Tricks, Strategies, an...
Dig this: Would you like to know what we thought of being an infinite golfer? Check out our Golfinity review! Golfinity offers unlimited ways to test your skills at golf. Here are a few ways to make sure your score doesn’t get too high and your... | Read more »
Dark Hearts, The Sequel to Haunting Meli...
Dark Hearts, The Sequel to Haunting Melissa, is Available Now Posted by Jessica Fisher on November 21st, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Meowza! Toyze Brings Talking Tom to Life...
Meowza! | Read more »
Square Enix Announces New Tactical RPG f...
Square Enix Announces New Tactical RPG for Mobile, Heavenstrike Rivals. Posted by Jessica Fisher on November 21st, 2014 [ permalink ] With their epic stories and gorgeous graphics, | Read more »
Quest for Revenge (Games)
Quest for Revenge 1.0.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0.0 (iTunes) Description: The great Kingdom of the west has fallen. The gods ignore the prayers of the desperate. A dark warlord has extinguished... | Read more »
Threadz is a New Writing Adventure for Y...
Threadz is a New Writing Adventure for You and Your Friends Posted by Jessica Fisher on November 21st, 2014 [ permalink ] In the tradition of round-robin storytelling, | Read more »
SteelSeries Stratus XL Hardware Review
Made by: SteelSeries Price: $59.99 Hardware/iOS Integration Rating: 4 out of 5 stars Usability Rating: 4.5 out of 5 stars Reuse Value Rating: 4.25 out of 5 stars Build Quality Rating: 4.5 out of 5 stars Overall Rating: 4.31 out of 5 stars | Read more »
ACDSee (Photography)
ACDSee 1.0.0 Device: iOS iPhone Category: Photography Price: $1.99, Version: 1.0.0 (iTunes) Description: Capture, perfect, and share your photos with ACDSee. The ACDSee iPhone app combines an innovative camera, a powerful photo... | Read more »
ProTube for YouTube (Entertainment)
ProTube for YouTube 2.0.2 Device: iOS Universal Category: Entertainment Price: $1.99, Version: 2.0.2 (iTunes) Description: ProTube is the ultimate, fully featured YouTube app. With it's highly polished design, ProTube offers ad-free... | Read more »

Price Scanner via MacPrices.net

15″ 2.2GHz Retina MacBook Pro on sale for $17...
 B&H Photo has the 2014 15″ 2.2GHz Retina MacBook Pro on sale today for $1749. Shipping is free, and B&H charges NY sales tax only. B&H will also include free copies of Parallels Desktop... Read more
27-inch 3.5GHz 5K iMac in stock today and on...
 B&H Photo has the new 27″ 3.5GHz 5K iMac in stock today and on sale for $2299 including free shipping plus NY sales tax only. Their price is $200 off MSRP, and it’s the lowest price available... Read more
21-inch 1.4GHz iMac on sale for $979, save $1...
B&H Photo has the new 21″ 1.4GHz iMac on sale for $979.99 including free shipping plus NY sales tax only. Their price is $120 off MSRP. B&H will also include free copies of Parallels Desktop... Read more
13-inch 1.4GHz/256GB MacBook Air on sale for...
B&H Photo has lowered their price on the 13″ 1.4GHz/256GB MacBook Air to $1059.99 including free shipping plus NY sales tax only. Their price is $140 off MSRP, and it’s the lowest price for this... Read more
Save up to $400 with Apple refurbished 2014 1...
The Apple Store has restocked 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 shipping... Read more
New 13-inch 1.4GHz MacBook Air on sale for $8...
 Adorama has the 2014 13″ 1.4GHz/128GB MacBook Air on sale for $899.99 including free shipping plus NY & NJ tax only. Their price is $100 off MSRP. B&H Photo has the 13″ 1.4GHz/128GB MacBook... Read more
Apple Expected to Reverse Nine-Month Tablet S...
Apple and Samsung combined accounted for 62 percent of the nearly 36 million branded tablets shipped in 3Q 2014, according to early vendor shipment share estimates from market intelligence firm ABI... Read more
Stratos: 30 Percent of US Smartphone Owners t...
Stratos, Inc., creator of the Bluetooth Connected Card Platform, has announced results from its 2014 Holiday Mobile Payments Survey. The consumer survey found that nearly one out of three (30 percent... Read more
2014 1.4GHz Mac mini on sale for $449, save $...
 B&H Photo has lowered their price on the new 1.4GHz Mac mini to $449.99 including free shipping plus NY tax only. Their price is $50 off MSRP, and it’s the lowest price available for this new... Read more
Check Apple prices on any device with the iTr...
MacPrices is proud to offer readers a free iOS app (iPhones, iPads, & iPod touch) and Android app (Google Play and Amazon App Store) called iTracx, which allows you to glance at today’s lowest... Read more

Jobs Board

*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
*Apple* Solutions Consultant (ASC)- Retail S...
**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, *Apple* Financial Services...
**Job Summary** Apple Financial Services (AFS) offers consumers, businesses and educational institutions ways to finance Apple purchases. We work with national and Read more
*Apple* Store Leader Program - College Gradu...
Job Description: Job Summary As an Apple Store Leader Program agent, you can continue your education as you major in the art of leadership at the Apple Store. You'll Read more
*Apple* Retail - Multiple Positions (US) - A...
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
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.