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!)

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Kodi 15.0.beta1 - Powerful media center...
Kodi (was XBMC) is an award-winning free and open-source (GPL) software media player and entertainment hub that can be installed on Linux, OS X, Windows, iOS, and Android, featuring a 10-foot user... Read more
DiskCatalogMaker 6.4.12 - Catalog your d...
DiskCatalogMaker is a simple disk management tool which catalogs disks. Simple, light-weight, and fast. Finder-like intuitive look and feel. Super-fast search algorithm. Can compress catalog data... Read more
Macs Fan Control 1.3.0.0 - Monitor and c...
Macs Fan Control allows you to monitor and control almost any aspect of your computer's fans, with support for controlling fan speed, temperature sensors pane, menu-bar icon, and autostart with... Read more
Lyn 1.5.11 - Lightweight image browser a...
Lyn is a lightweight and fast image browser and viewer designed for photographers, graphic artists and Web designers. Featuring an extremely versatile and aesthetically pleasing interface, it... Read more
NeoOffice 2014.11 - Mac-tailored, OpenOf...
NeoOffice is a complete office suite for OS X. With NeoOffice, users can view, edit, and save OpenOffice documents, PDF files, and most Microsoft Word, Excel, and PowerPoint documents. NeoOffice 3.x... Read more
LaunchBar 6.4 - Powerful file/URL/email...
LaunchBar is an award-winning productivity utility that offers an amazingly intuitive and efficient way to search and access any kind of information stored on your computer or on the Web. It provides... Read more
Remotix 3.1.4 - Access all your computer...
Remotix is a fast and powerful application to easily access multiple Macs (and PCs) from your own Mac. Features Complete Apple Screen Sharing support - including Mac OS X login, clipboard... Read more
DesktopLyrics 2.6.6 - Displays current i...
DesktopLyrics is an application that displays the lyrics of the song currently playing in "iTunes" right on your desktop. The lyrics for the song have to be set in iTunes; DesktopLyrics does nothing... Read more
VOX 2.5.1 - Music player that supports m...
VOX is a beautiful music player that supports many filetypes. The beauty is in its simplicity, yet behind the minimal exterior lies a powerful music player with a ton of features and support for all... Read more
Microsoft Remote Desktop 8.0.18 - Connec...
With Microsoft Remote Desktop, you can connect to a remote PC and your work resources from almost anywhere. Experience the power of Windows with RemoteFX in a Remote Desktop client designed to help... Read more

Biz Builder Delux (Games)
Biz Builder Delux 1.0.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0.0 (iTunes) Description: Ah, there's nothing like the rhythmic bustle of a burgeoning business burg... especially when you're the one building it... | Read more »
Auroch Digital is Bringing Back Games Wo...
| Read more »
Carbo - Handwriting in the Digital Age...
Carbo - Handwriting in the Digital Age 1.0 Device: iOS Universal Category: Productivity Price: $3.99, Version: 1.0 (iTunes) Description: | Read more »
Draggy Dead (Games)
Draggy Dead 1.1 Device: iOS Universal Category: Games Price: $.99, Version: 1.1 (iTunes) Description: Ditch your dead end job and take up a rewarding career in Grave Robbing today!Guide the recently deceased to a fun filled life of... | Read more »
Bad Dinos (Games)
Bad Dinos 1.0.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.0 (iTunes) Description: | Read more »
The Apple Watch isn't Great as a Fi...
| Read more »
Show the World What You See With Stre.am...
Live broadcasting is getting popular on mobile devices, which is why you can now get Stre.am, by Infinite Takes. [Read more] | Read more »
PhotoTime's 2.1 Update Adds Apple W...
The latest PhotoTime update is adding even more functionality to the handy photo organizing app. Yep, including Apple Watch support. [Read more] | Read more »
Oh My Glob! Adventure Time Puzzle Quest...
Finn and Jake are taking over D3 Go!'s popular puzzle game series in the upcoming Adventure Time Puzzle Quest. [Read more] | Read more »
Earthcore: Shattered Elements - Tips, Tr...
At first glance, Earthcore: Shattered Elements seems like a rather simple card-battling game. Once you’re introduced to skills that will change quite a bit. Even more so once you start to acquire hero cards. But it’s not so complicated that we... | Read more »

Price Scanner via MacPrices.net

Memorial Day Weekend Sale: New 27-inch 3.3GHz...
Best Buy has the new 27″ 3.3GHz 5K iMac on sale for $1899.99 this weekend. Choose free shipping or free local store pickup (if available). Sale price for online orders only, in-store prices may vary... Read more
OtterBox Maximizes Portability, Productivity...
From the kitchen recipe book to the boarsroom presentation, the OtterBox Agility Tablet System turns tablets into one of the most versatile pieces of handheld technology available. Available now, the... Read more
Launch of New Car App Gallery and Open Develo...
Automatic, a company on a mission to bring the power of the Internet into every car, has announced the launch of the Automatic App Gallery, an app store for nearly every car or truck on the road... Read more
Memorial Day Weekend Sale: 13-inch 1.6GHz Mac...
Best Buy has the new 13″ 1.6GHz/128GB MacBook Air on sale for $849 on their online store this weekend. Choose free shipping or free local store pickup (if available). Sale price for online orders... Read more
Memorial Day Weekend Sale: 27-inch 3.5GHz 5K...
Best Buy has the 27″ 3.5GHz 5K iMac on sale for $2099.99 this weekend. Choose free shipping or free local store pickup (if available). Sale price for online orders only, in-store prices may vary.... Read more
Sale! 16GB iPad mini 3 for $349, save $50
B&H Photo has the 16GB iPad mini 3 WiFi on sale for $349 including free shipping plus NY sales tax only. Their price is $50 off MSRP, and it’s the lowest price available for this model. Read more
Price drop on 2014 15-inch Retina MacBook Pro...
B&H Photo has dropped prices on 2014 15″ Retina MacBook Pros by $200. Shipping is free, and B&H charges NY sales tax only: - 15″ 2.2GHz Retina MacBook Pro: $1799.99 save $200 - 15″ 2.5GHz... Read more
With a Mission to Make Mobile Free, Scratch W...
Scratch Wireless, claiming to be the world’s first truly free mobile service, has announced the availability of a new Scratch-enabled Android smartphone, the Coolpad Arise. The smartphone is equipped... Read more
First-Ever Titanium Alloy Curved iPhone 6 Scr...
One of the most common problems with mobile phones is damage to the screens. The slightest drop can cause a dreaded spider web of gashes and cracks in the glass panel surface that can cost $hundreds... Read more
Preorder new 12-inch MacBook, $10 off, save o...
Adorama has new 12″ Retina MacBooks available for preorder for $10 off MSRP including free shipping plus NY & NJ sales tax only. For a limited time, Adorama will include a free Apple USB-C to USB... Read more

Jobs Board

*Apple* Solutions Consultant - Retail Sales...
**Job Summary** As an Apple Solutions Consultant (ASC) you are the link between our customers and our products. Your role is to drive the Apple business in a retail Read more
*Apple* TV Live Streaming Frameworks Test En...
**Job Summary** Work and contribute towards the engineering of Apple 's state-of-the-art products involving video, audio, and graphics in Interactive Media Group (IMG) at 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
Partner Marketing Manager, Merchant- *Apple*...
**Job Summary** The Apple Pay partner marketing team is looking for a marketing manager to develop and drive US marketing programs with our merchant partners. The right Read more
Technical Project Manager - *Apple* Pay - A...
**Job Summary** Apple Pay is seeking an experienced technical PM to…manage the on boarding of new merchants for the Apple Pay platform in the US Within this role you Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.