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

Posterino 3.3.5 - Create posters, collag...
Posterino offers enhanced customization and flexibility including a variety of new, stylish templates featuring grids of identical or odd-sized image boxes. You can customize the size and shape of... Read more
Skim 1.4.28 - PDF reader and note-taker...
Skim is a PDF reader and note-taker for OS X. It is designed to help you read and annotate scientific papers in PDF, but is also great for viewing any PDF file. Skim includes many features and has a... Read more
Apple macOS Sierra 10.12.4 - The latest...
With Apple macOS Sierra, Siri makes its debut on Mac, with new features designed just for the desktop. Your Mac works with iCloud and your Apple devices in smart new ways, and intelligent... Read more
Apple Numbers 4.1 - Apple's spreads...
With Apple Numbers, sophisticated spreadsheets are just the start. The whole sheet is your canvas. Just add dramatic interactive charts, tables, and images that paint a revealing picture of your data... Read more
Xcode 8.3 - Integrated development envir...
Xcode includes everything developers need to create great applications for Mac, iPhone, iPad, and Apple Watch. Xcode provides developers a unified workflow for user interface design, coding, testing... Read more
Dropbox 22.4.24 - Cloud backup and synch...
Dropbox is an application that creates a special Finder folder that automatically syncs online and between your computers. It allows you to both backup files and keep them up-to-date between systems... Read more
Merlin Project 4.2.0 - Project managemen...
Merlin Project is the leading professional project management software for OS X. If you plan complex projects on your Mac, you won’t get far with a simple list of tasks. Good planning raises... Read more
Postbox 5.0.12 - Powerful and flexible e...
Postbox is a new email application that helps you organize your work life and get stuff done. It has all the elegance and simplicity of Apple Mail, but with more power and flexibility to manage even... Read more
Apple Pages 6.1 - Apple's word proc...
Apple Pages is a powerful word processor that gives you everything you need to create documents that look beautiful. And read beautifully. It lets you work seamlessly between Mac and iOS devices, and... Read more
RapidWeaver 7.3.2 - Create template-base...
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

Hearthstone celebrates the upcoming Jour...
Hearthstone gets a new expansion, Journey to Un'Goro, in a little over a week, and they'll be welcoming the Year of the Mammoth, the next season, at the same time. There's a lot to be excited about, so Blizzard is celebrating in kind. Players will... | Read more »
4 smart and stylish puzzle games like Ty...
TypeShift launched a little over a week ago, offering some puzzling new challenges for word nerds equipped with an iOS device. Created by Zach Gage, the mind behind Spelltower, TypeShift boasts, like its predecessor, a sleak design and some very... | Read more »
The best deals on the App Store this wee...
Deals, deals, deals. We're all about a good bargain here on 148Apps, and luckily this was another fine week in App Store discounts. There's a big board game sale happening right now, and a few fine indies are still discounted through the weekend.... | Read more »
The best new games we played this week
It's been quite the week, but now that all of that business is out of the way, it's time to hunker down with some of the excellent games that were released over the past few days. There's a fair few to help you relax in your down time or if you're... | Read more »
Orphan Black: The Game (Games)
Orphan Black: The Game 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: Dive into a dark and twisted puzzle-adventure that retells the pivotal events of Orphan Black. | Read more »
The Elder Scrolls: Legends is now availa...
| Read more »
Ticket to Earth beginner's guide: H...
Robot Circus launched Ticket to Earth as part of the App Store's indie games event last week. If you're not quite digging the space operatics Mass Effect: Andromeda is serving up, you'll be pleased to know that there's a surprising alternative on... | Read more »
Leap to victory in Nexx Studios new plat...
You’re always a hop, skip, and a jump away from a fiery death in Temple Jump, a new platformer-cum-endless runner from Nexx Studio. It’s out now on both iOS and Android if you’re an adventurer seeking treasure in a crumbling, pixel-laden temple. | Read more »
Failbetter Games details changes coming...
Sunless Sea, Failbetter Games' dark and gloomy sea explorer, sets sail for the iPad tomorrow. Ahead of the game's launch, Failbetter took to Twitter to discuss what will be different in the mobile version of the game. Many of the changes make... | Read more »
Splish, splash! The Pokémon GO Water Fes...
Niantic is back with a new festival for dedicated Pokémon GO collectors. The Water Festival officially kicks off today at 1 P.M. PDT and runs through March 29. Magikarp, Squirtle, Totodile, and their assorted evolved forms will be appearing at... | Read more »

Price Scanner via MacPrices.net

13-inch MacBook Airs, Apple refurbished, in s...
Apple has Certified Refurbished 2016 13″ MacBook Airs available starting at $849. An Apple one-year warranty is included with each MacBook, and shipping is free: - 13″ 1.6GHz/8GB/128GB MacBook Air: $... Read more
12-inch Retina MacBooks on sale for $1199, sa...
B&H has 12″ 1.1GHz Retina MacBooks on sale for $100 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 12″ 1.1GHz Space Gray Retina MacBook: $1199 $100 off MSRP - 12″ 1.1GHz... Read more
Save up to $260 with Apple refurbished 12-inc...
Apple has Certified Refurbished 2016 12″ Retina MacBooks available for $200-$260 off MSRP. Apple will include a standard one-year warranty with each MacBook, and shipping is free. The following... Read more
13-inch 2.7GHz Retina MacBook Pro on sale for...
B&H Photo has the 2015 13″ 2.7GHz/128GB Retina Apple MacBook Pro on sale for $170 off MSRP. Shipping is free, and B&H charges NY tax only: - 13″ 2.7GHz/128GB Retina MacBook Pro (MF839LL/A): $... Read more
15-inch 2.2GHz Retina MacBook Pro on sale for...
B&H Photo has the 2015 15″ 2.2GHz Retina MacBook Pro (MJLQ2LL/A) on sale for $1799.99 including free shipping plus NY sales tax only. Their price is $200 off MSRP. Read more
Save up to $160 with Apple refurbished 9-inch...
Apple has Certified Refurbished 9″ and 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 9″... Read more
Apple Chip Foundry TSMC To Begin A11 System-o...
Digitimes’ Steve Shen is reporting today that according to the Chinese-language Economic Daily News (EDN), chipmaker and major Apple supplier foundery Taiwan Semiconductor Manufacturing Company (TSMC... Read more
MacX MediaTrans 3.5 iOS Data Transfer Spring...
MacXDVD Software has announced general availability of the latest MacX MedTrans 3.5, featuring a new user interface (UI). MacX MediaTrans is ann iPhone manager that enables free data transfer between... Read more
Regular Price $19.95 DupeZap 4 Finder For OS...
Hyperbolic Software has announced the release of DupeZap 4.0.2, their modern duplicate finder developed exclusively for macOS. DupeZap 4 is an utility for Mac owners seeking to reclaim disk space... Read more
B-Eng Releases SSD Health Check for MVNe for...
Fehraltorf, Switzerland based B-Eng has announced the release and immediate availability of SSD Health Check for MVNe for MacBook Pro, their app that delivers important data and insights for MVNe... Read more

Jobs Board

Fulltime aan de slag als shopmanager in een h...
Ben jij helemaal gek van Apple -producten en vind je het helemaal super om fulltime shopmanager te zijn in een jonge en hippe elektronicazaak? Wil jij werken in Read more
*Apple* Mobile Master - Best Buy (United Sta...
**492889BR** **Job Title:** Apple Mobile Master **Location Number:** 000886-Norwalk-Store **Job Description:** **What does a Best Buy Apple Mobile Master do?** Read more
*Apple* Mobile Master - Best Buy (United Sta...
**492472BR** **Job Title:** Apple Mobile Master **Location Number:** 000470-Seattle-Store **Job Description:** **What does a Best Buy Apple Mobile Master do?** Read more
*Apple* Mobile Master - Best Buy (United Sta...
**492562BR** **Job Title:** Apple Mobile Master **Location Number:** 000853-Jackson-Store **Job Description:** **What does a Best Buy Apple Mobile Master do?** 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
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.