Theocacao: New Objective-C Tutorial

Cathy Shive — Apr 14, 08 5716

This is a really nice tutorial – Thanks!

Eimantas — Apr 14, 08 5717

Read it all. Nice. Noticed only a few typos, but will be waiting for updates ,)

Mike F. — Apr 14, 08 5718

Hello! I'm new to Cocoa and Objective-C, and have been finding your site and articles very useful.

When reading this, I noticed that in your section on Accessors (2), you state, "All instance variables are private in Objective-C by default."

However, in the Objective-C guide from Apple, they state, "By default, all unmarked instance variables (like name above) are @protected." (p.38)

That confused me slightly, but since you don't really mention the concept of protected vs. private variables in your tutorial, I'm also not sure whether it is an intentional simplification.

Thanks for your excellent work on the community's behalf!

Steven Degutis — Apr 14, 08 5719

In the tutorial, at the end, you give an example of a category which implements the -isURL method on NSString. Assuming the goal is to explain categories, and not a good method, you could probably replace the entire contents of that with this, in order to get the point of a category across less confusingly, and it would be functionally the same as your current method:

- (BOOL) isURL
{
    if ( [self hasPrefix: @"http://"] )
        return YES;
	
    // else
        return NO;
}

Steven Degutis — Apr 14, 08 5720

Mike F.

The instance variables are in fact protected, but this simply means that they are private to all external sources, but subclasses may access them.

For example, if I create a class Car that has an instance variable of type NSString called "model" then my subclass Lexus may access the "model" variable in its superclass Car, but any instance in the class GasStation cannot access that variable without using an accessor (getter).

Ewan — Apr 14, 08 5721

The last example on categories is to determine if a string is a HTTP URL. However, the introduction text says it's to replace bold tags with italics tags. Otherwise, this s a good tutorial and I enjoyed reading it.

Ian Athanasakis — Apr 14, 08 5722

It is a great tutorial. Well done.

cesar — Apr 14, 08 5723

hey scott,
the new tutorial is pretty cool! just have one question. How do you create your graphics??

Scott Stevenson — Apr 14, 08 5724

@Mike F: All instance variables are private in Objective-C by default

I actually meant private in the general sense, but yes, you're right. I'll probably change that. There were a lot of thing I cut in order to keep it a reasonable length, and this was on of them.

@Steven Degutis: In the tutorial, at the end, you give an example of a category which implements the -isURL method on NSString

Thanks for pointing this out. Silly mistake on my part.

@Ewan: However, the introduction text says it's to replace bold tags with italics tags

I'll fix that too, thanks.

@cesar: How do you create your graphics?

Just Photoshop and Illustrator.

Andy Lee — Apr 14, 08 5726

These are not just named arguments.

Not to be pedantic -- and maybe you're trying to keep things simple -- but they aren't named arguments at all. This has been brought up in several places; I posted a link to a simple proof of this
on cocoa-dev recently.

Again, maybe you're trying to keep things simple, or you think "named arguments" is a close enough description for beginners (there's time later to fine-tune their understanding), but IMO it's better not to feed the misconception.

In any case -- thanks for the tutorial!

Paul Robinson — Apr 15, 08 5735

These tutorial as beautifully presented and clearly written. Thanks for making another one!

Jon Kantro — Apr 16, 08 5736

This looks awesome. I've been learning C for about two months as well as reading The Object-Oriented Thought Process by Matt Weisfeld and they certainly have helped me tremendously in understanding C and object-oriented programming in general. This tutorial looks like it will make my transition into Obj-C not necessarily easier but more pleasant, so thanks!

Andras Puiz — Apr 16, 08 5740

Great tutorial. Maybe you should say a word about NSString constants with the @"" notation (which you start using very early on), they may confuse true beginners.

Danny — Apr 18, 08 5749

Thanks for this. Your tutorials are always first class and this covers some of the new 2.0 material in very straightforward way.

Marco Masser — Apr 19, 08 5750

Nice article. But as noted before, I think you should elaborate on the @"" string syntax and maybe on the fact that objects you create with methods other than -init... (e.g. [NSString string]) are simply autoreleased in the constructor by convention.
Also, there are two errors in section 7 "More on Memory Management." The following line appears twice in the section:


NSNumber* value1 = [[Photo alloc] initWithFloat:8.75];

The +alloc method should be sent to the NSNumber class, of course.

And one small grammatical error: In section 9 "Properties," there's a verb ("feel"?) missing in the second paragraph, counted from the bottom.

Scott Stevenson — Apr 19, 08 5751

@Andras Puiz and @Marco Masser:
I agree about needing to call out the @"string" syntax. I need to figure out where it fits in the flow.

@Marco Masser: the fact that objects you create with methods other than -init... (e.g. [NSString string]) are simply autoreleased in the constructor by convention.

The trickiest topic in an entry-level Objective-C tutorial is memory management. I tried to simplify it down to its essence, and I think talking about the conventions behind factory methods would be too much detail. The mechanics don't change how they use the class, so I don't think it's an essential skill to get started.

Also, there are two errors in section 7 "More on Memory Management [...] And one small grammatical error: In section 9 "Properties,"

Both fixed. Thanks for finding them.

Marco Masser — Apr 20, 08 5754

OK, I agree on that point. It's just that I finally understood the elegance behind the autorelease mechanics only after I started to write my own classes with factory methods. At first I had memory leaks or double releases until I got my head around that convention. But as you said, that's probably too much detail for an entry-level Objective-C tutorial.

Marco Masser — Apr 20, 08 5755

[...] I think talking about the conventions behind factory methods would be too much detail.

OK, I agree on that point. It's just that I finally understood the elegance behind the autorelease mechanics only after I started to write my own classes with factory methods. At first I had memory leaks or double releases until I got my head around that convention. But as you said, that's probably too much detail for an entry-level Objective-C tutorial.

Marco Masser — Apr 20, 08 5756

Ah... sorry for those formatting problems and so many posts...

I just found that the the -isURL method in the downloadable Xcode project is still the "old" one.

Danny — May 03, 08 5763

Scott, I have an ObjC class with a C++ class pointer which is instantiated in init and deleted in release. How can I preserve this mechanism with Garbage Collection when dealloc is never called?

Scott Stevenson — May 04, 08 5767

@Danny: How can I preserve this mechanism with Garbage Collection when dealloc is never called?

Garbage-collected objects can implement -finalize. It's not a direct replacement for dealloc, but it will probably do what you need.

kamelito — May 08, 08 5805

Hi,

We've to wait, I'm sure it'll worth it...

Check it out :

http://www.amazon.com/Programming-Objective-C-2-0-Developers-Library/dp/0321566157/ref=sr_1_3?ie=UTF8&s=books&qid=1210279077&sr=1-3

Joe — May 23, 08 5895

Hi Scott,

Can you explain further how copy and retain work?
Thanks ahead of time; I'm having a hard time getting this one.

Scott Stevenson — May 24, 08 5910

@Joe: Can you explain further how copy and retain work?

Do you have a specific question? I'm not sure if I could explain in more clearly here as a comment than I did in the actual tutorial. Does the concept not make sense at all, or is there one part you're stuck on?

Joe — May 25, 08 5915

Ok, so

retain increments the reference count of an object every time it's called. And including it as an attribute in the property declaration is the equivalent of

 
[[caption setCaption:newCaption]retain];

so retain is called every time the setter is called?

And copy is another way to create an object and would look like


newObject = [oldObject copy];

and the new object comes with a reference count of 1?

Joe — May 28, 08 5943

Sorry, that's not right is it?
I didn't understand why retain would be called over and over.

So, does including retain in the property declaration mean that the autorelease > retain sequence will be synthesized? or is that synthesized by default? in which case, what would retain do?

Specifically, I'm having trouble understanding the difference between retain and assign in property declarations.

I've looked at the documentation on this several times, and it's kind of vague (for me).

Scott Stevenson — May 29, 08 5948

@Joe: Specifically, I'm having trouble understanding the difference between retain and assign in property declarations.

If the instance variable is called "myName", assign would do something roughly equivalent to this:
myName = newName;

Retain would do this:

[myName autorelease];
myName = [newName retain];

Copy would do this:

[myName autorelease];
myName = [newName copy];

This isn't necessarily the exact code that gets generated, but it has the same basic effect.

You also asked if the retain keyword would be the equivalent of "[[caption setCaption:newCaption]retain];". The answer is no, but I'm not sure how to quickly explain why that is the case. I hope the above clears that up.

If you have garbage collection on, assign and retain are the same thing.

Jason Barry — Jul 02, 08 6136

This is a very good beginner's tutorial. I think I found a typo in chapter 4 under Nested Messages. It says

"Avoid nested nesting more than two message calls on a single line, as it easily gets unreadable."

I believe the second word nested should be removed. Thanks for the great tutorial.

Paul — Aug 14, 08 6247

Hi,
I wanted to thank you for post this tutorial on Objective-C. I was just asked to write an app for the Iphone and I've never used objective-c or cocoa before...in fact I've never booted up a Mac computer before, so its all new to me right now :)

Dan Ribar — Jun 05, 09 6803

Scott - thanks for this post. It has helped me a lot!

One of the things i'm stuck on is [using your example] that I can't set and then read a variable from an outside class. It keeps going back to null....

So if I set 'caption' from one method
and try to get the value that is on 'caption' from another method, it is gone. I know there is discussion above, but i'm not too sure what the resolution is.

I am using something like this to read the 'caption' value:

method1============================
photo* myPhoto = [[photo alloc]init];

myPhoto.caption=@"some caption";
NSLog(@"caption: %@",myPhoto.caption);
==================================
in this example all is well.

now in the same class I try this method and the value is now null

method2============================
NSLog(@"caption: %@",myPhoto.caption);
==================================

dude - not too sure what I'm missing. Must be part of the asp.net to objective-c cycle... arghhh..

any thoughts?

Dan Ribar

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