View Full Version : Confusion on instance variables being in the implementation file.

Feb 13, 2012, 11:57 AM
Hi guys,

I was referencing one of my Objective-C books* and noticed that there were instance variables defined in the implementation section of a class. I assume because the users in the class don't need to know about the instance variables, just the interface file but it leads me to two questions:

1. When did this start? I'm pretty sure its new as I've never seen it before. (Some internet research suggests its new in Xcode 4.2, but I can't find anything in Apple's docs about it).

2. If it's Xcode only, wouldn't this be a bad way to do things since some other IDE's support Objective-C, but not necessarily putting the ivars in the implementation section?

The book is the newest version (4.0) of Steve Kochan's programming in Objective-C. I have and read the 2.0 version and the classes do not have ivars in the implementation section, but I couldn't find in the new version where this difference, and the reasoning behind it is mentioned.

Feb 13, 2012, 12:12 PM
Maybe post some code of what you mean ? Are you sure these are "instance" variables ?

Usually, when doing this for methods, it's to make them "private". For variables, you can do so in the interface section using the @private directive, so no need to stick them in the implementation file.

However, I do use variables in implementation files, as a global symbol. These I use as "class variables", variables that survive and are accessible by all instances of the class or by class methods.

Feb 13, 2012, 12:15 PM
Here is some code straight from the book:

@interface Fraction : NSObject


-(void) print;
-(void) setNumerator: (int) n;
-(void) setDenominator: (int)n;


@implementation Fraction
int numerator;
int denominator;

NSLog(@"%i / %i", numerator, denominator);

numerator = n;

denominator = n;


I did see in this version of the book it confirms that the change is new to Xcode 4.2, but that it will explain the changes later. I use this book only for reference so I'm not sure where it explains the changes (I haven't found it yet).

Feb 13, 2012, 01:09 PM
This is a new feature of the most recent clang (aka Apple LLVM) compiler, specifically. It's useful to hide private state of classes.

Feb 13, 2012, 03:56 PM
I forgot about this feature. I like having only public things in the header, and this makes it even more cleaner. FWIW, it works with the Snow Leopard Xcode 4.2 build.