How should private and public members be implemented in objective-c?

Question

I had some discussion related to the use of properties and instance variables at work, therefore I would like to find a wiki answer for that. Now, I know there\'s no real pr

Answer 1

By using class extensions you can have private properties.

A class extension syntax is simple:

Inside the .m-file, that has the class, create a unnamed category:

.h

@interface OverlayViewController : UIViewController 
- (IBAction)moreButtonClicked:(id)sender;
- (IBAction)cancelButtonClicked:(id)sender;
@end

.m

#import "OverlayViewController.h"

@interface OverlayViewController ()
@property(nonatomic) NSInteger amount;
@property(retain,nonatomic)NSArray *colors;
@end

@implementation OverlayViewController
@synthesize amount = amount_;
@synthesize colors = colors_;

//…

@end

Now you got all the aspects of properties for private members, without exposing them to public. There should be no overhead to synthesized properties to written getter/setters, as the compiler will create more or less the same at compile time.

Note that this code uses synthesized ivars. No ivar declaration in the header is needed.

There is a nice cocoawithlove article, about this approach.

You also ask why to use properties for private ivars. There are several good reasons:

• properties take care for ownership and memory management.
• at any point in future you can decide, to write a custom getter/setter. i.e. to reload a tableview, once a NSArray ivar was newly set. If you used properties consequently, no other changes are needed.
• Key Value Coding support properties.
• public readonly properties can be re-declared to private readwrite properties.

---

Since LLVM 3 it is also possible, to declare ivars in class extensions

@interface OverlayViewController (){
    NSInteger amount;
    NSArray *colors;
}
@end

or even at the implementation block

@implementation OverlayViewController{
    NSInteger amount;
    NSArray *colors;
}
//…
@end

see "WWDC2011: Session 322 - Objective-C Advancements in Depth" (~03:00)