Objective C: Why do we declare ivars in the .h member area if @property seems to do it automatically?

Question

In implementing an interface it seems the common method in tutorials and literature is to declare an ivar and then set the @property then @synthesize.

@interface MyClass : NSObject {
NSString *myString;
}

@property (nonatomic, retain) NSString *myString;
@end

However, omitting the explicit declaration and just putting @property has the same effect.

@interface MyClass: NSObject {
}

@property (nonatomic, retain) NSString *myString;
@end

So how come most people use @property and an explicit declaration? Is it bad form not to?

Answer 1

It used to be necessary. There are two different versions of the Objective-C runtime: a 32-bit-only "legacy" runtime (the old one) and a 32/64-bit runtime (the new 32-bit runtime is only used on iOS devices and for the iOS simulator).

I think the only place this is still necessary is when you're running the app in 32-bit mode (either 10.5 or 10.6). Everywhere else (64-bit Leopard, 64-bit Snow Leopard, Lion, iOS) uses the newer runtime that has "automatic ivar synthesis", and the resulting ivars are referred to as "synthesized ivars".

Answer 2

Some platforms support synthesized instance variables, some don't. Explicitly declaring instance variables makes your code valid in more places, and it used to be outright necessary until very recently, so people still do it. In a few years, they probably won't anymore.