Do declared properties require a corresponding instance variable?

Question

Do properties in Objective-C 2.0 require a corresponding instance variable to be declared? For example, I'm used to doing something like this:

MyObject.h

@interface MyObject : NSObject { NSString *name; } @property (nonatomic, retain) NSString *name; @end 

MyObject.m

@implementation @synthesize name; @end 

However, what if I did this instead:

MyObject.h

@interface MyObject : NSObject { } @property (nonatomic, retain) NSString *name; @end 

Is this still valid? And is it in any way different to my previous example?

Answer 1

If you are using the Modern Objective-C Runtime (that's either iOS 3.x or greater, or 64-bit Snow Leopard or greater) then you do not need to define ivars for your properties in cases like this.

When you @synthesize the property, the ivar will in effect be synthesized also for you. This gets around the "fragile-ivar" scenario. You can read more about it on Cocoa with Love

Answer 2

In your interface, you can formally declare an instance variable between the braces, or via @property outside the braces, or both. Either way, they become attributes of the class. The difference is that if you declare @property, then you can implement using @synthesize, which auto-codes your getter/setter for you. The auto-coder setter initializes integers and floats to zero, for example. IF you declare an instance variable, and DO NOT specify a corresponding @property, then you cannot use @synthesize and must write your own getter/setter.

You can always override the auto-coded getter/setter by specifying your own. This is commonly done with the managedObjectContext property which is lazily loaded. Thus, you declare your managedObjectContext as a property, but then also write a -(NSManagedObjectContext *)managedObjectContext method. Recall that a method, which has the same name as an instance variable/property is the "getter" method.

The @property declaration method also allows you other options, such as retain and readonly, which the instance variable declaration method does not. Basically, ivar is the old way, and @property extends it and makes it fancier/easier. You can refer to either using the self. prefix, or not, it doesn't matter as long as the name is unique to that class. Otherwise, if your superclass has the same name of a property as you, then you have to say either like self.name or super.name in order to specify which name you are talking about.

Thus, you will see fewer and fewer people declare ivars between the braces, and instead shift toward just specifying @property, and then doing @synthesize. You cannot do @synthesize in your implementation without a corresponding @property. The Synthesizer only knows what type of attribute it is from the @property specification. The synthesize statement also allows you to rename properties, so that you can refer to a property by one name (shorthand) inside your code, but outside in the .h file use the full name. However, with the really cool autocomplete that XCode now has, this is less of an advantage, but is still there.

Hope this helps clear up all the confusion and misinformation that is floating around out there.