objective-c interface - declaring variable vs just property?

Question

In Obj-c when declaring a variable within @interface

@interface: NSObject{ MyObject* myObject}

@property (unsafe, nonatomic) MyObject* myObj

Answer 1

@property defines an interface, not an implementation. In your case, you're defining a readwrite property. This means that you're promising to implement -myObject and -setMyObject:. This has nothing to do with ivars.

Now, the most common way to implement those methods is by having them be backed by an ivar. As a convenience, ObjC lets you automatically generate the required methods with an ivar store using @synthesize myObject=myObject_; This says "create the required methods for the property myObject using an automatically created ivar called myObject_." The ivar myObject_ is a real ivar, and you can access it normally (though you generally shouldn't; you should use accessors).

Instead of using @synthesize, you could just implement -myObject and -setMyObject:. You could even use @dynamic myObject; to tell the compiler "don't worry about the implementations for this property; it'll be handled correctly at runtime."

There are a few differences between @property and just declaring methods, but in principle, this line:

@property (nonatomic, readwrite, strong) MyObject* myObject;

is conceptually the same as this:

- (MyObject *)myObject;
- (void)setMyObject:(MyObject *)anObject;

Declaring the ivar yourself has no real impact here. You still need to implement the methods somehow. If your named ivar is the same as the ivar @synthesize is using, then @synthesize just won't create a new ivar.

As a matter of practice, I discourage people from declaring ivars anymore. I recommend just using public and private properties with @synthesize to create any needed ivars. If you must have a manual ivar for some reason, then I recommend declaring them in the @implementation block rather than the @interface.

Answer 2

Skipping declaring the ivar is perfectly fine--but you will not be able to see the ivar's value in the Xcode IDE. One day Apple may fix this.

You will be able to "po" the ivar to inspect it in GDB or lldb.