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.