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

There really is not a clean, safe, zero overhead, solution to this which is directly supported by the language. Many people are content with the current visibility features, while many feel they are lacking.

The runtime could (but does not) make this distinction with ivars and methods. First class support would be best, IMO. Until then, we have some abstraction idioms:

Option A

Is bad - everything's visible. I don't agree that it is a good approach, and that is not OOD (IMO). If everything is visible, then your class should either:

• support all cases for how the client may use your class (usually unreasonable or undesirable)
• or you provide them with a ton of rules via documentation (doc updates are likely to go unnoticed)
• or the accessors should have no side effects (not OOD, and frequently translates to 'do not override accessors')

Option B

Has the deficiencies of Option A,, and like Option A, members may be accessed by key.

Option C

This is slightly safer. Like all the others, you can still use keyed access, and subclasses may override your accessors (even if unknowingly).

Option D

One approach to this is to write your class as a wrapper over over an implementation type. You can use an ObjC type or a C++ type for this. You may favor C++ where speed is important (it was mentioned in the OP).

A simple approach to this would take one of the forms:

// inner ObjC type
@class MONObjectImp;

@interface MONObject : NSObject
{
@private
 MONObjectImp * imp;
}
@end


// Inner C++ type - Variant A
class MONObjectImp { ... };

@interface MONObject : NSObject
{
@private
 MONObjectImp imp;
}
@end


// Inner C++ type - Variant B
class MONObjectImp;

@interface MONObject : NSObject
{
@private
 MON::t_auto_pointer imp;
}
@end

(Note: Since this was originally written, the ability to declare ivars in the @implementation block has been introduced. You should declare your C++ types there if it isn't necessary to support older toolchains or the 'fragile' 32-bit OS X ABI).

C++ Variant A is not as 'safe' as the others, because it requires the class' declaration visible to the client. In the other cases, you can declare and define the Imp class in the implementation file -- hiding it from clients.

Then you can expose the interface you choose. Of course, clients can still access your members if they really want to via the runtime. This would be easiest for them to do safely with the ObjC Imp type -- the objc runtime does not support C++ semantics for members, so clients would be asking for UB (IOW it's all POD to the runtime).

The runtime cost for the ObjC implementation is to write a new type, to create a new Imp instance for each instance, and a good amount of doubling of messaging.

The C++ type will cost practically nothing, apart from the allocation (Variant B).

Option E

Other approaches often dissociate ivars from interfaces. While this is a good thing, it's also very unusual for ObjC types. ObjC types/designs often maintain close relations to their ivars and accessors -- so you'll face resistance from some other devs.