Question

I’ve looked through a bunch of posts on this subject. Maybe I didn’t run across “the one” and someone will point me in that direction. The question is simple and probably has a simple answer.

If you have two ivars, say, “public_ivar” and “private_ivar”, where/how should you declare them so that what is public is public and what is private is not exposed in any way to anyone looking at the header file?

Same question in the case of “public_method” and “private_method”.

I like clean header files (in other languages) that only expose the methods and ivars I want someone else to see. You should be able to publish your header file and not run into the danger of someone accessing something they are not supposed to. How do you do that in objective-C.

For example, let’s say that I decide that I need to use an ivar to keep track of some data, a counter or somthing like that, between various class methods that all need access to this information. If that ivar is declared conventionally in the header under @interface its existence is publicly advertised and it is usable by anyone creating an instance of the class. The ideal scenario would be that this ivar would not be visible at all outside of the class implementation.

Answer 1

You can declare instance variables or declared properties in a class extension. Since a class extension is declared in an implementation file (i.e., not a header file), they won’t be visible to someone inspecting the header file. For instance, in the header file:

@interface SomeClass : NSObject
@end

and in the implementation file:

@interface SomeClass ()
@property (nonatomic, assign) int privateInt;
@end

@implementation SomeClass

@synthesize privateInt;
…
@end

or

@interface SomeClass () {
    int privateInt;
}
@end

@implementation SomeClass
…
@end

Note that there’s nothing preventing access to private/class extension instance variables (or the accessor methods for properties declared in a class extension) during runtime. I’ve written a rather detailed post about this as an answer to another question on Stack Overflow: Does a private @property create an @private instance variable?

Edit: Instance variables in class extensions were presented in WWDC 2010 session 144.

Edit: “Using the Clang/LLVM 2.0 compiler, you can also declare properties and instance variables in a class extension.”

http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/ObjectiveC/Chapters/ocCategories.html#//apple_ref/doc/uid/TP30001163-CH20-SW1