Objective-C: what is private what is not?

Question

Why are people using

@interface ViewController : UIViewController 
{
@private
    UIButton* button_;
}

@private declarations in public headers? Declaring a variable inside an implementation yields the same result, doesn't it? It feels strange to me, I thought a public header should only contain really public members. What to do with protected members?

@implementation ViewController
    UIButton* button_;
@end

The only difference I know of is that this variable is only visible inside the current compilation unit (the .m file, right?)

Does the same hold true for methods? I could compile fine with proper method ordering or forward declarations. Why do people care to declare categories for private methods? For testing purposes only?

Answer 1

@private is referring only to the iVars.

By default you can access ivars of an instance like so:- id iShouldNotDoThis = foo->bar;

@private means you can't access the ivar like that and have to use the access methods.

id thisIsBetter = [foo bar];

Nothing to do with private categories or methods.

Answer 2

The variable declaration inside the @implementation block will create a global variable, and not an instance variable. Instance variables need to be defined as part of the @interface.

While you can create an additional @interface block, by means of a category or extension, but it can only contain method declarations, and not instance variables.

Answer 3

I would say that while it might "feel" wrong to you to put private instance variables in a supposedly public header, I wouldn't worry about it.

Take a look at pretty much any header file for a Cocoa class (except for the cluster classes), and you'll see that Apple declares their instance variables in their public header files.

Since Apple is OK with it, I don't think you have much to worry about. =)

FYI: All instance variables are protected by default.

Answer 4

Does the same hold true for methods?

No, methods are visible to any part of the program. If you know the selector you can callit.

I could compile fine with proper method ordering or forward declarations. Why do people care to declare categories for private methods? For testing purposes only?

Private categories are a form of forward declaration. You can think of them as if they were C prototypes.

Andrew