Question

I left the original, so people can understand the context for the comments. Hopefully, this example will better help explain what I am after.

Can I create a class in Obj-C that has file-scope visibility?

For example, I have written a method-sqizzling category on NSNotificationCenter which will automatically remove any observer when it deallocs.

I use a helper class in the implementation, and to prevent name collision, I have devised a naming scheme. The category is NSNotificationCenter (WJHAutoRemoval), so the private helper class that is used in this code is named…

WJH_NSNotification_WJHAutoRemoval__Private__BlockObserver

That’s a mouthful, and currently I just do this…

#define BlockObserver WJH_NSNotification_WJHAutoRemoval__Private__BlockObserver

and just use BlockObserver in the code.

However, I don’t like that solution.

I want to tell the compiler, “Hey, this class is named Bar. My code will access it as Bar, but I’m really the only one that needs to know. Generate a funky name yourself, or better yet, don’t even export the symbol since I’m the only one who should care.”

For plain C, I would is “static” and for C++ “namespace { }”

What is the preferred/best/only way to do this in Obj-C?

Original Question

I want to use a helper class inside the implementation of another. However, I do not want external linkage. Right now, I’m just making the helper class name painfully unique so I will not get duplicate linker symbols.

I can use static C functions, but I want to write a helper class, with linker visibility only inside the compilation unit.

For example, I’d like to have something like the following in multiple .m files, with each “Helper” unique to that file, and no other compilation unit having linker access. If I had this in 10 different files, I’d have 10 separate classes.

@interface Helper : NSObject
...
@end

@implementation Helper : NSObject
...
@end

I have been unable to find even a hint of this anywhere, and my feeble attempts at prepending “static” to the interface/implementation were wrought with errors.

Thanks!

Answer 1

I don’t believe you will be able to do what you want because of the Objective-C Runtime. All of your classes are loaded into the runtime and multiple classes with the same name will conflict with each other.

Objective-C is a dynamic language. Unlike other languages which bind method calls at compile time, Objective-C does method resolution at invocation (every invocation). The runtime finds the class in the runtime and then finds the method in the class. The runtime can’t support distinct classes with the same name and Objective-C doesn’t support namespaces to seperate your classes.

If your Helper classes are different in each case they will need distinct class names (multiple classes with the same name sounds like a bad idea to me, in any language). If they are the same then why do you want to declare them separately.

I think you need to rethink your strategy as what you are trying to do doesn’t sound very Objective-C or Cocoa.