How does one publish a static void NSString in a protocol?

Question

I have a defined a custom error domain that I want to publish in a protocol. The domain is defined in the implementation file like this:

//in the .m file
static void *MyErrorDomain = (void *)@"MyErrorDomain";

The .h file implements a protocol, and I would like to publish MyErrorDomain there as well. I can't, however, figure out the correct form. The one that gives the fewest errors is:

// in the protocol    
static extern void * TBPluginErrorDomain;

The Xcode error is: "Multiple storage classes in declaration specifiers".

I thought that the static void construction turns MyErrorDomain into a kind of function, but "static extern void TBPluginErrorDomain();" only incremented the number of errors. In short, I'm wandering in a morass of ignorance and all guidance will be greatly appreciated.

Answer 1

Protocols only define the names and parameters of methods to be implemented by other classes. They do not contain any data or functional code.

If you want custom data returned by a protocol method, you should define a class that provides the data and have one of the protocol's required methods return and instance of that class. That will force those implementing the protocol to #import the class.

Answer 2

At the top-level, static means “not directly accessible (without a pointer) outside this file” (file scope). extern means “defined in some other object file”. static extern does not really make any sense. It is like asking for access to a private member from another class. You do not want to use static here.

Just make a normal definition in your .m:

void *MyErrorDomain = @"MyErrorDomain";

And make an extern declaration in your .h:

extern void *MyErrorDomain;

The be sure to link in the .o from the .m whenever you link together something that uses the variable (after including the .h file). In Xcode, in the Targets tab of the Get Info window for the .m file, be sure to check each target that uses the variable (maybe slightly different in newer versions, mine is old!).

You can probably add in const (const … and extern const …), but NSString instances are immutable, so it is not totally necessary. You might make it into a const pointer though, so the pointer can not be directly changed at runtime (without casting away the constness). Together you have const void * const MyErrorDomain in the definition. Just prefix it with extern in the declaration.

Also, depending on your purposes, you might consider using a NSString * instead of void *.