What is the best way to define string constants in an objective-c protocol?

Question

I have defined a protocol that all my plug-ins must implement. I would also like the plug-ins to all use certain strings, like MyPluginErrorDomain. With integers this is quite easily achieved in an enum, but I can't figure out how to do the same with strings. Normally, in classes I would define

extern NSString * const MyPluginErrorDomain;

in the .h file and in the .m file:

NSString * const MyPluginErrorDomain = @"MyPluginErrorDomain";

but that doesn't work very well in a protocol, because then each plug-in would have to provide its own implementation which defeats the purpose of having a constant.

I then tried

#define MYPLUGIN_ERROR_DOMAIN @"MyPluginErrorDomain"

but the implementing classes in the plug-in can't seem to see the #define. Who knows a good solution?

Answer 1

You can declare them in the header with the protocol (but outside the protocol interface itself), then define them in an implementation file for the protocol (obviously it wouldn't have an @implementation section - just your NSString definitions).

Or have a separate .h/.m pair that is just for the string constants (the protocol header can import the string constants header).