__typeof self in Objective-C generics?

Question

Is this - or how is this - possible with the new Objective-C generics syntax?

@class MyHelperClass<T>;

@interface NSObject (Extension)
    @property (nonatomic,readonly) MyHelperClass<MYTYPE *> * helper;
@end

The question is:

What is the syntax for placeholder MYTYPE * - and does it exist - that would make this work with any subclass of NSObject. Meaning, that on a UIView instance, the helper property would return a MyHelperClass<UIView *> and on an NSString instance, it would return MyHelperClass<NSString *>? __typeof self does not work as self is not defined in an interface.

Suggestions are highly appreciated! :)

Answer 1

I don't think this is possible. The most obvious way that I can think of to do this would be the following:

@interface NSObject (Extension)

- (MyHelperClass<instancetype> *)a;

@end

According to the clang documentation,

instancetype is a contextual keyword that is only permitted in the result type of an Objective-C method

This is a compiler error, however, so perhaps the documentation should read "instancetype is only permitted as the result type of an Objective-C method."

My next thought was to cheat by writing some Swift code

protocol Test {
  var a: Array<Self> {get}
}

then importing it into an Objective-C file to see what the preprocessor did to turn it into Objective-C. Sadly, according to the documentation here, generics in Swift can't be accessed from Objective-C -- because they aren't an Objective-C language feature! This document has actually been updated for Swift 2.0, according to the revision history. Probably they still aren't supported because the Objective-C type system isn't as expressive as Swift's even with the currently available generics.

In summary, barring the existence of some obscure clang extension I don't know about, I think there's no possible way to do this.