Objective-C: How do you access parent properties from subclasses?

If I have this class defined, how do I access the someObject property in subclasses without compiler errors?

@interface MyBaseClass
  // someObject property not declared here because I want it to be scoped 
  // protected. Only this class instance and subclass instances should be
  // able to see the someObject property.
@end

// This is a private interface extension...properties declared here
// won't be visible to subclasses. However, I don't see any way to 
// declare protected properties...
@interface MyBaseClass (private)
   @property (nonatomic, readwrite, retain) NSObject *someObject;
@end

@interface MySubclass : MyBaseClass 
@end

@implementation MySubclass

- (id) init {
    // Try to do something with the super classes' someObject property. 
    // Always throws compile errors.

    // Semantic Issue: Property 'someObject' not found 
    // object of type 'MySubclass *'
    self.someObject = nil; 

}
@end

I'm obviously not understanding how inheritance works in objective-c. Could someone enlighten me?

Solution 1:

The solution you're after is to declare the MyBaseClass private property in a class extension:

@interface MyBaseClass ()
@property (nonatomic, readwrite, retain) NSObject *someObject;
@end

You are then free to make that declaration both in MyBaseClass and in MySubclass. This lets MySubclass know about these properties so that its code can talk about them.

If the repetition bothers you, put the class extension in a .h file of its own and import it into both .m files.

I will give an example from my own code. Here is MyDownloaderPrivateProperties.h:

@interface MyDownloader ()
@property (nonatomic, strong, readwrite) NSURLConnection* connection;
@property (nonatomic, copy, readwrite) NSURLRequest* request;
@property (nonatomic, strong, readwrite) NSMutableData* mutableReceivedData;
@end

There is no corresponding .m file and that's all that's in this file; it is, as it were, purely declarative. Now here's the start of MyDownloader.m:

#import "MyDownloader.h"
#import "MyDownloaderPrivateProperties.h"
@implementation MyDownloader
@synthesize connection=_connection;
@synthesize request=_request;
@synthesize mutableReceivedData=_mutableReceivedData;
// ...

And here's the start of its subclass MyImageDownloader.m:

#import "MyImageDownloader.h"
#import "MyDownloaderPrivateProperties.h"

Problem solved. Privacy is preserved, as these are the only classes that import MyDownloaderPrivateProperties.h so they are the only classes that know about these properties as far as the compiler is concerned (and that's all that privacy is in Objective-C). The subclass can access the private properties whose accessors are synthesized by the superclass. I believe that's what you wanted to accomplish in the first place.

Solution 2:

that's how you access them. how you declare them is what's biting you:

@interface MyBaseClass : NSObject
@property (nonatomic, readwrite, retain) NSObject *someObject;
@end

this is the normal way to declare a new objc class.

by adding the parentheses (instead of declaring the superclass - NSObject in this case), you have declared a class extension, which is probably not visible to the subclass (via inclusion).

you will probably never need to declare a root class in objc:

@interface MyBaseClass // << superclass omitted
@property (nonatomic, readwrite, retain) NSObject *someObject;
@end

NSObject (or a subclass of, assuming you're target apple's systems) should be the base class unless you're very experienced and know what a root class is for.

class extensions are often used to 'simulate' private interfaces. by simulate, the compiler doesn't enforce this, as it would be enforced in other languages. for example, all messages are still dynamic, although the subclass may (unknowingly) override methods in your extensions, if declared with the same selector.

Solution 3:

Judging by the () after your base class name, it looks like you are declaring a private interface extension within your class implementation, is this the case? If so the variable will only be accessible from within that class implementation.

Does your MyBaseClass inherits from NSObject directly?

If so, you need to declare the someObject property in your interface file, as in:

@interface MyBaseClass : NSObject
{
}
@property (nonatomic, retain) NSObject *someObject;

And then synthesize it like you are already doing.