http://bhapca.blogspot.hk/2012/11/how-to-access-backing-ivars-for.html

How to access backing ivars for inherited properties in Objective-C
I had a superclass that declared and synthesized a read only property, meant to be set in its subclasses. The code was something like this:

@interface A : NSObject
@property (nonatomic, readonly) NSUInteger prop;
@end

@implementation A
@synthesize prop = _prop;
@end

@interface B : A
@end

@implementation B

• (void)establishValueForProp
  {
  _prop = 1; // PROBLEM !!!
  }

@end

The instance variable backing the property seemed to be invisible in the subclass. The error was pretty unclear when compiling an iOS app: “Use of undeclared identifier ‘_prop'”. To get around the error I did the following:

@interface A : NSObject {
NSUInteger _prop;
}
@property (nonatomic, readonly) NSUInteger prop;
@end

That is, I explicitly declared the backing variable in the declaration of the superclass.

Interestingly, when I was preparing this blog post I compiled the code above as a “Command Line Tool” type of project, not an iOS app. The error was different, dare I say better, more telling of the root cause: “Instance variable ‘_prop’ is private”. So it’s a matter of the visibility of the ivar, so perhaps a better fix would be:

@interface A : NSObject {
@protected
NSUInteger _prop;
}
@property (nonatomic, readonly) NSUInteger prop;
@end

Kind of interesting though that properties are synthesized by default as private ivars. Learned something new.
Posted by Bogdan Hapca at 12:09 PM
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Labels: iOS, Objective-C
1 comment:

Liviu Macsen said…
Thank your Bogdan for your solution. I found that you can also synthesize in subclass in order to solve this problem.

@implementation B

@synthesize prop = _prop;

• (void)establishValueForProp
  {
  _prop = 1; // Working !!!
  }

@end