Why can't I see the inherited instance variable in subclass?

Question

I have read in iOS Programming Fundamentals by Matt Neuberg that instance variables are protected, meaning that other classes, except for subclasses of this one, can't see them.

I have a parent class A where I define an ivar list. (A.m)

@interface A ()

@end

@implementation A
{
     NSArray *list;
}

@end

Class B extends A (B.h)

#import "A.h"

@interface B:A

@end

(B.m)

@interface B ()

@end

@implementation B

list = 
...

@end

I want to use ivar list in child class B but the compiler doesn't see that the was reference declared in the parent class. I have tried explicitly using @protected but that doesn't work. I don't want to expose ivar list on the public interface. It's an internal structure that is a common element of all subclasses. How can I do this?

You have to declare the instance variable in A.h so that B can see it, and you have to declare it as @protected — Darren
– Darren, Commented May 28, 2015 at 18:45

jlehr · Accepted Answer · 2015-05-28 19:49:59Z

1

Instance variables that are declared outside a class's public interface (in other words, the @interface section) are private by default. You can add a visibility modifier to the declaration to change the visibility of one or more ivars like so:

@implementation A
{
     NSNumber *_ivarWithDefaultVisibility;

@protected
     NSArray *_list;
     NSString *_anotherIvarWithProtectedVisibility;
}

(Note that according to Apple's Cocoa coding guidelines, ivar names should be prefixed with an underscore.)

answered May 28, 2015 at 19:49

jlehr

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1 Comment

Rob Napier Over a year ago

This @implementation block is in the A.m file, so B won't be able to see it (the compiler doesn't have access to A.m when it's building B.m). You'd have to declare these ivars in the @interface block in A.h if you were going to do it this way.

Rob Napier · Accepted Answer · 2015-05-28 20:48:00Z

This is not the usual way to implement protected properties in ObjC. (@protected and @private are very seldom used in Cocoa.) First, use a property, not an ivar. It will make it much cleaner. Declare it this way:

A.h

@interface A : NSObject
// Public interface goes here
@end

A.m

// Declare the property in a class extension inside the implementation file.
// This is the idiomatic way to create a "private" property.
@interface A ()
@property (nonatomic, readwrite, strong) NSArray *list;
@end

A+protected.h

// Enumerated any methods that should be accessible to subclasses here.
// Properties are just a special way of defining methods.
// The word "Protected" is just a category name.

@interface A (Protected)
@property (nonatomic, readwrite, strong) NSArray *list;
@end

B.h

#import "A.h"   
@interface B : A
...
@end

B.m

#import "B.h"
#import "A+protected.h"

// ... now you can use self.list ...

This approach allows you to create any kind of "protected" method, not just properties, and is also the technique used to create "friend" classes.

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