Subclass of class with synthesized readonly property cannot access instance variable in Objective-C

Question

In the superclass MyClass:

@interface MyClass : NSObject

@property (nonatomic, strong, readonly) NSString *pString;

@end

@implementation MyClass

@synthesize pString = _pString;

@end

In the subclass MySubclass

@interface MySubclass : MyClass

@end

@implementation MySubclass

- (id)init {
    if (self = [super init]) {
        _pString = @"Some string";
    }
    return self;
}

The problem is that the compiler doesn't think that _pString is a member of MySubclass, but I have no problem accessing it in MyClass.

What am I missing?

Solution 1

The instance variable _pString produced by @synthesize is private to MyClass. You need to make it protected in order for MySubclass to be able to access it.

Add an ivar declaration for _pString in the @protected section of MyClass, like this:

@interface MyClass : NSObject {
    @protected
    NSString *_pString;
}

@property (nonatomic, strong, readonly) NSString *pString;

@end

Now synthesize the accessors as usual, and your variable will become accessible to your subclass.

Solution 2

I am familiar with this problem. You synthesize the variable in your .m class, so it is not imported along with the header since the _pString variable will be created as part of the implementation, and not the interface. The solution is to declare _pString in your header interface and then synthesize it anyway (it will use the existing variable instead of creating a private one).

@interface MyClass : NSObject
{
    NSString *_pString; //Don't worry, it will not be public
}

@property (nonatomic, strong, readonly) NSString *pString;

@end

Comments

[TeaCupApp]

try [super setPString:] or [self setPString:]

[Dinesh]

tried to remove readonly property like below @property (nonatomic, strong) NSString *pString;

[borrrden]

Yes, you will...if it is from a subclass that has access to the underlying variable.

[Sergey Kalinichenko]

@Owl You will not be able to set the string through the property, but you will be able to set the string to the backing variable, which will have the effect of pString property changing its value. You need to set _pString = @"abc", not self.pString = @"abc".

[borrrden]

@Owl what dasblinken said! You don't HAVE to set the variable through its property interface. This way objects that don't inherit from the class cannot change the variable. It is readonly except to children.

[borrrden]

Just a semantic question about stack...does "oldest" answer go by creation time or edit time?

[tacos_tacos_tacos]

This was exactly what I was looking for. Thanks

[SmileBot]

Good answer. I would personally use a double under score to indicate that you are accessing a protected ivar on a superclass and explicitly @synthesize it:

[Rudolf Real]

A small reminder: in the subclass use the ivar (_myVariable) instead of the accessor (self.myVariable).