KVC valueForKey return a __NSCFConstantString instead a __NSCFString

Question

I use KVC in my projects.

And, in one of my classes, I wrote the property :

@property ( nonatomic, strong )             NSString            *notes;

I want to put a NSString object in that property :

And before setting the value, I want to test the class name of the destination.

a = [ newContainer valueForKey:@"notes"];
if( a != nil && ![ b isKindOfClass:[ a class ] ] )

// here b is the new NSString value

The result is that xcode indicates that the classes aren't the same !

(gdb) po [ b class ]
__NSCFString

(gdb) po [ a class ]
__NSCFConstantString

I read that is not very important and that __NSCFConstantString is a private subclass of NSString.

But, in my case, I need to check all properties of my object before updating it.

And I don't want to had in my code :

// OK, classes aren't the same ... 
// ---- BUT WE MUST test it again to know if a is a NSString and b a subclass of NSString or anything else ...

beurk !

Is anyone have the same problem ?

Thanks a lot for your help !

Answer 1

Three points:

1- You should check against [NSString class], the publicly exposed class of your property, not against the class of the current value of your property.

Imagine what happens when you check against the value class, instead of the property class: after setting your property to a NSMutableString, which is a perfect instance of NSString, you could not any longer set it to a regular NSString (since NSString is not a subclass of NSMutableString). Your current problem is a variant of the one described in this paragraph, which may be easier to understand.

2- The test should be done in the class that owns the property, not outside as you are doing now. Because only that class is entitled to know about the type of object it accepts.

3- So. Use the standard validateValue:forKey:error: method, which is your friend here. This method would be implemented by the class, and it would make sure notes is a NSString. Outside of the class, you would not check the type of the value directly, but ask the container class to validate it.

And voilà !