inherit an immutable copy property as mutable retain - objective-c

Question

I am having the following code:
#interface Room : NSObject
{
#protected
NSMutableDictionary* mCustomProperties;
}
#property (readonly, copy) NSDictionary* CustomProperties;
#end
#interface MutableRoom : Room
{
}
#property (readwrite, retain) NSMutableDictionary* CustomProperties;
#end
These properties are then implemented later by accessing mCustomProperties.
I know, that it is not allowed to redeclare the memory management attribute or the data type of a property in a subclass to differ from the ones in the base class.
However, I want to achieve the following:
- Give only readonly access to the dictionary in the immutable base class. As the member is a NSMutableDictionary, just returning it as a retained instance of NSDictionary would mean, that it simple cast would make it mutable again, even unintended, when the user of the property stores the access variable somewhere as an NSObject instance and then recovers the original type of it back later. Therefor I want to return the mutable dictionary by immutable copy in the readonly property.
- Give full readwrite access to the dictionary in the mutable sub class. As you should not only be able to store another dictionary there, but also to just change the content of the existing one, I would like the property in the subclass not only to be readwrite, but also to be retained, instead of copied, so that the original values will be accessed, and to access it as an NSMutableDictionary, not as a NSDictionary, so that one can add, remove or change entries of the dictionary through the property.
Is there any more elegant way than suppressing the warning or using two differently named properties?

Answer 1

when declaring a mutable/immutable pair of a class cluster, i often find it simplest to not inherit from another, but to use an inner class for the implementation, like so:
// Room.h
#interface Room : NSObject
// ...
#end
#interface MutableRoom : Room
// ...
#end
// MONRoom.h
#class RoomImp;
#interface MONRoom : Room
{
#private
RoomImp * imp;
}
#property (readonly, copy) NSDictionary* CustomProperties;
#end
#interface MONMutableRoom : MutableRoom
{
#private
RoomImp * imp;
}
#property (readwrite, retain) NSMutableDictionary* CustomProperties;
#end
// RoomImp.h
#interface RoomImp : NSObject
{
#private
NSMutableDictionary* mCustomProperties;
}
// ...
#end