An example of an immutable property with mutable storage.

PrivateMutableProperty.h

@interface MYClass : NSObject

@property (nonatomic, readonly) NSArray *apples;
@property (nonatomic, readonly) NSArray *oranges;

@end

PrivateMutableProperty.m

@interface MYClass () {
    // Option 1: private ivar
    NSMutableArray *_apples;
}

// Option 2: private property
@property (nonatomic) NSMutableArray *mutableOranges;
@end

@implementation MYClass

- (NSArray *)apples {
    return _apples.copy;
}

- (NSArray *)oranges {
    return self.mutableOranges.copy;
    // or, return _mutableOranges.copy
}

@end