Question

After trying to access ivars using KVC, I have noticed that there was no protection on private and protected ivars. It doesn’t matter what I put a in front of the ivar (private or protected keyword) – an ivar is always a public ivar when using KVC method “setValue”. Here is my code where all of the seven ivars and properties are changeble outside the class instance:

//************ interface file ***************//
@interface MyClass : NSObject {
@public    
  NSNumber *public_num;
@protected 
  NSNumber *protected_num;
@private 
  NSNumber *private_num;
  NSNumber *private_property;
}
@property (retain) NSNumber *public_property;
@property (retain) NSNumber *private_property;
@end

//********* implementation file *********//
@interface MyClass(){
@private
  NSNumber *very_private_num;
}
@property (retain) NSNumber *very_private_property;
@end

@implementation MyClass
@synthesize public_property, private_property, very_private_property;
@end

//****** main **********//
MyClass *myClass = [[MyClass alloc] init];

[myClass setValue:[NSNumber numberWithInt:1] forKey:@"public_num"];
[myClass setValue:[NSNumber numberWithInt:2] forKey:@"protected_num"];
[myClass setValue:[NSNumber numberWithInt:3] forKey:@"private_num"];
[myClass setValue:[NSNumber numberWithInt:4] forKey:@"public_property"];
[myClass setValue:[NSNumber numberWithInt:5] forKey:@"private_property"];
[myClass setValue:[NSNumber numberWithInt:6] forKey:@"very_private_num"];
[myClass setValue:[NSNumber numberWithInt:7] forKey:@"very_private_property"];

NSNumber *l_public_num = [myClass valueForKey:@"public_num"];
NSNumber *l_protected_num = [myClass valueForKey:@"protected_num"];
NSNumber *l_private_num = [myClass valueForKey:@"private_num"];
NSNumber *l_public_property = [myClass valueForKey:@"public_property"];
NSNumber *l_private_property = [myClass valueForKey:@"private_property"];
NSNumber *l_very_private_num = [myClass valueForKey:@"very_private_num"];
NSNumber *l_very_private_property = [myClass valueForKey:@"very_private_property"];

NSLog(@"public_num = %@, protected_num = %@, private_num = %@, public_property = %@, private_property = %@, very_private_num = %@, very_private_property = %@", l_public_num, l_protected_num, l_private_num, l_public_property, l_private_property, l_very_private_num, l_very_private_property);

The result of the output> public_num = 1, protected_num = 2, private_num = 3, public_property = 4, private_property = 5, very_private_num = 6, very_private_property = 7.

Even if the ivar declared at private interface, it is still changeable outside the class. So how do I have to enforce encapsulation and “to protect my ivars from evil other programmers” 🙂

Answer 1

NSObject conforms to the NSKeyValueCoding informal protocol. This defines setValue:forKey: and valueForKey:. setValue:forKey: and valueForKey: search for a way to access the value of the key according to specific search rules which includes directly accessing the instance variable. This direct accessing is controlled by accessInstanceVariablesDirectly method which is a part of the NSKeyValueCoding informal protocol, which by default returns YES, allowing those methods to directly access the instance variables and as a result not really making them private as such. They are still private from direct access.

To resolve this, you will have have to override the methods mentioned above and defined in the NSKeyValueCoding informal protocol to prevent their access.

As mentioned by Abizern, properties for private variables are still accessible since Objective-C has no concept of private methods.