Objective-C basics: subclassing and member variable accessability

Question

Hi,

Code below is pseudo code. Imagine a class "Fruit" which has a factory method to create a fruit.

interface Fruit
{
}
+(Fruit*) createFruit:
{
  return [[Fruit alloc] init autorelease];
}

Now I want to subclass the Fruit to get an Apple:

interface Apple : Fruit
{
int iSeeds;
}
+(Apple*) createAppleWithColor: (int) iSeeds
{
Apple* oApple = [Apple createFruit:];
oApple.iSeeds = iSeeds;
return oApple;
}

Questions:

• How can I make "iSeeds" private so it cannot be changed from outside? If I add a "private" keyword it does not build anymore.
• Still I want to set iSeeds from inside my Apple's factory method.
• I want users allow to READ the content of iSeeds. So I suppose I should have a getter but I can't get it to work. I always get some error about "LValue assignment".
• The Fruit's createFruit is making use of autorelease. Does the Apple have to reatin/release anything?

René

Answer 1

A couple things.

1. Your createFruit method is wrong. It should be:

   + (Fruit *) createFruit {
     //autorelease, because the Create Rule only applies to CoreFoundation functions
     return [[[Fruit alloc] init] autorelease];
   }

2. Instance variables are @protected by default. That means they can be directly accessed by the class and any subclasses. You can make it @protected, @private, @public, and @package. You do so like this:

   @interface Apple : Fruit {
     @private
     int iSeed
   }
   ...
   @end

3. If you want an instance variable to be readonly, then don't declare it as @public, but make a getter for it. For example:

   - (int) iSeeds {
     return iSeeds;
   }

4. However, since the instance variable is readonly, you can't set it externally. The way around this is to give the Apple a iSeed value during initialization:

   - (id) initWithSeeds:(int)aniSeedValue {
     if (self = [super init]) {
       iSeed = aniSeedValue;
     }
     return self;
   }

   Then make your createAppleWithColor:(int)iSeeds method like:

   + (Fruit *) createAppleWithColor:(int)iSeeds {
     return [[[Apple alloc] initWithSeeds:iSeeds] autorelease];
   }

5. Finally, you should read the Naming Conventions guide and the Memory Management guide.

Answer 2

One more thing. If you have a factory method which applies to a class and its subclasses, you can (probably should) do this:

+(Fruit*) fruit
{
    [[[self alloc] init] autorelease];
}

This means that if you invoke the method with a subclass, you'll get an object of the right type e.g.

Fruit* apple = [Apple fruit];

Of course, you'll need to provide an init method for Apple that provides a suitable default value for iSeeds and then invokes -initWithSeeds: