iPhone -- init method of an abstract class

Question

I want to create classes Car, Vehicle, and Airplane with the following properties:

• Car and Airplane are both subclasses of Vehicle.
• Car and Airplane both have an initWithString method.
• The acceptable input strings for Car's and Airplane's initWithString methods do not overlap.
• Vehicle is "almost abstract", in the sense that any initialized instance should be either a Car or an Airplane.
• It is possible to pass a string into Vehicle and get back an instance of Car, an instance of Airplane, or nil, depending on the input string.

Any particular design pattern I should prefer? In particular for Vehicle's initWithString and/or newVehicleWithString methods.

Answer 1

Referring to a subclass from a superclass is not a good idea.

If you really have to do this, you should at least go with a class method like vehicleWithString:.

In fact, I doubt that the other approach (using vehicle initWithString: to create instances of Car or Airplane) would work.

Answer 2

What you need is the "class cluster" pattern. Your Vehicle initWithString: method could look something like this:

- (id) initWithString:(NSString *)mode {
  // note that we don't call [super init] in a class cluster. 
  // instead, we have to release self because it's an unwanted Vehicle instance
  [self release];
  if([mode isEqualToString:@"wheels"]) {
    return [[Car alloc] initWithString:@"wheels"];
  }
  if([mode isEqualToString:@"wings"]) {
    return [[Airplane alloc] initWithString:@"wings"];
  }
  return nil;  //alternately, raise NSInvalidArgumentException
}