Using (id) in Objective-C

Question

I have a function that I want to operate on two different custom objects. My first thought was to accept the argument as an (id) and operate on the id object. I can't quite seem to figure out how to do that, however.

Both classes (say apples and oranges) have interface variables:

NSDecimalNumber *count;

I want to do something similar to this:

-(NSDecimalNumber*)addCount:(id)addObject{

    return [count decimalNumberByAdding:addObject.count];
}

I can't seem to figure out the syntax to make that happen. Is this the proper approach, or would it be better to subclass (from say a fruit class) and operate on the parent class?

-(NSDecimalNumber*)addCount:(Fruit*)addFruit{

    return [count decimalNumberByAdding:addFruit.count];
}

Answer 1

While you can send a message to any object (id) - property accessors require that the compiler be aware of the type you are dealing with - this is because property accessors are syntactic sugar around calling specific getter and setter methods.

You have a few of ways of working around this:

1. Instead of accessing the count property, call the corresponding [getCount] methods.

2. If the different classes have different versions of this method, you can use a runtime type check:

3. Provide a base class for both types so that you can pass in something more specific than (id).

4. Define and implement a Protocol that both objects implement that defines a count property (or method).

Example of a dynamic type check:

if( [object isKindOfClass:[Apple Class] )
   // call one overload of getCount
else if( [object isKindOfClass:[Orange Class] )
   // call another overload of getCount

Personally, I favor strong typing in my code because it makes it easier to understand the intent. It also allows the IDE to support your coding effort with intellisense, static analysis, and refactoring features. So, in your case, I would use either #3 or #4 as an approach - depending on whether inheritance is really appropriate for the problem.

Answer 2

You should try not to access instance variables from another class.

In Objective-C it's enough that the two objects respond to the same selector (say count), however that would give you a compiler warning.

There are two ways you can get rid of this warning: either by subclassing from a common Fruit class or by having your two classes conform to a protocol. I'd go with the protocol:

@protocol FruitProtocol

- (NSDecimalNumber *)count;

@end

@interface Orange : NSObject<FruitProtocol>
@end

@interface Apple : NSObject<FruitProtocol>
@end

Then your method can look like this:

-(NSDecimalNumber*)addCount:(id<FruitProtocol>)addFruit {
    return [count decimalNumberByAdding:[addFruit count]];
}

Here you are saying that your addCount expects any object that conforms to the FruitProtocol protocol, and hence can respond to the count selector, so the compiler will accept it.

Answer 3

The fact that you are trying to access 'addFruit.count' is the problem. The dot syntax is only for properties declared with @property (or for structs). If you change it to

[addFruit count]

and add

-(NSDecimalNumber*)count
{
     return [[count retain] autorelease];
}

to each class, then it would work. However, you will notice you'll get a warning saying 'id' may not respond to the 'count' message, and unless you can be absolutely sure the items sent to this method implement a 'count' method, this is a problematic approach.

I agree with pgb's approach. You should define a protocol, and declare both classes to implement that protocol. This eliminates the problem of not knowing whether the object will respond to 'count' or not, as you now have a 'contract' of sorts.

If you want to keep the dot syntax with a property, you can declare it in the protocol:

@protocol FruitProtocol

@property(readonly) NSDecimalNumber * count;

- (NSDecimalNumber *)count

@end

and then, your function would be:

-(NSDecimalNumber*)addCount:(id<FruitProtocol>)addObject{

    return [count decimalNumberByAdding:addObject.count];

}