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 quit

Answer 1

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
@end

@interface Apple : NSObject
@end

Then your method can look like this:

-(NSDecimalNumber*)addCount:(id)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.