Question

I’m reading an Objective-C book and I have a question that the book doesn’t seem to really answer.

Let’s say I have two custom-made classes.

The first class is called ClassA. It has both the .h and .m files of course. The second class is called ClassB. It also has both .h and .m files.

Somewhere in the code, ‘ClassA’ has this method:

-(IBAction)displaySomeText:(id)sender {
    ClassB *myNumber = [[ClassB  alloc]init];

    NSString *numberString = [myNumber storedNumberAsString];
    // storedNumberAsString is just a method that returns a string object that holds
    // myVariable.

    [textView insertText:numberString];
    //textView is a object  I created that just displays some text on screen.

    [myNumber release];
}

The book tells me that ClassB should have a method:

-(id)init {
    [super init]; //I know why this is done, the book explains it well.
    myVariable = 42; // I created this variable already in the ClassB .h file 
    return self;
}

Now, when in the Interface Builder I click the buttons I connected, etc. It works, the number displayed is 42.

My question is, why do I have to create an -(id)init method for ClassB, if I can do the following in ClassA‘s method:

-(IBAction)displaySomeText:(id)sender {
    ClassB *myNumber = [[ClassB alloc]init];
    myNumber.myVariable = 42; //I just do this to skip the -(id)init method.

    NSString *numberString = [myNumber storedNumberAsString];
    [textView insertText:numberString];
    [myNumber release]; 
}

Doing this, it still displays the same value: 42. I can change it to whatever I like. So why not just use the init inherited from NSObject and just do the simple way myNumber.myVariable = 42?

Answer 1

Suppose that the value of the instance variable were something more complicated than an integer. Suppose it involved reading a string from a file, or getting some information over the network, or just doing some arithmetic. In that case, it wouldn’t make sense to have ClassA be responsible for setting that value correctly. That would break the encapsulation that makes it useful to have separate classes in the first place.

In this extremely simple case, you’re quite right, there may be no reason to have a custom initializer for ClassB, but in general, a class should itself be responsible for its state being set up correctly. Foisting that responsibility off on other classes means that those others need to know about the internals of the first, meaning the two may be too tightly coupled.

In some cases, the value of the ivar might be a piece of information that is known only to ClassA, or needs to be calculated based on such a piece of information. Then you should create a custom initializer for ClassB which receives that value, e.g., - (id) initWithInteger: This would become the “designated initializer”, and you would then override -[ClassB init] to call it with some reasonable default value.