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Hide the default Objective-C initializer

Sep 4, 2013  ·  ios

Languages like Java and C# let you override and hide the default constructors of a class. Let’s take a look at how to do the same in Objective-C.

Hiding the default constructors of a class is particularly useful for constructor injection, to ensure that a class can not be created without providing it with required values.

Objective C, however, uses the init pattern when initializing class instances. It’s all good, but have some implications.

Consider a class that can be initialized with a required component, as such:

- (id)initWithMoreStuff:(id)moreStuff {
   self = [self init];
   if (self) {
      self.moreStuff = moreStuff;
   }
   return self;
}

The big problem with this, is that if the class requires moreStuff, a developer can still call the default initializer to create an instance, where moreStuff is nil.

One way to enforce initWithMoreStuff is to throw exceptions if another initializer is used. However, this is not good, since incorrect usages are not detected until the app crashes at runtime. We instead want make initWithMoreStuff the only available initializer.

This is easy. In the .h file, just annotate the default initializer with unavailable, like this:

- (id)init __attribute__((unavailable("A descriptive reason")));

Now, you will get a compilation error if you try to use the default initializer.

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