Hide the default Objective-C initializer

Sep 4, 2013 · Follow on Twitter and Mastodon ios

Languages like Java and C# let you override and hide the default constructors of a class, to ensure that developers can only create valid instances of it. Let’s take a look at how to do the same in Objective-C.

Init where are you

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 a set of required components. 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 force initWithMoreStuff to be used, is to throw an exception whenever an instance is created with the default initializer. However, this is not good, since developers can still use the default initializer, with the incorrect usage not being detected until the app crashes at runtime.

What we want is to hide the default initializer and make initWithMoreStuff the only available one.

Achieving 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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