Static constructor equivalent in Objective-C?

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The +initialize method is called automatically the first time a class is used, before any class methods are used or instances are created. You should never call +initialize yourself.

I also wanted to pass along a tidbit I learned that can bite you down the road: +initialize is inherited by subclasses, and is also called for each subclasses that doesn’t implement an +initialize of their own. This can be especially problematic if you naively implement singleton initialization in +initialize. The solution is to check the type of the class variable like so:

+ (void) initialize {
  if (self == [MyParentClass class]) {
    // Once-only initializion
  }
  // Initialization for this class and any subclasses
}

All classes that descend from NSObject have both +class and -class methods that return the Class object. Since there is only one Class object for each class, we do want to test equality with the == operator. You can use this to filter what should happen only once ever, versus once for each distinct class in a hierarchy (which may not yet exist) below a given class.

On a tangential topic, it’s worth learning about the following related methods, if you haven’t already:

  • – isMemberOfClass:(Class)aClass (true only for aClass itself)
  • – isKindOfClass:(Class)aClass (true for aClass and children)
  • + isSubclassOfClass:(Class)aClass (same as above, but a class method)

Edit: Check out this post by @bbum that explains more about +initialize: https://web.archive.org/web/20201108095221/http://www.friday.com/bbum/2009/09/06/iniailize-can-be-executed-multiple-times-load-not-so-much/

Also, Mike Ash wrote a nice detailed Friday Q&A about the +initialize and +load methods:
https://www.mikeash.com/pyblog/friday-qa-2009-05-22-objective-c-class-loading-and-initialization.html

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