I'm new to Objective C and I haven't been able to find out if there is the equivalent of a static constructor in the language, that is a static method in a class that will automatically be called before the first instance of such class is instantiated. Or do I need to call the Initialization code myself?
Thanks
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:
aClass
itself)aClass
and children)Edit: Check out this post by @bbum that explains more about +initialize
: 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
There is the +initialize class method that will be called before a class is used.
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