Class methods which create new instances

by

They are class methods, not static methods1. This specific type, creating autoreleased objects, can be referred to as “factory methods” (formerly also “convenience constructors”), and they are discussed in the Concepts in ObjC Guide. They go something like this:

+ (instancetype)whatsisWithThingummy: (Thingummy *)theThingummy {
    return [[self alloc] initWithThingummy:theThingummy];
}

Where Whatsis is your class, and Thingummy is another class which your class uses.

If you’re not compiling with ARC, the convention is to autorelease the instance before returning it.

The instancetype keyword was introduced by Clang for these kinds of methods; combined with self (which is the class object itself2 in a class method) it allows correct subclass behavior: the method produces an instance of the class which received the message.3 instancetype allows the compiler to do more strict typechecking than id.

An illustration of this usage in subclasses from the framework: +[NSString stringWithFormat:] returns an NSString instance, whereas +[NSMutableString stringWithFormat:], returns an instance of the subclass NSMutableString, without NSMutableString being required to explicitly override the method.

As discussed by the [Fundamentals][1] doc, there are other uses for these factory methods, such as accessing a singleton, or appraisal of the necessary memory allocation before it’s performed (possible, but less convenient, with a standard alloc/init pair).

1“Static methods” in Java or C++, “class methods” in Objective-C. There’s no such thing as static methods in ObjC

2Whereas in an instance method self is, sensibly, a reference to the instance.

3Previously, like the usual initialization methods (initWith...), you would have used id as the return type. Using a specific class name unnecessarily forces subclasses to override the method.

More Related Contents:

  1. When to use -retainCount?
  2. What is the difference between class and instance methods?
  3. Why is @autoreleasepool still needed with ARC?
  4. Retain cycle on `self` with blocks
  5. Why would you use an ivar?
  6. What’s the difference between “class method” and “static method”?
  7. How does the NSAutoreleasePool autorelease pool work?
  8. Weak NSString variable is not nil after setting the only strong reference to nil
  9. In which situations do we need to write the __autoreleasing ownership qualifier under ARC?
  10. Why has NSNumber such strange retainCounts?
  11. How is release handled for @synthesized retain properties?
  12. With ARC, what’s better: alloc or autorelease initializers?
  13. Tagged pointers in Objective-C
  14. What is the cost of using autorelease in Cocoa?
  15. CoreData: error: Failed to call designated initializer on NSManagedObject class
  16. Does @”some text” give an autoreleased or retain 1 object back?
  17. What increases an object’s retain count?
  18. What is the difference between a __weak and a __block reference?
  19. Objective-C: Property / instance variable in category
  20. Check that a input to UITextField is numeric only
  21. Using NSPredicate to filter an NSArray based on NSDictionary keys
  22. Changing a managed object property doesn’t trigger NSFetchedResultsController to update the table view
  23. Cocoa Core Data efficient way to count entities
  24. Implementing -hash / -isEqual: / -isEqualTo…: for Objective-C collections
  25. How do I download and save a file locally on iOS using objective C? [duplicate]
  26. How to make subscripts and superscripts using NSAttributedString?
  27. How does a Block capture the variables outside of its enclosing scope?
  28. Add/Delete UITableViewCell with animation?
  29. Using @[array, of, items] vs [NSArray arrayWithObjects:]
  30. Listen to a value change of my text field

Leave a Comment