Class methods which create new instances

4 Solutions Collect From Internet About “Class methods which create new instances”

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.

The objects returned from factory methods should be autoreleased, meaning they’ll be cleaned up when the associated autorelease pool is drained. This means that you don’t own the returned objects unless you copy or retain them. Following is an example of a factory method:

+ (id)myClassWithString:(NSString *)string {
    return [[[MyClass alloc] initWithString:string] autorelease];
}

These methods are simply returning an autoreleased version of the object.

+(MyClass*)class
{
  MyClass* object = [[MyClass alloc] init];
  return [object autorelease];
}

The modern way to do this with ARC and the latest complier is:

+ (instancetype) myClassWithString:(NSString *)string {
     return [[MyClass alloc] initWithString:string];
}
  • No need to autorelease with ARC.
  • instancetype provides better compile time checks whilst making subclassing possible.