The use of [[self alloc] init] when writing factory methods

Question

I'm having trouble understanding the use of [[self alloc] init] when writing factory methods. I understand that factory methods are convenience methods to create instances of a class, and that they do the alloc, init, and autorelease for you. I can see how this is formed, for example in the declaration of an NSArray property with the factory method arrayWithArray:, or array, etc. called on it to set it up. I can obviously see how this is different to an outright (explicit) call to alloc and init.

My issue with this is that I don't understand factory methods at a deeper level. I came across an explanation online that said instead of calling alloc and init explicitly, a class factory method could be used to basically encapsulate something like this:

+(instancetype)createWithString:(NSString *)string
{
    return [[self alloc] initWithString:string];
}

But how do instancetype and [self alloc] effectively allow for subclasses to make use of the class factory method?

I'm not sure exactly what you're asking. Does [Class methods which create new instances](http://stackoverflow.com/q/5987969) explain anything for you? — jscs, Feb 24 '15 at 19:37
this approach has no issues with subclasses because `self` if resolved dynamically at runtime and will "point" to the "active class". `NSArray`, `NSNumber`, `NSString` are special cases called class clusters ([official doc](https://developer.apple.com/library/ios/documentation/General/Conceptual/CocoaEncyclopedia/ClassClusters/ClassClusters.html)) — tkanzakic, Feb 24 '15 at 19:45
@Josh: Yep, that is really helpful. I'm just done reading this page and found it very useful: http://qualitycoding.org/factory-method/ — cheznead, Feb 24 '15 at 20:39
Just one question. Say you do an explicit alloc - when using ARC, do you need to worry about autorelease anymore? — cheznead, Feb 24 '15 at 20:40
@shinnyWhack: Nope, not necessary to call `autorelease` when using ARC. (ARC will insert that call for you.) — mipadi, Feb 24 '15 at 21:40

score 5 · Accepted Answer · answered Feb 24 '15 at 20:06

instancetype is a keyword that says "the return type of this method is the type of the class that this method was called on" (or a subclass). So, if you call [Baseclass createWithString:], the return type is Baseclass *. However, let's say you create a subclass that does not override this method. If you call [Subclass createWithString:], the return type is Subclass * (not Baseclass *).
When a class receives a message, self points to the Class object. So when calling [Baseclass createWithString:], self will point to the Baseclass object. However, when calling [Subclass createWithString:], self will point to Subclass instead, so if Subclass defines its own alloc or initWithString: methods (that is, it overrides them) its versions will be called instead.

The use of [[self alloc] init] when writing factory methods

1 Answers1