Prevent class from being allocated in iOS - ios

Question

I'm wondering how it is possible to create a class in Objective-C that cannot be allocated (or init)?
I'm trying to replicate the SKDownload Apple class (StoreKit framework) and I noticed that the documentation doesn't mention any alloc or init methods.
This may be missing in the documentation but present in the code ? (some .h declaration missing to prevent us from using this method?). I've tried to +alloc-init this class and either alloc and init return null.
What I'm trying to achieve is a class B that possesses only getters, that can only be created by a class A (just like factory methods would do - only A can create and return B instances, but the user cannot create B directly).
Example:
#interface A : NSObject
// Only A "+createB" class method can create B instances
+ (Second *)createBWithValue:(int)value;
#end
#interface B : NSObject
- (id)init; // return nil
+ (id)alloc; // return nil
- (int)value; // this returns the value passed from A
#end
Question 1: How would you proceed to create such classes? Is is possible ?
Question 2: How can - (int)value on B return the value passed in the class method in A? (knowing that +alloc, -init and/or other memory allocation methods may be nullified since users cannot create B classes directly - also custom initWithValue methods in B may be unavailable too)
I'm a little confused on how the Apple engineers do that..are there hidden methods that Apple doesn't want me to use?
Thanks you.

Answer 1

I noticed that the documentation doesn't mention any alloc or init methods
From the documentation:
Your app never directly creates a SKDownload object. Instead, after a payment is processed, your app reads the transaction object’s
downloads property to retrieve an array of SKDownload objects
associated with the transaction.
are there hidden methods that Apple doesn't want me to use?
Yes. There are a great many hidden methods that Apple doesn't want you to use.
So, the idea is that you're not supposed to create instances of SKDownload yourself, and for that reason the initialization method isn't part of the public API. That doesn't mean that there isn't a way to create SKDownload instances, just that there's not a public way for you to create them.
It's pretty common for classes to limit allocation or initialization. Singleton classes, for example, will often override +allocWithZone: such that it only allocates a new object if one hasn't already been created; if one exists, it returns that object instead (and the initializer basically does nothing).
Hide the initializer.
If you want to create a class like that yourself, override the inherited initialization method(s) (probably just -init) so that they return nil or maybe even throw an exception. Implement your own initializer, but keep it out of the public interface for the class:
// B.h
#interface B : NSObject
#property(readonly) int value;
#end;
// B.m
#interface B ()
#property(readwrite) int value;
-(id)secretInitializerWithValue:(int)v;
#end
#implementation B
-(id)secretInitializerWithValue:(int)v
{
if ((self == [super init])) { // not really necessary since NSObject's -init doesn't do anything
_value = v;
}
}
-(id)init
{
#throw [NSException exceptionWithName:#"Don't do that!"
reason:#"You shouldn't instantiate B yourself."
userInfo:nil];
}
#end
Private mutable subclass.
Another option is to create an immutable class with the read-only accessors you want, and a private mutable subclass that has read-write accessors. You can implement -copyWithZone: in the mutable subclass so that you get an immutable object when you make a copy. This all amounts to about the same thing I described above -- either way, you're hiding the creation mechanism from the client.