super allocWithZone having some doubts in singleton class concept

Question

I am new in Objective-C and I am trying to create a singleton class based on Apple\'s documentation.

+ (MyGizmoClass*)sharedManager
{
    if (sharedGizmoManager          


        

Answer 1

(1.) What is super in a 'static function'? In Objective-C, +methods are properly called class methods, -methods are called instance methods. These +methods are not true static methods because Objective-C classes are themselves objects of an opaque type called Class. So both super and self are defined in +methods. super sends messages to the superclass of MyGizmoClass, but when called from a +method super looks for an equivalent +method, and when called from an -method super looks for a corresponding -method.
self in +methods of MyGizmoClass returns MyGizmoClass which is a Class, whereas in -methods self is a pointer to a MyGizmoClass instance.

(2.) The method +class returns self.isa. Yes [super class] invokes the superclass's:+class method, however, when self is passed up to +methods neither its value nor its type are modified (whereas self's type is cast to the superclass when it is passed up through -methods). So when an implementation of the +class method up the chain asks for self.isa, it gets the same value, MyGizmoClass.
To be certain, you can verify that super does call +class in superclasses by deriving MyGizmoClass from a MyGizmoSuperClass where you can place an override:

    @interface MyGizmoSuperClass : NSObject
    @end
    @implementation MyGizmoSuperClass
    +(Class) class {
        NSLog(@"yes it calls MyGizmoSuperClass:class");
        return [super class];
    }
    @end
    @interface MyGizmoClass : MyGizmoSuperClass 
    +(Class) classviasuper;
    @end
    @implementation MyGizmoClass
    +(Class) classviasuper {
        return [super class]; //which version of class will super call?
    }
    @end
    int main(int argc, char *argv[])
    {
       NSLog(@"returned %@",[MyGizmoClass classviasuper]); 
    }

prints

yes it calls MyGizmoSuperClass:class
returned MyGizmoClass

(3.) Again super calls the superclass version of allocWithZone but the self value passed to the method still points to a MyGizmoClass, and since allocWithZone returns an object of the receiver's class, you get a MyGizmoClass back.

(4.) You can easily verify super is different to self. If you implement [self allocWithZone:NULL] your code will call MyGizmoClass's implementation of allocWithZone and loop indefinitely. With [super allocWithZone:NULL], the superclass's version gets called.