regarding the "Class" class in obj-c?

Question

Please observe this method declaration:

NSString *NSStringFromClass(Class aClass);

I want to understand the Class class, but I can't find docs about it because I don't know how to create a good enough regular expression and google with it. The fact that the class name is Class and not NSClass makes me think Class is not a real class (like java.lang.Class). But, I don't know, and so I want to research this. But, I can't find docs.

If I need to ask a specific question: is "Class" in obj-c a real class? what are its methods and properties?

It's one of those things that is well-hidden in the (rather poorly structured) Apple documentation, but it is pretty much analogous to Java's `Class` (to the extent that anything is analogous between Objective-C and Java). — Hot Licks, Sep 07 '13 at 19:38

score 5 · Accepted Answer · edited Jun 20 '20 at 09:12

Quoting from Cocoa with Love

Every object has a class. This is a fundamental object-oriented concept but in Objective-C, it is also a fundamental part of the data. Any data structure which has a pointer to a class in the right location can be treated as an object. In Objective-C, an object's class is determined by its isa pointer. The isa pointer points to the object's Class.

And as a proof of it, here's the declaration of id as a pointer to the objc_object struct.

typedef struct objc_object {
    Class isa;
} *id;

So here we get to the point. What is a Class?

Let's look at the definition

Class is defined as follows (it may actually vary depending on the runtime, but let's keep it simple)

struct objc_class {
    Class isa;
}
typedef struct objc_class *Class;

As you can see a Class has a isa pointer as well. It looks suspiciously like the objc_object definition and the reason is simple: Class is in fact an object.

But what is the class of a Class? It's - by definition - a meta-class.

According to the same source (in bold the part that tackles your question directly),

The meta-class, like the Class before it, is also an object. This means that you can invoke methods on it too. Naturally, this means that it must also have a class.

All meta-classes use the base class' meta-class (the meta-class of the top Class in their inheritance hierarchy) as their class. This means that for all classes that descend from NSObject (most classes), the meta-class has the NSObject meta-class as its class.

Following the rule that all meta-classes use the base class' meta-class as their class, any base meta-classes will be its own class (their isa pointer points to themselves). This means that the isa pointer on the NSObject meta-class points to itself (it is an instance of itself).

For further reading on the subject, here's another great explanation by Greg Parker.

Thank you. Using these docs, I was able to find this thread which was extremely useful as well: http://stackoverflow.com/questions/11265375/in-cocoa-how-is-the-class-type-defined — davewp, Sep 08 '13 at 14:20

regarding the "Class" class in obj-c?

1 Answers1