What is a meta-class in Objective-C?

http://www.cocoawithlove.com/2010/01/what-is-meta-class-in-objective-c.html

In this post, I look at one of the stranger concepts in Objective-C — the meta-class. Every class in Objective-C has its own associated meta-class but since you rarely ever use a meta-class directly, they can remain enigmatic. I'll start by looking at how to create a class at runtime. By examining the "class pair" that this creates, I'll explain what the meta-class is and also cover the more general topic of what it means for data to be an object or a class in Objective-C.

Creating a class at runtime

The following code creates a new subclass of NSError at runtime and adds one method to it:

Class newClass =
    objc_allocateClassPair([NSError class], "RuntimeErrorSubclass", 0);
class_addMethod(newClass, @selector(report), (IMP)ReportFunction, "v@:");
objc_registerClassPair(newClass);

The method added uses the function named ReportFunction as its implementation, which is defined as follows:

void ReportFunction(id self, SEL _cmd)
{
    NSLog(@"This object is %p.", self);
    NSLog(@"Class is %@, and super is %@.", [self class], [self superclass]);

    Class currentClass = [self class];
    for (int i = 1; i < 5; i++)
    {
        NSLog(@"Following the isa pointer %d times gives %p", i, currentClass);
        currentClass = object_getClass(currentClass);
    }

    NSLog(@"NSObject's class is %p", [NSObject class]);
    NSLog(@"NSObject's meta class is %p", object_getClass([NSObject class]));
}

On the surface, this is all pretty simple. Creating a class at runtime is just three easy steps:

1. Allocate storage for the "class pair" (using objc_allocateClassPair).
2. Add methods and ivars to the class as needed (I've added one method using class_addMethod).
3. Register the class so that it can be used (using objc_registerClassPair).

However, the immediate question is: what is a "class pair"? The function objc_allocateClassPair only returns one value: the class. Where is the other half of the pair?

I'm sure you've guessed that the other half of the pair is the meta-class (it's the title of this post) but to explain what that is and why you need it, I'm going to give some background on objects and classes in Objective-C.

What is needed for a data structure to be an object?

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.

In fact, the basic definition of an object in Objective-C looks like this:

typedef struct objc_object {
    Class isa;
} *id;

What this says is: any structure which starts with a pointer to a Class structure can be treated as an objc_object.

The most important feature of objects in Objective-C is that you can send messages to them:

[@"stringValue"
    writeToFile:@"/file.txt" atomically:YES encoding:NSUTF8StringEncoding error:NULL];

This works because when you send a message to an Objective-C object (like the NSCFStringhere), the runtime follows object's isa pointer to get to the object's Class (the NSCFStringclass in this case). The Class then contains a list of the Methods which apply to all objects of that Class and a pointer to the superclass to look up inherited methods. The runtime looks through the list of Methods on the Class and superclasses to find one that matches the message selector (in the above case, writeToFile:atomically:encoding:error on NSString). The runtime then invokes the function (IMP) for that method.

The important point is that the Class defines the messages that you can send to an object.

What is a meta-class?

Now, as you probably already know, a Class in Objective-C is also an object. This means that you can send messages to a Class.

NSStringEncoding defaultStringEncoding = [NSString defaultStringEncoding];

In this case, defaultStringEncoding is sent to the NSString class.

This works because every Class in Objective-C is an object itself. This means that the Classstructure must start with an isa pointer so that it is binary compatible with the objc_objectstructure I showed above and the next field in the structure must be a pointer to the superclass (or nil for base classes).

As I showed last week, there are a couple different ways that a Class can be defined, depending on the version of the runtime you are running, but yes, they all start with an isafield followed by a superclass field.

typedef struct objc_class *Class;
struct objc_class {
    Class isa;
    Class super_class;
    /* followed by runtime specific details... */
};

However, in order to let us invoke a method on a Class, the isa pointer of the Class must itself point to a Class structure and that Class structure must contain the list of Methods that we can invoke on the Class.

This leads to the definition of a meta-class: the meta-class is the class for a Class object.

Simply put:

• When you send a message to an object, that message is looked up in the method list on the object's class.
• When you send a message to a class, that message is looked up in the method list on the class' meta-class.

The meta-class is essential because it stores the class methods for a Class. There must be a unique meta-class for every Class because every Class has a potentially unique list of class methods.

What is the class of the meta-class?

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).

Inheritance for classes and meta-classes

In the same way that the Class points to the superclass with its super_class pointer, the meta-class points to the meta-class of the Class' super_class using its own super_classpointer.

As a further quirk, the base class' meta-class sets its super_class to the base class itself.

The result of this inheritance hierarchy is that all instances, classes and meta-classes in the hierarchy inherit from the hierarchy's base class.

For all instances, classes and meta-classes in the NSObject hierarchy, this means that all NSObject instance methods are valid. For the classes and meta-classes, all NSObject class methods are also valid.

All this is pretty confusing in text. Greg Parker has put together an excellent diagram of instances, classes, meta-classes and their super classes and how they all fit together.

Experimental confirmation of this

To confirm all of this, let's look at the output of the ReportFunction I gave at the start of this post. The purpose of this function is to follow the isa pointers and log what it finds.

To run the ReportFunction, we need to create an instance of the dynamically created class and invoke the report method on it.

id instanceOfNewClass =
    [[newClass alloc] initWithDomain:@"someDomain" code:0 userInfo:nil];
[instanceOfNewClass performSelector:@selector(report)];
[instanceOfNewClass release];

Since there is no declaration of the report method, I invoke it using performSelector: so the compiler doesn't give a warning.

The ReportFunction will now traverse through the isa pointers and tell us what objects are used as the class, meta-class and class of the meta-class.

Getting the class of an object: the ReportFunction uses object_getClass to follow the isa pointers because the isa pointer is a protected member of the class (you can't directly access other object's isa pointers). The ReportFunction does not use the class method to do this because invoking the class method on a Class object does not return the meta-class, it instead returns the Class again (so [NSString class] will return the NSString class instead of the NSString meta-class).

This is the output (minus NSLog prefixes) when the program runs:

This object is 0x10010c810.
Class is RuntimeErrorSubclass, and super is NSError.
Following the isa pointer 1 times gives 0x10010c600
Following the isa pointer 2 times gives 0x10010c630
Following the isa pointer 3 times gives 0x7fff71038480
Following the isa pointer 4 times gives 0x7fff71038480
NSObject's class is 0x7fff710384a8
NSObject's meta class is 0x7fff71038480

Looking at the addresses reached by following the isa value repeatedly:

• the object is address 0x10010c810.
• the class is address 0x10010c600.
• the meta-class is address 0x10010c630.
• the meta-class's class (i.e. the NSObject meta-class) is address 0x7fff71038480.
• the NSObject meta-class' class is itself.

The value of the addresses is not really important except that it shows the progress from class to meta-class to NSObject meta-class as discussed.

Conclusion

The meta-class is the class for a Class object. Every Class has its own unique meta-class (since every Class can have its own unique list of methods). This means that all Class objects are not themselves all of the same class.

The meta-class will always ensure that the Class object has all the instance and class methods of the base class in the hierarchy, plus all of the class methods in-between. For classes descended from NSObject, this means that all the NSObject instance and protocol methods are defined for all Class (and meta-class) objects.

All meta-classes themselves use the base class' meta-class (NSObject meta-class for NSObjecthierarchy classes) as their class, including the base level meta-class which is the only self-defining class in the runtime.