In object-oriented programming, a metaclass is a class whose instances are classes. Just as an ordinary class defines the behavior of certain objects, a metaclass defines the behavior of certain classes and their instances. Not all object-oriented programming languages support metaclasses. Among those that do, the extent to which metaclasses can override any given aspect of class behavior varies. Metaclasses can be implemented by having classes be first-class citizen, in which case a metaclass is simply an object that constructs classes. Each language has its own metaobject protocol, a set of rules that govern how objects, classes, and metaclasses interact.
class Car(object): __slots__ = ['make', 'model', 'year', 'color'] def __init__(self, make, model, year, color): self.make = make self.model = model self.year = year self.color = color @property def description(self): """ Return a description of this car. """ return "%s %s %s %s" % (self.color, self.year, self.make, self.model)
At run time,
Car itself is an instance of
type. The source code of the
Car class, shown above, does not include such details as the size in bytes of
Car objects, their binary layout in memory, how they are allocated, that the
__init__ method is automatically called each time a
Car is created, and so on. These details come into play not only when a new
Car object is created, but also each time any attribute of a
Car is accessed. In languages without metaclasses, these details are defined by the language specification and can't be overridden. In Python, the metaclass -
type- controls these details of
Car's behavior. They can be overridden by using a different metaclass instead of
The above example contains some redundant code to do with the four attributes
color. It is possible to eliminate some of this redundancy using a metaclass. In Python, a metaclass is most easily defined as a subclass of
class AttributeInitType(type): def __call__(self, *args, **kwargs): """ Create a new instance. """ # First, create the object in the normal default way. obj = type.__call__(self, *args) # Additionally, set attributes on the new object. for name, value in kwargs.items(): setattr(obj, name, value) # Return the new object. return obj
This metaclass only overrides object creation. All other aspects of class and object behavior are still handled by
Now the class
Car can be rewritten to use this metaclass. This is done in Python 2 by assigning to
__metaclass__ within the class definition:
class Car(object): __metaclass__ = AttributeInitType __slots__ = ['color', 'year', 'make', 'model'] @property def description(self): """ Return a description of this car. """ return " ".join(str(getattr(self, attr, "Unknown")) for attr in self.__slots__)
In Python 3 you provide a named argument, metaclass=M to the class definition instead:
class Car(object, metaclass=AttributeInitType): __slots__ = ['color', 'year', 'make', 'model'] @property def description(self): """ Return a description of this car. """ return " ".join(str(getattr(self, attr, "Unknown")) for attr in self.__slots__)
Car objects can then be instantiated like this:
new_car = Car(make='Toyota', model='Prius', year=2005, color='Green') old_car = Car(make='Ford', model='Prefect', year=1979)
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In Smalltalk, everything is an object. Additionally, Smalltalk is a class based system, which means that every object has a class that defines the structure of that object (i.e. the instance variables the object has) and the messages an object understands. Together this implies that a class in Smalltalk is an object and that therefore a class needs to be an instance of a class (called metaclass).
As an example, a car object
c is an instance of the class
Car. In turn, the class
Car is again an object and as such an instance of the metaclass of
Car class. Note the blank in the name of the metaclass. The name of the metaclass is the Smalltalk expression that, when evaluated, results in the metaclass object. Thus evaluating
Car class results in the metaclass object for
Car whose name is
Car class (one can confirm this by evaluating
Car class name which returns the name of the metaclass of
Class methods actually belong to the metaclass, just as instance methods actually belong to the class. When a message is sent to the object
2, the search for the method starts in
Integer. If it is not found it proceeds up the superclass chain, stopping at Object whether it is found or not.
When a message is sent to
Integer the search for the method starts in
Integer class and proceeds up the superclass chain to
Object class. Note that, so far, the metaclass inheritance chain exactly follows that of the class inheritance chain. But the metaclass chain extends further because
Object class is the subclass of
Class. All metaclasses are subclasses of Class.
In early Smalltalks, there was only one metaclass called
Class. This implied that the methods all classes have were the same, in particular the method to create new objects, i.e.,
new. To allow classes to have their own methods and their own instance variables (called class instance variables and should not be confused with class variables), Smalltalk-80 introduced for each class
C their own metaclass
C class. This means that each metaclass is effectively a singleton class.
Since there is no requirement that metaclasses behave differently from each other, all metaclasses are instances of only one class called
Metaclass. The metaclass of
Metaclass is called
Metaclass class which again is an instance of class
The superclass hierarchy for metaclasses parallels that for classes, except for class
Object. ALL metaclasses are subclasses of
Object class superclass == Class.
Like conjoined twins, classes and metaclasses are born together.
Metaclass has an instance variable
thisClass, which points to its conjoined class. Note that the usual Smalltalk class browser does not show metaclasses as separate classes. Instead the class browser allows to edit the class together with its metaclass at the same time.
The names of classes in the metaclass hierarchy are easily confused with the concepts of the same name. For instance:
Objectis the base class that provides common methods for all objects; "an object" is an integer, or a widget, or a
Classis the base of the metaclasses that provides common methods for all classes (though it is not a metaclass itself); "a class" is something like
Metaclassprovides common methods for all metaclasses.
Four classes provide the facilities to describe new classes. Their inheritance hierarchy (from Object), and the main facilities they provide are:
- Object - default behavior common to all objects, like class access
Ruby purifies the Smalltalk-80 concept of metaclasses by introducing eigenclasses, removing the
Metaclass class, and (un)redefining the class-of map. The change can be schematized as follows:
Note in particular the correspondence between Smalltalk's implicit metaclasses and Ruby's eigenclasses of classes. The Ruby eigenclass model makes the concept of implicit metaclasses fully uniform: every object x has its own meta-object, called the eigenclass of x, which is one meta-level higher than x. The "higher order" eigenclasses usually exist purely conceptually – they do not contain any methods or store any (other) data in most Ruby programs.
According to the Ruby's introspection method named
class, the class of every class (and of every eigenclass) is constantly the
Class class. As a consequence,
Class is the only class that has classes as instances, similarly to Java or Scala. (This also means that subclassing of
Class is disallowed.) Following the standard definition of metaclasses we can conclude that the
Class class is the only metaclass in Ruby. This seems to contradict the correspondence between Ruby and Smalltalk, since in Smalltalk-80, every class has its own metaclass. The discrepancy is based on the disagreement between the
class introspection method in Ruby and Smalltalk. While the map x ↦ x.
class coincides on terminal objects, it differs in the restriction to classes. As already mentioned above, for a class
x, the Ruby expression
x.class evaluates constantly to
Class. In Smalltalk-80, if
x is a class then the expression
x class corresponds to the Ruby's
x.singleton_class – which evaluates to the eigenclass of
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Metaclasses in Objective-C are almost the same as those in Smalltalk-80—not surprising since Objective-C borrows a lot from Smalltalk. Like Smalltalk, in Objective-C, the instance variables and methods are defined by an object's class. A class is an object, hence it is an instance of a metaclass.
Like Smalltalk, in Objective-C, class methods are simply methods called on the class object, hence a class's class methods must be defined as instance methods in its metaclass. Because different classes can have different sets of class methods, each class must have its own separate metaclass. Classes and metaclasses are always created as a pair: the runtime has functions
objc_registerClassPair() to create and register class-metaclass pairs, respectively.
There are no names for the metaclasses; however, a pointer to any class object can be referred to with the generic type
Class (similar to the type
id being used for a pointer to any object).
Because class methods are inherited through inheritance, like Smalltalk, metaclasses must follow an inheritance scheme paralleling that of classes (e.g. if class A's parent class is class B, then A's metaclass's parent class is B's metaclass), except that of the root class.
Unlike Smalltalk, the metaclass of the root class inherits from the root class (usually
NSObject using the Cocoa framework) itself. This ensures that all class objects are ultimately instances of the root class, so that you can use the instance methods of the root class, usually useful utility methods for objects, on class objects themselves.
Since metaclass objects do not behave differently (you cannot add class methods for a metaclass, so metaclass objects all have the same methods), they are all instances of the same class—the metaclass of the root class (unlike Smalltalk). Thus, the metaclass of the root class is an instance of itself. The reason for this is that all metaclasses inherit from root class; hence, they must inherit the class methods of the root class.
Support in languages and tools
The following are some of the most prominent programming languages that support metaclasses.
- Common Lisp, via CLOS
- Delphi and other versions of Object Pascal influenced by it
- Perl, via the metaclass pragma, as well as Moose
Some less widespread languages that support metaclasses include OpenJava, OpenC++, OpenAda, CorbaScript, ObjVLisp, Object-Z, MODEL-K, XOTcl, and MELDC. Several of these languages date from the early 1990s and are of academic interest.
- Ira R. Forman and Scott Danforth (1999). Putting Metaclasses to Work. ISBN 0-201-43305-2.
- IBM Metaclass programming in Python, parts 1, 2 and 3
- Artima Forum: Metaclasses in Python 3.0 (part 1 of 2) (part 2 of 2)
- David Mertz. "A Primer on Python Metaclass Programming". ONLamp. Retrieved June 28, 2006.
- "The Ruby Object Model: Comparison with Smalltalk-80".
- Paolo Perrotta. Metaprogramming Ruby. Pragmatic Bookshelf. ISBN 978-1-934356-47-0.
- "Java Reflection in Action, Part 2".
- Cocoa with Love: What is a meta-class in Objective-C?
- An implementation of mixins in Java using metaclasses