Parent class vs. metaclass

There is a distinct difference between the notions of "parent" (or base) class and "metaclass." Both notions are related to the fact that a class defines the methods and variables of its instances, which are therefore called instance methods and instance variables.

A parent of a given class is a class from which the given class is derived by subclassing. (Thus, the given class is called a child or a subclass of the parent.) A parent class is a class from which instance methods and instance variables are inherited. For example, the parent of class "Dog" might be class "Animal". Hence, the instance methods and variables introduced by "Animal" (such as methods for breathing and eating, or a variable for storing an animal's weight) would also apply to instances of "Dog", because "Dog" inherits these from "Animal", its parent class. As a result, any given dog instance would be able to breath and eat, and would have a weight.

A metaclass is a class whose instances are class objects, and whose instance methods and instance variables (as described above) are therefore the methods and variables of class objects. For this reason, a metaclass is said to define class methods-the methods that a class object performs. For example, the metaclass of "Animal" might be "AnimalMClass", which defines the methods that can be invoked on class "Animal" (such as, to create Animal instances-objects that are not classes, like an individual pig or cat or elephant or dog).

Note: It is important to distinguish the methods of a class object (that is, the methods that can be invoked on the class object, which are defined by its metaclass) from the methods that the class defines for its instances.

To summarize: the parent of a class provides inherited methods that the class's instances can perform; the metaclass of a class provides class methods that the class itself can perform. These distinctions are further summarized below: The distinctions between parent class and metaclass are summarized in A class has both parent classes and a metaclass.
A class has both parent classes and a metaclass

Any class "C" has both a metaclass and one or more parent class(es).

• The parent class(es) of "C" provide the inherited instance methods that individual instances (objects "O{i}") of class "C" can perform. Instance methods that an instance "O{i}" performs might include (a) initializing itself, (b) performing computations using its instance variables, (c) printing its instance variables, or (d) returning its size. New instance methods are defined by "C" itself, in addition to those inherited from C's parent classes.

• The metaclass "M"defines the class methods that class "C" can perform. For example, class methods defined by metaclass "M" include those that allow "C" to (a) inherit its parents' instance methods and instance variables, (b) tell its own name, (c) create new instances, and (d) tell how many instance methods it supports. These methods are inherited from SOMClass. Additional methods supported by "M" might allow "C" to count how many instances it creates.

• Each class "C" has one or more parent classes and exactly one metaclass. (The single exception is SOMObject, which has no parent class.) Parent class(es) must be explicitly identified in the IDL declaration of a class. (SOMObject is given as a parent if no subsequently-derived class applies.) If a metaclass is not explicitly listed, the SOM run time will determine an applicable metaclass.

• An instance of a metaclass is always another class object. For example, class "C" is an instance of metaclass "M". SOMClass is the SOM-provided metaclass from which all subsequent metaclasses are derived.

A metaclass has its own inheritance hierarchy (through its parent classes) that is independent of its instances' inheritance hierarchies. For example, suppose a series of classes is defined (or derived), stemming from SOMObject. The child class (or subclass) at the end of this line ("C[2]") inherits instance methods from all of its ancestor classes (here, SOMObject and "C[1]"). An instance created by "C2" can perform any of these instance methods. In an analogous manner, a line of metaclasses can be defined, stemming from SOMClass. Just as a new class is derived from an existing class (such as SOMObject), a new metaclass is derived from an existing metaclass (such as SOMClass). Parentclassesandmetaclasseseachhavetheirown
independent inheritance hierrarchies