12.5: Every Metaclass Inherits from Class and Behavior

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Andrew P. Black, Stéphane Ducasse, Oscar Nierstrasz, Damien Pollet

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Every metaclass is-a class, hence inherits from Class. Class in turn inherits from its superclasses, ClassDescription and Behavior. Since everything in Smalltalk is-an object, these classes all inherit eventually from Object. We can see the complete picture in Figure \(\PageIndex{1}\).

Where is new defined? To understand the importance of the fact that metaclasses inherit from Class and Behavior, it helps to ask where new is defined and how it is found. When the message new is sent to a class it is looked up in its metaclass chain and ultimately in its superclasses Class, ClassDescription and Behavior as shown in Figure \(\PageIndex{2}\).

The question “Where new is defined?” is crucial. new is first defined in the class Behavior, and it can be redefined in its subclasses, including any of the metaclass of the classes we define, when this is necessary. Now when a message new is sent to a class it is looked up, as usual, in the metaclass of this class, continuing up the superclass chain right up to the class Behavior, if it has not been redefined along the way.

Figure \(\PageIndex{2}\): new is an ordinary message looked up in the metaclass chain.

Note that the result of sending TranslucentColor new is an instance of TranslucentColor and not of Behavior, even though the method is looked-up in the class Behavior! new always returns an instance of self, the class that receives the message, even if it is implemented in another class.

TranslucentColor new class    →    TranslucentColor "not Behavior"

A common mistake is to look for new in the superclass of the receiving class. The same holds for new:, the standard message to create an object of a given size. For example, Array new: 4 creates an array of 4 elements. You will not find this method defined in Array or any of its superclasses. Instead you should look in Array class and its superclasses, since that is where the lookup will start.

Responsibilities of Behavior, ClassDescription and Class. Behavior provides the minimum state necessary for objects that have instances: this includes a superclass link, a method dictionary, and a description of the instances(i.e.,representation and number). Behavior inherits from Object, so it, and all of its subclasses, can behave like objects.

Behavior is also the basic interface to the compiler. It provides methods for creating a method dictionary, compiling methods, creating instances (i.e., new, basicNew, new:, and basicNew:), manipulating the class hierarchy (i.e., superclass:, addSubclass:), accessing methods (i.e., selectors, allSelectors, compiledMethodAt:), accessing instances and variables (i.e., allInstances, instVarNames ...), accessing the class hierarchy (i.e., superclass, subclasses) and querying (i.e., hasMethods, includesSelector, canUnderstand:, inheritsFrom:, isVariable).

ClassDescription is an abstract class that provides facilities needed by its two direct subclasses, Class and Metaclass. ClassDescription adds a number of facilities to the basis provided by Behavior: named instance variables, the categorization of methods into protocols, the notion of a name (abstract), the maintenance of change sets and the logging of changes, and most of the mechanisms needed for filing-out changes.

Class represents the common behaviour of all classes. It provides a class name, compilation methods, method storage, and instance variables. It provides a concrete representation for class variable names and shared pool variables (addClassVarName:, addSharedPool:, initialize). Class knows how to create instances, so all metaclasses should inherit ultimately from Class.