1. 1 CLOS: Common Lisp Object System Basic principles of CLOS Classes, Instances and Slots Inheritance Generic functions Methods Multi-methods Comparison CLOS and Java

2. 2 CLOS: Classes, Instances and Slots Classes –Common Lisp types Instances of a class –Share structure, behavior, and type Slots –Determine the structure of a class –Have a name and a value –Can be read and written by slot accessors –Local slots maintain a separate value for each instance –Shared slots maintain a common (class-wide) value

3. 3 CLOS: Superclasses Allow classes to be built from other classes New class inherits structure and behavior from superclasses Classes can inherit structure and behavior from multiple superlasses.

4. 4 CLOS Elements: Generic functions Syntactically identical to normal CL functions Semantic difference 1 –Normal functions specify interface and implementation –Generic functions specify only interface Implementation is distributed among a set of methods Semantic difference 2 –Implementation of a normal function doesn't vary from call to call –Implementation of a generic function varies depending upon the class of its arguments Behavior of a generic function involves both –Selection of methods, and –Composition of methods

5. 5 CLOS Elements: Methods The underlying implementation of generic functions –Methods are similar to ordinary Lisp functions, but –They are not called directly Method “role” determines its part in generic function –Primary methods perform bulk of work Only one primary method is called each time a generic function is called It provides the return value of the generic function –Before methods called before the primary –After methods called after the primary –Around methods “sandwich” the primary

6. 6 CLOS Elements: Inheritance Inheritance is the sharing of characteristics and behavior among a set of classes Both slots and methods can be shared among classes Multiple inheritance creates problem of differently defined methods or slots with the same name in different ancestors CLOS computes a class precedence list to determine which method to inherit –A class always has precedence over its superclasses –The order in which classes are listed in a class definition sets determines their precedence

7. 7 Example class hierarchy window with-label window with-border primary method: refresh Code to Clear Window window after method: refresh Code to draw border window- with-border after method: refresh Code to draw label window- with-label generic function refresh

8. 8 Example refresh invocation Instance of window-with-label argument refresh implementation side effects value Implementation: primary method: refresh Code to Clear Window window after method: refresh Code to draw label window- with-label

9. 9 Method combination The process of determining the implementation associated with a generic function for a given set of arguments is called generic dispatch –Find all applicable methods –Sort them by order of precedence –Call one or more of them Method ordering –All before methods in most-specific-first order –The most specific primary method –All after methods in most-specific-last order

10. 10 Example: window-with-border instance before method for window-with-border before method for window most specific primary method after method for window after method for window-with-border

11. 11 Slot access and method combination Slot accessors are simply primary methods – (defclass triangle (shape) ((side-a :accessor side-a) (side-b :accessor side-b) (side-c :accessor side-c) (area :reader area))) We can update the area slot automatically whenever a side changes value by defining an :after method – (defmethod (setf side-a) :after (new-length (tri triangle)) (setf (area tri) ))

12. 12 Multi-methods Methods can be written to specialize on multiple parameters. Multi-methods allow methods to “belong” to more than one class simultaneously. basic- product basic- OS LifeAdventureUnixWindows (defmethod install((sw basic-product) (os basic-OS)) (restore-product sw os) (compile-product sw os) (configure-site sw os) (verify-product sw os))

13. 13 Meta-object protocol (MOP) Used to define default model of OO above. Allows implementation of alternative paradigms Specifies: –What a subclass inherits from its superclasses –How a subclass determines the precedence of superclasses –How generic functions dispatch Classes are themselves instances of a meta-class Meta-classes specify the structure and behavior of classes Meta-classes inherit structure and behavior from their super meta-classes

14. 14 CLOS vs. Java Java does not support generic functions –Java methods analogous to CLOS methods –Java does not explicitly support member function composition Java does not support multi-methods –Methods owned by a single class Java does not provide a meta-object protocol –Inheritance and dispatch procedure hardwired. Java provides access control (public, private..)