3.2.2 Relationship between classes and methods

The relationship between classes and methods in Dylan is different from that in C++ and Smalltalk, among other languages.

Comparison to C++ and Smalltalk: In C++ and Smalltalk, a class contains the equivalent of methods. In Dylan, a class does not contain methods; instead, a method belongs to a generic function. This design decision enables these powerful features of Dylan:

You can define methods on built-in classes (because you do not have to modify the class definition to define a method intended for use on the class). For an example, see Section 6.1, page 75. More generally, you can define a method for a class that you did not define.
You can write multimethods. In a multimethod, the method dispatch is based on the classes of more than one argument to a generic function. For an introduction to method dispatch, see Section 5.5, page 63. For information about multimethods, see Chapter 6, Multimethods.
You can restrict generic functions to operate on specific classes of objects.

In Dylan, a method belongs to a generic function, as shown in Figure 3.1, page 30. Although methods are independent of classes, methods operate on instances of classes. A method states the types of objects for which it is applicable by the type constraint of each of its required parameters. Consider the say-greeting method defined earlier:

define method say-greeting (greeting :: <integer>);
  format-out("Your lucky number is %s.\n", greeting);
end;

This method operates on instances of the <integer> class. Notice how easy and convenient it is to define a method intended for use on the built-in class <integer>.