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A class definition can restrict the values of any slot to a particular C-style type. It can specify whether a slot is private (the default, meaning that it cannot be accessed except by that methods of that class), protected (like private, except that the slot can also be accessed by subclasses of the class), or public (meaning get and put methods for the slot are generated automatically), and it can specify an initial value. The class definition also may contain method clauses, which determine how instances of the class will respond to messages. A class definition may also specify one or more superclasses and which methods are to be inherited.
The point object created in the previous example had two floating
point slots, named x
and y
, with initial values of 1.0 and
2.0, respectively. As we have seen, the point
class also defined
put and get methods for x
and y
, as well as a send method for
printing the object. The put and get methods for x
and y
can be
automatically generated simply by declaring the slots public
, but
the print
method must be explicitly written. In addition, in
order to be able to create instances of this class, we must define a
create
method, as explained in obj-scl-meth.
We also provide a second create
method, taking two arguments,
allowing us to specify an x
and y
value when we first create a
point object.
:- class point = [public x:float = 1.0, public y:float = 2.0]. Self <- create. Self <- create(X, Y) :- Self << x(X), Self << y(Y). Self <- print(Stream) :- Self >> x(X), Self >> y(Y), format(Stream, '(~w,~w)', [X,Y]). :- end_class point.
The variable name Self
in these clauses is arbitrary—any
variable to the
left of the message operator in the head of a method clause refers to
the instance of the class receiving the message.