Instance Methods

Instance methods allow each object in a class to have its own method for handling a specified message. For example, in a push-button class it would be convenient for each instance (each push-button) to have its own method for responding to being pressed.

The declaration

     :- instance_method Name/Arity, ....

inside a class definition states that the message Name/Arity supports instance methods. If the class definition defines a method for this message, it will be treated as a default method for the message.

The define_method/3 predicate installs a method for an object of the class, and the undefine_method/3 predicate removes that method.

Suppose that the date_stamp class, defined earlier, declared an instance method to print the year of a date_stamp instance.

     :- instance_method print_year/1.
     Self <- print_year(Stream) :-
             Self >> year(Y0),
             Y1 is YO + 1970,
             format(Stream, "~d", [Y1]).

The arithmetic is necessary because UNIX dates are based on January 1, 1970.

If a particular date_stamp object's date were to be printed in Roman numerals, it could be given a different print_year method, using the define_method/3 predicate.

     | ?- create(date_stamp, DateObj),
     		   print_roman_year(Stream, DateObj)).

If this date_stamp object is created in 1994, then a print_year message sent to it would print the current year as


Defining the predicate print_roman_year/2 is left as an exercise. It must be able to access the year slot of a date_stamp object. Because it is not defined by a method clause within the class definition, print_roman_year/2 cannot use the get_slot/2 predicate.

None of instance_method/1, define_method/3, undefine_method/3 specify a message operator. Instance methods can only be defined for send messages.