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Logtalk - Object oriented extension to Prolog
Release 2.20.1

Copyright (c) 1998-2004 Paulo Moura.  All Rights Reserved.
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% start by loading the example:

| ?- logtalk_load(loader).
...


% print the (public and protected) interface of each class:

| ?- (object, abstract_class, class)::print.

Object: object

  interface:
    new/1
    delete/1
    instances/1
    metaclass/0
    abstract_class/0
    strict_instance/0
    print/0

Object: abstract_class

  interface:
    new/1
    delete/1
    instances/1
    metaclass/0
    abstract_class/0
    strict_instance/0
    print/0

Object: class

  interface:
    new/1
    delete/1
    instances/1
    metaclass/0
    abstract_class/0
    strict_instance/0
    print/0

yes


% class is the metaclass of all classes:

| ?- class::instances(Instances), class::metaclass.

Instances = [class,abstract_class,object]
yes


% create an abstract_class, check it and print its interface:

| ?- abstract_class::new(ac), ac::abstract_class, ac::print.

Object: ac

  interface:
    metaclass/0
    abstract_class/0
    strict_instance/0
    print/0

yes


% try to create an instance of the abstract class:

| ?- ac::new(i).

uncaught exception: error(existence_error(predicate_declaration,new(i)),ac::new(i),user)


% create a new instantiable class and print its interface:

| ?- class::new(c), c::print.

Object: c

  interface:
    new/1
    delete/1
    instances/1
    metaclass/0
    abstract_class/0
    strict_instance/0
    print/0

yes


% create an instance of the new class:

| ?- c::new(i), c::instances(Instances).

Instances = [i]
yes


% because c does not declare any predicates, its instances have no interface:

| ?- i::current_predicate(Predicate).

no


% create an instance of object, root of the inheritance graph, and print its interface:

| ?- object::new(j), j::print.

Object: j

  interface:
    strict_instance/0
    print/0

yes


% delete the dynamic objects that we created:

| ?- c::delete(i), class::delete(c), abstract_class::delete(ac), object::delete(j).

yes