..
   This file is part of Logtalk <https://logtalk.org/>  
   Copyright 1998-2021 Paulo Moura <pmoura@logtalk.org>
   SPDX-License-Identifier: Apache-2.0

   Licensed under the Apache License, Version 2.0 (the "License");
   you may not use this file except in compliance with the License.
   You may obtain a copy of the License at

       http://www.apache.org/licenses/LICENSE-2.0

   Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   See the License for the specific language governing permissions and
   limitations under the License.


A reflective class-based system
===============================

When compiling an object, Logtalk distinguishes prototypes from instance
or classes by examining the object relations. If an object instantiates
and/or specializes another object, then it is compiled as an instance or
class, otherwise it is compiled as a prototype. A consequence of this is
that, in order to work with instance or classes, we always have to
define root objects for the instantiation and specialization hierarchies
(however, we are not restricted to a single hierarchy). The best
solution is often to define a reflective class-based system [Maes87]_,
where every class is also an object and, as such, an instance of some class.

In this example, we are going to define the basis for a reflective
class-based system, based on an extension of the ideas presented in
[Cointe87]_. This extension
provides, along with root objects for the instantiation and
specialization hierarchies, explicit support for abstract classes
[Moura94]_.

.. _classes:

Defining the base classes
-------------------------

We will start by defining three classes: ``object``, ``abstract_class``,
and ``class``. The class ``object`` will contain all predicates common
to all objects. It will be the root of the inheritance graph:

::

   :- object(object,
       instantiates(class)).

       % predicates common to all objects

   :- end_object.

The class ``abstract_class`` specializes ``object`` by adding predicates
common to all classes. It will be the default meta-class for abstract
classes:

::

   :- object(abstract_class,
       instantiates(class),
       specializes(object)).

       % predicates common to all classes

   :- end_object.

The class ``class`` specializes ``abstract_class`` by adding predicates
common to all instantiable classes. It will be the root of the
instantiation graph and the default meta-class for instantiable classes:

::

   :- object(class,
       instantiates(class),
       specializes(abstract_class)).

       % predicates common to all instantiable classes

   :- end_object.

Note that all three objects are instances of class ``class``. The
instantiation and specialization relationships are chosen so that each
object may use the predicates defined in itself and in the other two
objects, with no danger of :term:`message lookup` endless loops.

Summary
-------

-  An object that does not instantiate or specialize other objects is
   always compiled as a prototype.

-  An instance must instantiate at least one object (its class).
   Similarly, a class must at least specialize or instantiate other
   object.

-  The distinction between abstract classes and instantiable classes is
   an operational one, depending on the class inherited methods. A class
   is instantiable if inherits methods for creating instances.
   Conversely, a class is abstract if does not inherit any instance
   creation method.
