=head1 Translating Object Oriented Features

This document discusses the support for object oriented programming that the
.NET virtual machine provides, the support that Parrot provides and how to map
the .NET object model onto the Parrot one in a way that, as far as possible,
will allow other Parrot languages to interoperate with .NET languages.

=head2 The .NET Model

The .NET object model includes classes with many methods and fields. Methods
and fields may be static or per instance, and instance methods may be virtual
(that is, subclasses may provide seperate implementations), thus providing for
polymorphism.

Only single inheritance of classes is supported, however many interfaces may
be inherited (or in .NET terminology, implemented). An interface is a class
with no instance fields and where strictly every method is abstract. Abstract
methods are those with a declaration but no definition (that is, they have no
implementation). Abstract classes are also available and allow one or more
methods to be abstract while others may have an implementation. They can only
be singly inherited.

Details of object oriented constructs are stored in the meta-data tables. The
classes table provides a parent field to reference the parent class. Every
class must have a parent except System.Object. Interfaces may or may not have
a parent. Mappings of a class to the interfaces it implements are stored in
another table and details of fields in yet another.

=head2 The Parrot Model

Parrot provides two PMCs that support object orientation, namely ParrotClass
(which represents a class) and ParrotObject (which represents an object; that
is, the instantiation of a class). A class can inherit many other classes and
the inheritance hierachy may change at runtime.

Class creation and definition of the inheritance hierachy are performed through
a number of Parrot instructions. This means that classes are not "hard-wired"
at compile time, and only come into existence at runtime. It also means that
the parents of a particular class must all have been created before that class
can be.

A ParrotClass may have many attributes (runtime storage locations per instance
of the class that get inherited) and also many properties (runtime storage
locations per instance of the class that do not get inherited). These map to
what .NET calls fields.

While Parrot does have some support for interfaces stubbed in, this has not
yet been implemented.

It is possible to find out at runtime the dynamic type of an object (that is,
the class it really is), whether it is an instance of a particular class (for
example, if A inherits B then an object of type B is an instance of both A and
B) and whether a class has a certain method.


=head2 Mapping .NET OOP Onto Parrot OOP

=head3 Classes

=head3 Interfaces

Since Parrot currently lacks interfaces, they have to be "faked out" using
classes and taking advantage of multiple inheritance. An interface is translated
just as a class would be, but each of the methods simply contain code to throw
an exception if they are invoked stating that an interface has not been fully
implemented. The constructor also throws an exception to prevent instantiation
of interfaces. This provides all of the required semantics.

=head3 Methods

=head3 Fields

=head3 Class Inheritance

=head3 Interface Implementation

Since interfaces are really just classes in the translated code, implementing
the interface looks just like inheriting a class.

=head3 Instantiation

=head3 Constructors

=head3 Finalizers