Version 16 of XOTcl

Updated 2004-02-22 17:29:58

This is a page for information, tips and hits about the XOTcl object oriented extension.

Partly taken from the XOTcl homepage [L1 ]:

XOTcl combines the ideas of scripting and object-orientation in a way that preserves the benefits of both of them. It is equipped with several new language functionalities that help building and managing complex systems. It supports the following features:

  • Dynamic Object Aggregations, to provide dynamic aggregations through nested namespaces (objects).
  • Nested Classes, to reduce the interference of independently developed program structures.
  • Assertions, to reduce the interface and the reliability problems caused by dynamic typing and, therefore, to ease the combination of many components.
  • Per-object mixins, as a means to improve flexibility of mixin methods by giving an object access to several different supplemental classes, which may be changed dynamically.
  • Per-class mixins, as a means to improve flexibility of mixin methods to a class, all instances of the class have access to the mixed in methods like for multiple inheritance, but without the need of intersection classes.
  • Per-class and per-object filters, as a means of abstractions over method invocations to implement large program structures, like design patterns.
  • Dynamic Component Loading, XOTcl integrates the Tcl package loading with architectural support for integration with object-oriented constructs. Moreover, it provides tracking/tracing of component loading.
  • Dynamic read/write introspection and extensibility.
  • Meta-classes.

Zoran Vasiljevic describes: "... it runs leak free, does not crash, obeys to what's written in the manual, is thread-safe, can be used under AOLserver or under Tcl threading extension w/o problems, is loaded with features., checked with Purify... ... We have written 100K lines in Xotcl and I find it very valuable. I like it more than incrTcl since it ihas more of the dynamic nature of Tcl than incrTcl does. The incrTcl is more appealing to c++/java programmers. Xotcl should be more appealing to Tcl programmers, IMHO."

Gustaf Neumann GN writes to the XOTcl mailing list [L2 ] on 20Jul03:

...let me re-iterate the basic "philosophical" point of view. Languages like xotcl are object-oriented, while languages like java (and most of UML) is class-oriented.

Class-oriented means: look at the class and you know exactly how all of the instances look alike. The class is the first and primary language construct; the class is well the place where you specify the instance variables (there are no instance variables except those specified in the class). The only kind of individualism left in the objects is to let them differ by their state (the values of their instance variables). Changing classes (class migration) is conceptionally quite hard for this setup.

Object-oriented (in this distinction) means that the primary elements are objects, which keep all instance variables. classes my be used to specify the behavior of objects, they are container for methods and they control the life-cycle of objects. Objects are like the facts, and objects are like rules, that determine the behavior of the objects. Since the connection between objects and classes is rather loose, it is sufficient to define their relation through an association. Therefore it is quite easy to change the relation between objects and classes (and between classes and classes) dynamically. Objects have arbitrary individualism, they may have variables never used in any class, they may have private procs etc.

From the expressiveness, object-oriented languages can - in principle - be restricted to behave like class-oriented languages, but the other way around is much harder.

Useful code tidbits:

  • XOTcl Introductory Example. This example shows how XOTcl diffes from e.g. itcl by providing "open class definitions", object specific behavior and mixin classes (among other things) to achieve a more dynamic behaviour.
  • Cacheable class. This shows how filters can be used to build a transparent cache for specified methods. Because of the nature of XOTcl this can be dynamically mixed into objects and their class hierarchies and later removed when not necessary.
  • You will find some more code examples in the Category XOTcl Code and on the xotcl documentatiton page [L3 ]

See also XotclIDE, XOTcl Documentation Tool.

Category Object Orientation | Category Package