itcl(n) [incr Tcl] itcl(n)
NAME
itcl - object-oriented extensions to Tcl
DESCRIPTION
[incr Tcl] provides object-oriented extensions to Tcl, much as C] pro-
vides object-oriented extensions to C. The emphasis of this work, how-
ever, is not to create a whiz-bang object-oriented programming environ-
ment. Rather, it is to support more structured programming practices
in Tcl without changing the flavor of the language. More than anything
else, [incr Tcl] provides a means of encapsulating related procedures
together with their shared data in a namespace that is hidden from the
outside world. It encourages better programming by promoting the
object-oriented "library" mindset. It also allows for code re-use
through inheritance.
CLASES
The fundamental construct in [incr Tcl] is the class definition. Each
class acts as a template for actual objects that can be created. Each
object has its own unique bundle of data, which contains instances of
the "variables" defined in the class. Special procedures called "meth-
ods" are used to manipulate individual objects. Methods are just like
the operations that are used to manipulate Tk widgets. The "button"
widget, for example, has methods such as "flash" and "invoke" that
cause a particular button to blink and invoke its command.
Within the body of a method, the "variables" defined in the class are
automatically available. They need not be declared with anything like
the global command. Within another class method, a method can be
invoked like any other command-simply by using its name. From any
other context, the method name must be prefaced by an object name,
which provides a context for the data that the method can access.
Each class has its own namespace containing things that are common to
all objects which belong to the class. For example, "common" data mem-
bers are shared by all objects in the class. They are global variables
that exist in the class namespace, but since they are included in the
class definition, they need not be declared using the global command;
they are automatically available to any code executing in the class
context. A class can also create ordinary global variables, but these
must be declared using the global command each time they are used.
Classes can also have ordinary procedures declared as "procs". Within
another class method or proc, a proc can be invoked like any other com-
mand-simply by using its name. From any other context, the procedure
name should be qualified with the class namespace like "class-
Name::::proc". Class procs execute in the class context, and therefore
have automatic access to all "common" data members. However, they can-
not access object-specific "variables", since they are invoked without
reference to any specific object. They are usually used to perform
generic operations which affect all objects belonging to the class.
Each of the elements in a class can be declared "public", "protected"
or "private". Public elements can be accessed by the class, by derived
classes (other classes that inherit this class), and by external
clients that use the class. Protected elements can be accessed by the
class, and by derived classes. Private elements are only accessible in
the class where they are defined.
The "public" elements within a class define its interface to the exter-
nal world. Public methods define the operations that can be used to
manipulate an object. Public variables are recognized as configuration
options by the "configure" and "cget" methods that are built into each
class. The public interface says what an object will do but not how it
will do it. Protected and private members, along with the bodies of
class methods and procs, provide the implementation details. Insulat-
ing the application developer from these details leaves the class
designer free to change them at any time, without warning, and without
affecting programs that rely on the class. It is precisely this encap-
sulation that makes object-oriented programs easier to understand and
maintain.
The fact that [incr Tcl] objects look like Tk widgets is no accident.
[incr Tcl] was designed this way, to blend naturally into a Tcl/Tk
application. But [incr Tcl] extends the Tk paradigm from being merely
object-based to being fully object-oriented. An object-oriented system
supports inheritance, allowing classes to share common behaviors by
inheriting them from an ancestor or base class. Having a base class as
a common abstraction allows a programmer to treat related classes in a
similar manner. For example, a toaster and a blender perform different
(specialized) functions, but both share the abstraction of being appli-
ances. By abstracting common behaviors into a base class, code can be
shared rather than copied. The resulting application is easier to
understand and maintain, and derived classes (e.g., specialized appli-
ances) can be added or removed more easily.
This description was merely a brief overview of object-oriented pro-
gramming and [incr Tcl]. A more tutorial introduction is presented in
the paper included with this distribution. See the class command for
more details on creating and using classes.
NAMESPACES
[incr Tcl] now includes a complete namespace facility. A namespace is
a collection of commands and global variables that is kept apart from
the usual global scope. This allows Tcl code libraries to be packaged
in a well-defined manner, and prevents unwanted interactions with other
libraries. A namespace can also have child namespaces within it, so
one library can contain its own private copy of many other libraries.
A namespace can also be used to wrap up a group of related classes.
The global scope (named "::") is the root namespace for an interpreter;
all other namespaces are contained within it.
See the namespace command for details on creating and using namespaces.
MEGA-WIDGETS
Mega-widgets are high-level widgets that are constructed using Tk wid-
gets as component parts, usually without any C code. A fileselection-
box, for example, may have a few listboxes, some entry widgets and some
control buttons. These individual widgets are put together in a way
that makes them act like one big widget.
[incr Tk] is a framework for building mega-widgets. It uses [incr Tcl]
to support the object paradigm, and adds base classes which provide
default widget behaviors. See the itk man page for more details.
[incr Widgets] is a library of mega-widgets built using [incr Tk]. It
contains more than 30 different widget classes that can be used right
out of the box to build Tcl/Tk applications. Each widget class has its
own man page describing the features available.
KEYWORDS
class, object, object-oriented, namespace, mega-widget
itcl 3.0 itcl(n)
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