1itcl(n) [incr Tcl] itcl(n)
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8 itcl - object-oriented extensions to Tcl
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13 [incr Tcl] provides object-oriented extensions to Tcl, much as C++ pro‐
14 vides object-oriented extensions to C. The emphasis of this work, how‐
15 ever, is not to create a whiz-bang object-oriented programming environ‐
16 ment. Rather, it is to support more structured programming practices
17 in Tcl without changing the flavor of the language. More than anything
18 else, [incr Tcl] provides a means of encapsulating related procedures
19 together with their shared data in a namespace that is hidden from the
20 outside world. It encourages better programming by promoting the
21 object-oriented "library" mindset. It also allows for code re-use
22 through inheritance.
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26 The fundamental construct in [incr Tcl] is the class definition. Each
27 class acts as a template for actual objects that can be created. Each
28 object has its own unique bundle of data, which contains instances of
29 the "variables" defined in the class. Special procedures called "meth‐
30 ods" are used to manipulate individual objects. Methods are just like
31 the operations that are used to manipulate Tk widgets. The "button"
32 widget, for example, has methods such as "flash" and "invoke" that
33 cause a particular button to blink and invoke its command.
34 Within the body of a method, the "variables" defined in the class are
36 automatically available. They need not be declared with anything like
37 the global command. Within another class method, a method can be
38 invoked like any other command-simply by using its name. From any
39 other context, the method name must be prefaced by an object name,
40 which provides a context for the data that the method can access.
41 Each class has its own namespace containing things that are common to
43 all objects which belong to the class. For example, "common" data mem‐
44 bers are shared by all objects in the class. They are global variables
45 that exist in the class namespace, but since they are included in the
46 class definition, they need not be declared using the global command;
47 they are automatically available to any code executing in the class
48 context. A class can also create ordinary global variables, but these
49 must be declared using the global command each time they are used.
50 Classes can also have ordinary procedures declared as "procs". Within
52 another class method or proc, a proc can be invoked like any other com‐
53 mand-simply by using its name. From any other context, the procedure
54 name should be qualified with the class namespace like "class‐
55 Name::proc". Class procs execute in the class context, and therefore
56 have automatic access to all "common" data members. However, they can‐
57 not access object-specific "variables", since they are invoked without
58 reference to any specific object. They are usually used to perform
59 generic operations which affect all objects belonging to the class.
60 Each of the elements in a class can be declared "public", "protected"
62 or "private". Public elements can be accessed by the class, by derived
63 classes (other classes that inherit this class), and by external
64 clients that use the class. Protected elements can be accessed by the
65 class, and by derived classes. Private elements are only accessible in
66 the class where they are defined.
67 The "public" elements within a class define its interface to the exter‐
69 nal world. Public methods define the operations that can be used to
70 manipulate an object. Public variables are recognized as configuration
71 options by the "configure" and "cget" methods that are built into each
72 class. The public interface says what an object will do but not how it
73 will do it. Protected and private members, along with the bodies of
74 class methods and procs, provide the implementation details. Insulat‐
75 ing the application developer from these details leaves the class
76 designer free to change them at any time, without warning, and without
77 affecting programs that rely on the class. It is precisely this encap‐
78 sulation that makes object-oriented programs easier to understand and
79 maintain.
80 The fact that [incr Tcl] objects look like Tk widgets is no accident.
82 [incr Tcl] was designed this way, to blend naturally into a Tcl/Tk
83 application. But [incr Tcl] extends the Tk paradigm from being merely
84 object-based to being fully object-oriented. An object-oriented system
85 supports inheritance, allowing classes to share common behaviors by
86 inheriting them from an ancestor or base class. Having a base class as
87 a common abstraction allows a programmer to treat related classes in a
88 similar manner. For example, a toaster and a blender perform different
89 (specialized) functions, but both share the abstraction of being appli‐
90 ances. By abstracting common behaviors into a base class, code can be
91 shared rather than copied. The resulting application is easier to
92 understand and maintain, and derived classes (e.g., specialized appli‐
93 ances) can be added or removed more easily.
94 This description was merely a brief overview of object-oriented pro‐
96 gramming and [incr Tcl]. A more tutorial introduction is presented in
97 the paper included with this distribution. See the class command for
98 more details on creating and using classes.
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102 [incr Tcl] now includes a complete namespace facility. A namespace is
103 a collection of commands and global variables that is kept apart from
104 the usual global scope. This allows Tcl code libraries to be packaged
105 in a well-defined manner, and prevents unwanted interactions with other
106 libraries. A namespace can also have child namespaces within it, so
107 one library can contain its own private copy of many other libraries.
108 A namespace can also be used to wrap up a group of related classes.
109 The global scope (named "::") is the root namespace for an interpreter;
110 all other namespaces are contained within it.
111 See the namespace command for details on creating and using namespaces.
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116 Mega-widgets are high-level widgets that are constructed using Tk wid‐
117 gets as component parts, usually without any C code. A fileselection‐
118 box, for example, may have a few listboxes, some entry widgets and some
119 control buttons. These individual widgets are put together in a way
120 that makes them act like one big widget.
121 [incr Tk] is a framework for building mega-widgets. It uses [incr Tcl]
123 to support the object paradigm, and adds base classes which provide
124 default widget behaviors. See the itk man page for more details.
125 [incr Widgets] is a library of mega-widgets built using [incr Tk]. It
127 contains more than 30 different widget classes that can be used right
128 out of the box to build Tcl/Tk applications. Each widget class has its
129 own man page describing the features available.
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133 class, object, object-oriented, namespace, mega-widget
134itcl 3.0 itcl(n)