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 members 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 command-simply by using its name. From any other context, the procedure name should be qualified with the class namespace like "className::proc". Class procs execute in the class context, and therefore have automatic access to all "common" data members. However, they cannot 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 external 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. Insulating 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 encapsulation 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 appliances. 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 appliances) can be added or removed more easily.
This description was merely a brief overview of object-oriented programming 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.
See the namespace command for details on creating and using namespaces.
[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.
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