Provided by: itcl3-doc_3.4.4-2build1_all 
NAME
class - create a class of objects
SYNOPSIS
itcl::class className {
inherit baseClass ?baseClass...?
constructor args ?init? body
destructor body
method name ?args? ?body?
proc name ?args? ?body?
variable varName ?init? ?config?
common varName ?init?
public command ?arg arg ...?
protected command ?arg arg ...?
private command ?arg arg ...?
set varName ?value?
array option ?arg arg ...?
}
className objName ?arg arg ...?
objName method ?arg arg ...?
className::proc ?arg arg ...?
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DESCRIPTION
The fundamental construct in [incr Tcl] is the class definition. Each class acts as a template for
actual objects that can be created. The class itself is a namespace which contains things common to all
objects. Each object has its own unique bundle of data which contains instances of the "variables"
defined in the class definition. Each object also has a built-in variable named "this", which contains
the name of the object. Classes can also have "common" data members that are shared by all objects in a
class.
Two types of functions can be included in the class definition. "Methods" are functions which operate on
a specific object, and therefore have access to both "variables" and "common" data members. "Procs" are
ordinary procedures in the class namespace, and only have access to "common" data members.
If the body of any method or proc starts with "@", it is treated as the symbolic name for a C procedure.
Otherwise, it is treated as a Tcl code script. See below for details on registering and using C
procedures.
A class can only be defined once, although the bodies of class methods and procs can be defined again and
again for interactive debugging. See the body and configbody commands for details.
Each namespace can have its own collection of objects and classes. The list of classes available in the
current context can be queried using the "itcl::find classes" command, and the list of objects, with the
"itcl::find objects" command.
A class can be deleted using the "delete class" command. Individual objects can be deleted using the
"delete object" command.
CLASS DEFINITIONS
class className definition
Provides the definition for a class named className. If the class className already exists, or if
a command called className exists in the current namespace context, this command returns an error.
If the class definition is successfully parsed, className becomes a command in the current
context, handling the creation of objects for this class.
The class definition is evaluated as a series of Tcl statements that define elements within the class.
The following class definition commands are recognized:
inherit baseClass ?baseClass...?
Causes the current class to inherit characteristics from one or more base classes. Classes
must have been defined by a previous class command, or must be available to the auto-
loading facility (see "AUTO-LOADING" below). A single class definition can contain no more
than one inherit command.
The order of baseClass names in the inherit list affects the name resolution for class
members. When the same member name appears in two or more base classes, the base class
that appears first in the inherit list takes precedence. For example, if classes "Foo" and
"Bar" both contain the member "x", and if another class has the "inherit" statement:
inherit Foo Bar
then the name "x" means "Foo::x". Other inherited members named "x" must be referenced with their
explicit name, like "Bar::x".
constructor args ?init? body
Declares the args argument list and body used for the constructor, which is automatically
invoked whenever an object is created.
Before the body is executed, the optional init statement is used to invoke any base class
constructors that require arguments. Variables in the args specification can be accessed
in the init code fragment, and passed to base class constructors. After evaluating the
init statement, any base class constructors that have not been executed are invoked
automatically without arguments. This ensures that all base classes are fully constructed
before the constructor body is executed. By default, this scheme causes constructors to be
invoked in order from least- to most-specific. This is exactly the opposite of the order
that classes are reported by the info heritage command.
If construction is successful, the constructor always returns the object name-regardless of
how the body is defined-and the object name becomes a command in the current namespace
context. If construction fails, an error message is returned.
destructor body
Declares the body used for the destructor, which is automatically invoked when an object is
deleted. If the destructor is successful, the object data is destroyed and the object name
is removed as a command from the interpreter. If destruction fails, an error message is
returned and the object remains.
When an object is destroyed, all destructors in its class hierarchy are invoked in order
from most- to least-specific. This is the order that the classes are reported by the "info
heritage" command, and it is exactly the opposite of the default constructor order.
method name ?args? ?body?
Declares a method called name. When the method body is executed, it will have automatic
access to object-specific variables and common data members.
If the args list is specified, it establishes the usage information for this method. The
body command can be used to redefine the method body, but the args list must match this
specification.
Within the body of another class method, a method can be invoked like any other
command-simply by using its name. Outside of the class context, the method name must be
prefaced an object name, which provides the context for the data that it manipulates.
Methods in a base class that are redefined in the current class, or hidden by another base
class, can be qualified using the "className::method" syntax.
proc name ?args? ?body?
Declares a proc called name. A proc is an ordinary procedure within the class namespace.
Unlike a method, a proc is invoked without referring to a specific object. When the proc
body is executed, it will have automatic access only to common data members.
If the args list is specified, it establishes the usage information for this proc. The
body command can be used to redefine the proc body, but the args list must match this
specification.
Within the body of another class method or proc, a proc can be invoked like any other
command-simply by using its name. In any other namespace context, the proc is invoked
using a qualified name like "className::proc". Procs in a base class that are redefined in
the current class, or hidden by another base class, can also be accessed via their
qualified name.
variable varName ?init? ?config?
Defines an object-specific variable named varName. All object-specific variables are
automatically available in class methods. They need not be declared with anything like the
global command.
If the optional init string is specified, it is used as the initial value of the variable
when a new object is created. Initialization forces the variable to be a simple scalar
value; uninitialized variables, on the other hand, can be set within the constructor and
used as arrays.
The optional config script is only allowed for public variables. If specified, this code
fragment is executed whenever a public variable is modified by the built-in "configure"
method. The config script can also be specified outside of the class definition using the
configbody command.
common varName ?init?
Declares a common variable named varName. Common variables reside in the class namespace
and are shared by all objects belonging to the class. They are just like global variables,
except that they need not be declared with the usual global command. They are
automatically visible in all class methods and procs.
If the optional init string is specified, it is used as the initial value of the variable.
Initialization forces the variable to be a simple scalar value; uninitialized variables, on
the other hand, can be set with subsequent set and array commands and used as arrays.
Once a common data member has been defined, it can be set using set and array commands
within the class definition. This allows common data members to be initialized as arrays.
For example:
itcl::class Foo {
common boolean
set boolean(true) 1
set boolean(false) 0
}
Note that if common data members are initialized within the constructor, they get initialized
again and again whenever new objects are created.
public command ?arg arg ...?
protected command ?arg arg ...?
private command ?arg arg ...?
These commands are used to set the protection level for class members that are created when
command is evaluated. The command is usually method, proc, variable orcommon, and the
remaining arg's complete the member definition. However, command can also be a script
containing many different member definitions, and the protection level will apply to all of
the members that are created.
CLASS USAGE
Once a class has been defined, the class name can be used as a command to create new objects belonging to
the class.
className objName ?args...?
Creates a new object in class className with the name objName. Remaining arguments are passed to
the constructor of the most-specific class. This in turn passes arguments to base class
constructors before invoking its own body of commands. If construction is successful, a command
called objName is created in the current namespace context, and objName is returned as the result
of this operation. If an error is encountered during construction, the destructors are
automatically invoked to free any resources that have been allocated, the object is deleted, and
an error is returned.
If objName contains the string "#auto", that string is replaced with an automatically generated
name. Names have the form className<number>, where the className part is modified to start with a
lowercase letter. In class "Toaster", for example, the "#auto" specification would produce names
like toaster0, toaster1, etc. Note that "#auto" can be also be buried within an object name:
fileselectiondialog .foo.bar.#auto -background red
This would generate an object named ".foo.bar.fileselectiondialog0".
OBJECT USAGE
Once an object has been created, the object name can be used as a command to invoke methods that operate
on the object.
objName method ?args...?
Invokes a method named method on an object named objName. Remaining arguments are passed to the
argument list for the method. The method name can be "constructor", "destructor", any method name
appearing in the class definition, or any of the following built-in methods.
BUILT-IN METHODS
objName cget option
Provides access to public variables as configuration options. This mimics the behavior of the
usual "cget" operation for Tk widgets. The option argument is a string of the form "-varName",
and this method returns the current value of the public variable varName.
objName configure ?option? ?value option value ...?
Provides access to public variables as configuration options. This mimics the behavior of the
usual "configure" operation for Tk widgets. With no arguments, this method returns a list of
lists describing all of the public variables. Each list has three elements: the variable name,
its initial value and its current value.
If a single option of the form "-varName" is specified, then this method returns the information
for that one variable.
Otherwise, the arguments are treated as option/value pairs assigning new values to public
variables. Each variable is assigned its new value, and if it has any "config" code associated
with it, it is executed in the context of the class where it was defined. If the "config" code
generates an error, the variable is set back to its previous value, and the configure method
returns an error.
objName isa className
Returns non-zero if the given className can be found in the object's heritage, and zero otherwise.
objName info option ?args...?
Returns information related to a particular object named objName, or to its class definition. The
option parameter includes the following things, as well as the options recognized by the usual Tcl
"info" command:
objName info class
Returns the name of the most-specific class for object objName.
objName info inherit
Returns the list of base classes as they were defined in the "inherit" command, or an empty
string if this class has no base classes.
objName info heritage
Returns the current class name and the entire list of base classes in the order that they
are traversed for member lookup and object destruction.
objName info function ?cmdName? ?-protection? ?-type? ?-name? ?-args? ?-body?
With no arguments, this command returns a list of all class methods and procs. If cmdName
is specified, it returns information for a specific method or proc. If no flags are
specified, this command returns a list with the following elements: the protection level,
the type (method/proc), the qualified name, the argument list and the body. Flags can be
used to request specific elements from this list.
objName info variable ?varName? ?-protection? ?-type? ?-name? ?-init? ?-value? ?-config?
With no arguments, this command returns a list of all object-specific variables and common
data members. If varName is specified, it returns information for a specific data member.
If no flags are specified, this command returns a list with the following elements: the
protection level, the type (variable/common), the qualified name, the initial value, and
the current value. If varName is a public variable, the "config" code is included on this
list. Flags can be used to request specific elements from this list.
CHAINING METHODS/PROCS
Sometimes a base class has a method or proc that is redefined with the same name in a derived class.
This is a way of making the derived class handle the same operations as the base class, but with its own
specialized behavior. For example, suppose we have a Toaster class that looks like this:
itcl::class Toaster {
variable crumbs 0
method toast {nslices} {
if {$crumbs > 50} {
error "== FIRE! FIRE! =="
}
set crumbs [expr $crumbs+4*$nslices]
}
method clean {} {
set crumbs 0
}
}
We might create another class like SmartToaster that redefines the "toast" method. If we want to access
the base class method, we can qualify it with the base class name, to avoid ambiguity:
itcl::class SmartToaster {
inherit Toaster
method toast {nslices} {
if {$crumbs > 40} {
clean
}
return [Toaster::toast $nslices]
}
}
Instead of hard-coding the base class name, we can use the "chain" command like this:
itcl::class SmartToaster {
inherit Toaster
method toast {nslices} {
if {$crumbs > 40} {
clean
}
return [chain $nslices]
}
}
The chain command searches through the class hierarchy for a slightly more generic (base class)
implementation of a method or proc, and invokes it with the specified arguments. It starts at the
current class context and searches through base classes in the order that they are reported by the "info
heritage" command. If another implementation is not found, this command does nothing and returns the
null string.
AUTO-LOADING
Class definitions need not be loaded explicitly; they can be loaded as needed by the usual Tcl auto-
loading facility. Each directory containing class definition files should have an accompanying
"tclIndex" file. Each line in this file identifies a Tcl procedure or [incr Tcl] class definition and
the file where the definition can be found.
For example, suppose a directory contains the definitions for classes "Toaster" and "SmartToaster". Then
the "tclIndex" file for this directory would look like:
# Tcl autoload index file, version 2.0 for [incr Tcl]
# This file is generated by the "auto_mkindex" command
# and sourced to set up indexing information for one or
# more commands. Typically each line is a command that
# sets an element in the auto_index array, where the
# element name is the name of a command and the value is
# a script that loads the command.
set auto_index(::Toaster) "source $dir/Toaster.itcl"
set auto_index(::SmartToaster) "source $dir/SmartToaster.itcl"
The auto_mkindex command is used to automatically
generate "tclIndex" files.
The auto-loader must be made aware of this directory by appending the directory name to the "auto_path"
variable. When this is in place, classes will be auto-loaded as needed when used in an application.
C PROCEDURES
C procedures can be integrated into an [incr Tcl] class definition to implement methods, procs, and the
"config" code for public variables. Any body that starts with "@" is treated as the symbolic name for a
C procedure.
Symbolic names are established by registering procedures via Itcl_RegisterC(). This is usually done in
the Tcl_AppInit() procedure, which is automatically called when the interpreter starts up. In the
following example, the procedure My_FooCmd() is registered with the symbolic name "foo". This procedure
can be referenced in the body command as "@foo".
int
Tcl_AppInit(interp)
Tcl_Interp *interp; /* Interpreter for application. */
{
if (Itcl_Init(interp) == TCL_ERROR) {
return TCL_ERROR;
}
if (Itcl_RegisterC(interp, "foo", My_FooCmd) != TCL_OK) {
return TCL_ERROR;
}
}
C procedures are implemented just like ordinary Tcl commands. See the CrtCommand man page for details.
Within the procedure, class data members can be accessed like ordinary variables using Tcl_SetVar(),
Tcl_GetVar(), Tcl_TraceVar(), etc. Class methods and procs can be executed like ordinary commands using
Tcl_Eval(). [incr Tcl] makes this possible by automatically setting up the context before executing the
C procedure.
This scheme provides a natural migration path for code development. Classes can be developed quickly
using Tcl code to implement the bodies. An entire application can be built and tested. When necessary,
individual bodies can be implemented with C code to improve performance.
KEYWORDS
class, object, object-oriented