class(n) [incr Tcl] class(n) ______________________________________________________________________________
NAME
itcl::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 ...?
______________________________________________________________________________
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? ?-scope?
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. Flags can be specified with
varName in an arbitrary order. The result is a list of
the specific information in exactly the same order as the
flags are specified.
If no flags are given, this command returns a list as if
the followings flags have been specified:
-protection -type -name -init -value ?-config?
The -config result is only present if varName is a public
variable. It contains the code that is executed at
initialization of varName. The -scope flag gives the
namespace context of varName. Herewith the variable can
be accessed from outside the object like any other
variable. It is similar to the result of the itcl::scope
command.
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_RegisterObjC(). 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_FooObjCmd() 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_RegisterObjC(interp, "foo", My_FooObjCmd) != 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
itcl class(n)
tcl 8.6.17 - Generated Thu Aug 21 09:41:28 CDT 2025