class(3itcl)

bestie, welcome to the class(3itcl) page from [incr Tcl] 💖 this one is all about itcl::class, and it's giving main character energy.

NAME 💅

itcl::class — create a class of objects. That's the name, periodt.

SYNOPSIS ✨

okay so the shape of it is literally this, no cap:

       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. ngl, that's the whole vibe. Every 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. lowkey obsessed.

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. not me mentally separating those two, but okay.

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. iykyk.

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. that's the highkey useful part for debugging bestie.

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. vibe check: go query them.

A class can be deleted using the delete class command. Individual objects can be deleted using the delete object command. periodt.

✨ CLASS DEFINITIONS ✨

Okay bestie, so you wanna define a class? Say less. 💅

• class className definition — This gives you the definition for a class named className. If that class already exists, or if there's already a command called className hanging out in the current namespace context, this command is gonna throw an error, no cap. But if the class definition parses successfully, className becomes a command in the current context, and it handles making objects for this class. Periodt. 👑

The class definition gets evaluated as a series of Tcl statements that define elements within the class. Here are the class definition commands you can use, bestie:

• inherit baseClass ?baseClass...? — Causes the current class to inherit characteristics from one or more base classes. The classes must have been defined by a previous class command, or they need to be available to the auto-loading facility (see "AUTO-LOADING" below, iykyk). A single class definition can contain no more than one inherit command. That's the rule. 🚫

The order of baseClass names in the inherit list affects 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. So if classes "Foo" and "Bar" both contain the member "x", and another class has this 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". Slay. ✨

• 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. Simple as that. 💁‍♀️

• 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. So yeah, it's the reverse vibe. 🔄

• 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. No cap.

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 with 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. Got it? 💅

• 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. So no object-specific tea. ☕

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. Easy. ✨

• 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. So like, you got options. 🧚

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. Periodt. 💅

• 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. So they're like the group chat everyone can see. 👀

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. So watch out for that. 😭

• 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 or common, 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. So you can lock things down. 🔒

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🌈 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. Let's go. 🚀

• 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. So like, it cleans up after itself. 🧹

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 also be buried within an object name:

fileselectiondialog .foo.bar.#auto -background red

This would generate an object named ".foo.bar.fileselectiondialog0". So cute. 🎀

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💫 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. Bet. 💅

• 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 💅

okay bestie, every itcl object rolls up with these built-in methods, and they're lowkey the whole personality. let's get into it 💖

• objName cget option — gives you access to public variables as configuration options. it's giving "the usual cget operation for Tk widgets," fr. the option argument is a string shaped like -varName, and this method hands you back the current value of the public variable varName. slay.
• objName configure ?option? ?value option value ...? — same energy: public variables as configuration options, mimicking the usual configure operation for Tk widgets. here's the tea:
  • With no arguments, this method returns a list of lists describing all the public variables. each little list has three elements: the variable name, its initial value, and its current value.
  • If you pass a single option of the form -varName, then this method returns the info for that one variable. just the one, bestie.
  • Otherwise, the arguments are treated as option/value pairs, assigning new values to public variables. each variable gets assigned its new value, and if it has any config code associated with it, that code is executed in the context of the class where it was defined. and if that config code generates an error? 😭 the variable is set back to its previous value, and the configure method returns an error. no cap, it snaps right back.
• objName isa className — returns non-zero if the given className can be found in the object's heritage, and zero otherwise. simple as that, periodt.
• 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, plus all the options recognized by the usual Tcl info command:
  • objName info class — returns the name of the most-specific class for object objName. the main character of its lineage ✨
  • 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. it's the full family tree, bestie 🌸
  • 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. and 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 that list, iykyk.
  • 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 — any order you want, she's flexible 💅. 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 following 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.

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🔗 CHAINING METHODS/PROCS 🔗

sometimes a base class has a method or proc that gets redefined with the same name in a derived class. and honestly? that's a feature, not a bug. it's a way of making the derived class handle the same operations as the base class, but with its own specialized behavior. the glow-up, if you will 💖

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
    }
}

so we might create another class like SmartToaster that redefines the toast method. and 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]
    }
}

but instead of hard-coding the base class name, we can use the chain command like this — way cuter, ngl:

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. and if another implementation is not found? this command does nothing and returns the null string. iconic behavior honestly, no notes.

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🌈 AUTO-LOADING 🌈

class definitions need not be loaded explicitly — they can be loaded as needed by the usual Tcl auto-loading facility. so lazy, so valid, we love that for them 🦋

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. let her do the work ✨

and the auto-loader must be made aware of this directory by appending the directory name to the auto_path variable. once that's in place, classes will be auto-loaded as needed when used in an application. periodt.

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🧚 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. yes bestie, a whole crossover episode 💀

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, iykyk 📖. 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. she's got your back 🫶

and this scheme provides a natural migration path for code development — here's the roadmap:

• 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.

start cute, optimize later. the pipeline 💅

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🔑 KEYWORDS 🔑

class, object, object-oriented

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and that's a wrap on this whole page, bestie 🎀 this one came from the itcl crew and it's the class(3itcl) page — go forth and build something iconic with it 💖✨