## Tcl8.6.0/Tk8.6.0 Documentation > Tcl Commands, version 8.6.0 > class

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  NAME
    oo::class — class of all classes
  SYNOPSIS
  CLASS HIERARCHY
  DESCRIPTION
  CONSTRUCTOR
  DESTRUCTOR
  EXPORTED METHODS
    cls create name ?arg ...?
    cls new ?arg ...?
  NON-EXPORTED METHODS
    cls createWithNamespace name nsName ?arg ...?
  EXAMPLES
  SEE ALSO
  KEYWORDS

### NAME

oo::class — class of all classes

### SYNOPSIS

package require TclOO
oo::class method ?arg ...?

### CLASS HIERARCHY

oo::object
→ oo::class

### DESCRIPTION

Classes are objects that can manufacture other objects according to a pattern stored in the factory object (the class). An instance of the class is created by calling one of the class's factory methods, typically either create if an explicit name is being given, or new if an arbitrary unique name is to be automatically selected.

The oo::class class is the class of all classes; every class is an instance of this class, which is consequently an instance of itself. This class is a subclass of oo::object, so every class is also an object. Additional metaclasses (i.e., classes of classes) can be defined if necessary by subclassing oo::class. Note that the oo::class object hides the new method on itself, so new classes should always be made using the create method.

#### CONSTRUCTOR

The constructor of the oo::class class takes an optional argument which, if present, is sent to the oo::define command (along with the name of the newly-created class) to allow the class to be conveniently configured at creation time.

#### DESTRUCTOR

The oo::class class does not define an explicit destructor. However, when a class is destroyed, all its subclasses and instances are also destroyed, along with all objects that it has been mixed into.

#### EXPORTED METHODS

cls create name ?arg ...?
  This creates a new instance of the class cls called name (which is resolved within the calling context's namespace if not fully qualified), passing the arguments, arg ..., to the constructor, and (if that returns a successful result) returning the fully qualified name of the created object (the result of the constructor is ignored). If the constructor fails (i.e. returns a non-OK result) then the object is destroyed and the error message is the result of this method call.

cls new ?arg ...?
  This creates a new instance of the class cls with a new unique name, passing the arguments, arg ..., to the constructor, and (if that returns a successful result) returning the fully qualified name of the created object (the result of the constructor is ignored). If the constructor fails (i.e., returns a non-OK result) then the object is destroyed and the error message is the result of this method call.

  Note that this method is not exported by the oo::class object itself, so classes should not be created using this method.

#### NON-EXPORTED METHODS

The oo::class class supports the following non-exported methods:

cls createWithNamespace name nsName ?arg ...?
  This creates a new instance of the class cls called name (which is resolved within the calling context's namespace if not fully qualified), passing the arguments, arg ..., to the constructor, and (if that returns a successful result) returning the fully qualified name of the created object (the result of the constructor is ignored). The name of the instance's internal namespace will be nsName unless that namespace already exists (when an arbitrary name will be chosen instead). If the constructor fails (i.e., returns a non-OK result) then the object is destroyed and the error message is the result of this method call.

### EXAMPLES

This example defines a simple class hierarchy and creates a new instance of it. It then invokes a method of the object before destroying the hierarchy and showing that the destruction is transitive.

    oo::class create fruit {
        method eat {} {
            puts "yummy!"
        }
    }
    oo::class create banana {
        superclass fruit
        constructor {} {
            my variable peeled
            set peeled 0
        }
        method peel {} {
            my variable peeled
            set peeled 1
            puts "skin now off"
        }
        method edible? {} {
            my variable peeled
            return $peeled
        }
        method eat {} {
            if {![my edible?]} {
                my peel
            }
            next
        }
    }
    set b [banana new]
    $b eat               → prints "skin now off" and "yummy!"
    fruit destroy
    $b eat               → error "unknown command"

### SEE ALSO

oo::define, oo::object

### KEYWORDS

class, metaclass, object
Copyright © 2007 Donal K. Fellows