'''[http://www.tcl.tk/man/tcl/TclCmd/class.htm%|%oo::class]''', a [Tcl
commands%|%built-in] [Tcl] command, is the [class] of all classes in [TclOO].
All classes are instances of this class.  Consequently, oo::class is an
instance of itself.



** Documentation **

   [http://www.tcl.tk/man/tcl/TclCmd/class.htm%|%official reference]:   



** Description **

===
? [oo::object]
  ? [oo::class]
===



** Discussion **


[AMG]: Is there a compelling reason why [[oo::class new]] is hidden?

[DKF]: Because everyone treats classes as named entities with long lifetimes.
Expose it if you want.

[AMG]: How do I do this?  Sorry, I'm a total [TclOO] newb. :^)

[DKF]:

======
oo::define oo::class export new
======

Note that this does it for ''all'' classes. It might be better to do this
instead:

======
oo::define oo::class self export new
======

This only exports it for `oo::class` itself.

----

[AMG]: Can I define a destructor for a class?  Automatically deleting all
subclasses, instances, and "mixees" is good, but I'm working on a project that
needs some additional cleanup when a class is destroyed.

[DKF]: Yes. The simplest technique is to make your class an instance of a
subclass of `oo::class` and attach the destructor to that.

======
oo::class create class-with-destructor {
    superclass oo::class
    destructor {
        puts "bye!"
    }
}

class-with-destructor create MyClass {
    # ...
}
======

But if you've already got your class instance, do not despair! You can inject
the destructor through modifying the class instance's superclass with
[oo::objdefine]:

======
oo::objdefine MyClass class class-with-destructor
======

If you're changing objects' classes, remember that classes cannot be changed to
non-classes or ''vice versa'', and both [oo::object] and `oo::class` cannot
have their class changed (they're special, for the sake of sanity). ''(And this
is something where a mixin won't work; mixins are currently ignored for
destructor processing, and while maybe that shouldn't be so, it's how things
are now.)''

[AMG]: Wait a second, according to
[http://www.tcl.tk/man/tcl/TclCmd/define.htm#M8], "the destructor is called
when objects of the class are deleted".  I want a destructor for when the class
itself is deleted.

[DKF]: Yes. I know:

======
% oo::class create class-with-destructor {
    superclass oo::class
    destructor {
        puts "bye!"
    }
}
::class-with-destructor
% oo::class create foo {
  destructor { puts "killing [self]" }
}
::foo
% foo create bar
::bar
% oo::objdefine foo class class-with-destructor
% foo create bar2
::bar2
% bar2 destroy
killing ::bar2
% foo destroy
bye!
killing ::bar
======

Is that not what you want? (Note that class destructors are processed before
instances; guaranteeing order to be the other way is somewhat tricky…)

[AMG]: No, it's not what I want.  I want to schedule code to run when
[[class-with-destructor destroy]] is invoked, or the class is destroyed by
other means (e.g. [rename] to "").  I have external data associated with the
class (not the instances), and that external data needs to be cleaned up when
the class goes away.



** See Also **

   * [Semantics of the oo::class command]

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