TIP:		397
Title:		Extensible Object Copying
State:		Final
Type:		Project
Tcl-Version:	8.6
Vote:		Done
Post-History:	
Version:	$Revision: 1.4 $
Author:		Donal K. Fellows <donal.k.fellows@manchester.ac.uk>
Created:	13-Feb-2012
Keywords:	Tcl, TclOO, copy, clone

~ Abstract

This TIP proposes a mechanism whereby an object or class can provide
additional control over how it gets copied to a new one by the '''oo::copy'''
command.

~ Rationale

I always knew that TclOO's '''oo::copy''' command was not particularly
complete, in that only the C-level state was copied. The vast majority of
state associated with an object (notably including its variables) was not
copied on the grounds that the caller of '''oo::copy''' could do that for
themselves. However, this has not proved particularly workable in practice
once someone actually started to use the mechanism. It turns out that the
callers of '''oo::copy''' expect a full copy of the object to be created.

This is quite challenging because some things associated with an object are
tricky to copy correctly. For example, if an object has a number of Tk widgets
under its control, or has timer callbacks set up, these are very challenging
for a generic object copying scheme to detect (and generic copying of Tcl
commands is difficult anyway). Fortunately, we already have a mechanism for
handling such per-object variation: methods.

~ Proposed Change

I propose that the '''oo::copy''' command will internally call the newly
created object's '''<cloned>''' method (passing in the name of the source
object as the only argument) to allow for the customization of the copy. If
the method fails (throws an exception), the copy will be destroyed and the
error propagated.

 > ''copiedObject'' '''<cloned>''' ''sourceObject''

Furthermore, a default implementation of the '''<cloned>''' method will be
created in '''oo::object''' which will duplicate the variables and procedures
(note, not other commands) of the source to the copy. This method will not be
exported by default (nor should it be; the method is not intended to be called
directly).

~ Reference Implementation

See the "development-rfe3485060" branch of the tcloo repository:
https://core.tcl.tk/tcloo/timeline?r=development-rfe3485060

~ Copyright

This document has been placed in the public domain.