TIP #397: EXTENSIBLE OBJECT COPYING 
=====================================
 Version:      $Revision: 1.4 $
 Author:       Donal K. Fellows <donal.k.fellows_at_manchester.ac.uk>
 State:        Final
 Type:         Project
 Tcl-Version:  8.6
 Vote:         Done
 Created:      Monday, 13 February 2012
 URL:          https://tip.tcl-lang.org397.html
 Post-History: 

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 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: 
 <URL:https://core.tcl.tk/tcloo/timeline?r=development-rfe3485060> 

 COPYRIGHT 
===========

 This document has been placed in the public domain. 

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