From: Dave Thomas <Dave@...>
Date: 2000-06-19T12:03:17-05:00
Subject: [ruby-talk:03527] Re: Deep copy?

Conrad Schneiker <schneik@austin.ibm.com> writes:

> For the general education of other readers (e.g. myself), could someone
> please describe how deep copies are supported (in the sense of built-in
> features, if any) in other languages Python, Smalltalk, Eiffel, and
> Lisp/CLOS (or Scheme)?

The Smalltalk implementation is typical. We assume a protocol (in the
Smalltalk case it's simply support for the messages 'copy' and
'postCopy').


Objects are copied using

   fred  := bert copy.         # deep copy
   freda := betha shallowCopy.


'shallowCopy' is implemented in Object, and does the obvious stuff.


'copy' is also implemented in Object, but on its own it doesn't
work. Instead, every class that acts as a container has to add support 
for copying those contained objects. There are two ways of doing
this. The brute force way is to override the 'copy' method. Sometimes
this is required. However, the tidier way is to implement 'postCopy',
whose job is to copy just the contained objects. That's because the
default implementation of 'copy' is:

   copy
      ^self shallowCopy postCopy.

(so the 'shallowCopy' will copy the object itself, and the 'postCopy' then
handles the object specific stuff.) For example, postCopy for a bag in
Dolphin Smalltalk is implemented as

   postCopy
      super postCopy.
      original := contents.
      contents := original copyEmpty.
      original keysAndValuesDo: [:k :v | contents at: k put: v]




Regards


Dave