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From: barmar@think.com (Barry Margolin)
Newsgroups: comp.lang.lisp
Subject: Re: Problems Copying Structures
Keywords: defstruct structures death copying
Message-ID: <1990Dec8.180825.21702@Think.COM>
Date: 8 Dec 90 18:08:25 GMT
References: <1990Dec8.071142.5318@cec1.wustl.edu>
Sender: news@Think.COM
Organization: Thinking Machines Corporation, Cambridge MA, USA
Lines: 41

Ian Flanigan complained that when he copies a structure whose slots contain
arrays, and then modifies one of the arrays, the array in the other
structure is also modified.

Structure copiers are only supposed to copy the structure itself, not the
objects referenced by the structure.  They are like COPY-LIST rather than
COPY-TREE.  If you want more, you have to write your own copier function
that does precisely what you want.  There's no standard function that's
able to read the programmer's mind and determine which parts of a structure
should be copied and which should just be referenced.

Such a function is not too hard to write.  Given something like:

(defstruct my-structure
  (slot1 (make-array ...))
  (slot2 (make-array ...))
  (slot3 (make-array ...)))

(defun copy-my-structure-and-contents (my-s)
  (let ((old1 (my-structure-slot1 my-s))
	(old2 (my-structure-slot2 my-s))
	(old3 (my-structure-slot3 my-s)))
    (make-my-structure
      :slot1 (make-array (array-dimensions old1)
			 :element-type (array-element-type old1)
			 :initial-elements old1)
      :slot2 (make-array (array-dimensions old2)
			 :element-type (array-element-type old2)
			 :initial-elements old2)
      :slot3 (make-array (array-dimensions old3)
			 :element-type (array-element-type old3)
			 :initial-elements old3))))

Note that this still only copies one level more than the
automatically-generated copier.  If you also want to copy the objects
referenced in the arrays you'll have to do more.
--
Barry Margolin, Thinking Machines Corp.

barmar@think.com
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