Re: Destructors can copying objects

From	Yannick Tremblay <yannick@tremnblay.co.uk>
Newsgroups	comp.lang.c++
Subject	Re: Destructors can copying objects
Date	2011-04-05 11:36 +0000
Organization	A noiseless patient Spider
Message-ID	<ineusg$pf4$1@dont-email.me> (permalink)
References	<in76i3$105$2@reader1.panix.com> <incf69$9fr$1@dont-email.me> <indbq3$r87$2@reader1.panix.com>

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On 2011-04-04, Ruben Safir <ruben@mrbrklyn.com> wrote:
> On Mon, 04 Apr 2011 12:56:41 +0000, Yannick Tremblay wrote:
>
>>> ======================================= /* constructor */
>>> Node<unk>( unk value, Node<unk> *item_to_link_to = 0); Node ( const
>>> Node &other ); /* copy constructor - Need to write this! */
>> 
>> Stop here!
>> 
>> Could you clarify to us why the following comments is there: /* copy
>> constructor - Need to write this! */
>
> Because I haven't written it yet.
>
> The purpose of the question is to understand what is happening, not to necessarily
> fix the program.  This is not a homework assignment with a deadline.  I just want to
> have a better understanding of why the destructor caused a double delete.

This was a leading question.  

copy-constructor are not always needed for all classes.  However, for
some classes a copy constructor is absolutely needed.

Here you have a header file that explicitely says (correctly given the
your class contains raw pointers) that the copy contructor need to be
written.  I am not sure if you wrote it yourself as a reminder or if
it was a format that was specified externally from a template that
attempt to remind peoples that they should write copy-constructors.  

Regardless, despite the presence of this reminder, you have not
written the copy-constructor. That should very probably be your first
point of call.  

Your destructor cause double delete because your copy constructor and
your destructor are incompatible.  Write a copy-constructor that is
compatible with your destructor or write a copy-constructor and modify
your destructor so both are compatible and you will not have a bug
anymore.  

So essentially you have a Node that contains a pointer to value and a
pointer to next.  You have a destructor that delete the pointer to
value.

What currently happens if you copy a node?  
(hint: compiler generated copy constructor)
(hint: what will be the values of value_ and next_ in the copy and in
the original?)

What will happen if you delete this node copy?

What will happen if you delete the original node?

Yannick

Thread

Destructors can copying objects Ruben Safir <ruben@mrbrklyn.com> - 2011-04-02 12:58 +0000
  Re: Destructors can copying objects Volker Lukas <vlukas@gmx.de> - 2011-04-02 15:29 +0200
    Re: Destructors can copying objects Ruben Safir <ruben@mrbrklyn.com> - 2011-04-03 02:59 +0000
      Re: Destructors can copying objects "Balog Pal" <pasa@lib.hu> - 2011-04-03 11:32 +0200
        Re: Destructors can copying objects Leigh Johnston <leigh@i42.co.uk> - 2011-04-03 12:29 +0100
          Re: Destructors can copying objects Bart van Ingen Schenau <bart@ingen.ddns.info> - 2011-04-04 13:43 +0200
            Re: Destructors can copying objects Michael Doubez <michael.doubez@free.fr> - 2011-04-05 01:26 -0700
          Re: Destructors can copying objects Joshua Maurice <joshuamaurice@gmail.com> - 2011-04-04 14:03 -0700
            Re: Destructors can copying objects Ruben Safir <ruben@mrbrklyn.com> - 2011-04-04 21:10 +0000
            Re: Destructors can copying objects Leigh Johnston <leigh@i42.co.uk> - 2011-04-04 22:28 +0100
          Re: Destructors can copying objects Ruben Safir <ruben@mrbrklyn.com> - 2011-04-04 21:07 +0000
            Re: Destructors can copying objects Leigh Johnston <leigh@i42.co.uk> - 2011-04-04 22:31 +0100
              Re: Destructors can copying objects Leigh Johnston <leigh@i42.co.uk> - 2011-04-04 22:43 +0100
              Re: Destructors can copying objects Ruben Safir <ruben@mrbrklyn.com> - 2011-04-04 22:38 +0000
                Re: Destructors can copying objects Leigh Johnston <leigh@i42.co.uk> - 2011-04-04 23:42 +0100
                Re: Destructors can copying objects Ruben Safir <ruben@mrbrklyn.com> - 2011-04-04 22:54 +0000
      Re: Destructors can copying objects James Kanze <james.kanze@gmail.com> - 2011-04-03 10:07 -0700
        Re: Destructors can copying objects Ruben Safir <ruben@mrbrklyn.com> - 2011-04-04 21:14 +0000
        Re: Destructors can copying objects Ruben Safir <ruben@mrbrklyn.com> - 2011-04-04 21:17 +0000
          Re: Destructors can copying objects LR <lruss@superlink.net> - 2011-04-05 01:52 -0400
    Re: Destructors can copying objects Ruben Safir <ruben@mrbrklyn.com> - 2011-04-03 03:08 +0000
      Re: Destructors can copying objects Paavo Helde <myfirstname@osa.pri.ee> - 2011-04-03 02:11 -0500
      Re: Destructors can copying objects Volker Lukas <vlukas@gmx.de> - 2011-04-03 12:02 +0200
      Re: Destructors can copying objects Peter Remmers <p.remmers@expires-2011-04-30.arcornews.de> - 2011-04-03 12:14 +0200
        Re: Destructors can copying objects Roberto Waltman <usenet@rwaltman.com> - 2011-04-03 18:16 -0400
          Re: Destructors can copying objects Ruben Safir <ruben@mrbrklyn.com> - 2011-04-15 23:06 +0000
            Re: Destructors can copying objects LR <lruss@superlink.net> - 2011-04-15 19:19 -0400
        Re: Destructors can copying objects Ruben Safir <ruben@mrbrklyn.com> - 2011-04-15 23:03 +0000
          Re: Destructors can copying objects "Alf P. Steinbach /Usenet" <alf.p.steinbach+usenet@gmail.com> - 2011-04-16 01:47 +0200
            Re: Destructors can copying objects Peter Remmers <p.remmers@expires-2011-04-30.arcornews.de> - 2011-04-16 13:00 +0200
              Re: Destructors can copying objects "Alf P. Steinbach /Usenet" <alf.p.steinbach+usenet@gmail.com> - 2011-04-16 15:57 +0200
  Re: Destructors can copying objects LR <lruss@superlink.net> - 2011-04-02 09:51 -0400
  Re: Destructors can copying objects Yannick Tremblay <yannick@tremnblay.co.uk> - 2011-04-04 12:56 +0000
    Re: Destructors can copying objects Ruben Safir <ruben@mrbrklyn.com> - 2011-04-04 21:05 +0000
      Re: Destructors can copying objects Paavo Helde <myfirstname@osa.pri.ee> - 2011-04-04 21:56 -0500
      Re: Destructors can copying objects Yannick Tremblay <yannick@tremnblay.co.uk> - 2011-04-05 11:36 +0000
        Re: Destructors can copying objects Ruben Safir <ruben@mrbrklyn.com> - 2011-04-05 15:40 +0000
          Re: Destructors can copying objects Sana <sanatakos@gmail.com> - 2011-04-05 10:11 -0700
          Re: Destructors can copying objects Ruben Safir <ruben@mrbrklyn.com> - 2011-04-05 20:28 +0000
          Re: Destructors can copying objects Yannick Tremblay <yannick@tremnblay.co.uk> - 2011-04-06 11:12 +0000
      Re: Destructors can copying objects drew@furrfu.invalid (Drew Lawson) - 2011-04-05 13:02 +0000
        Re: Destructors can copying objects Ruben Safir <ruben@mrbrklyn.com> - 2011-04-05 15:51 +0000
          Re: Destructors can copying objects drew@furrfu.invalid (Drew Lawson) - 2011-04-05 17:47 +0000
            Re: Destructors can copying objects Ruben Safir <ruben@mrbrklyn.com> - 2011-04-05 20:25 +0000
              Re: Destructors can copying objects drew@furrfu.invalid (Drew Lawson) - 2011-04-05 20:37 +0000

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