Re: Destructors can copying objects

From	Ruben Safir <ruben@mrbrklyn.com>
Newsgroups	comp.lang.c++
Subject	Re: Destructors can copying objects
Date	2011-04-04 22:54 +0000
Organization	PANIX Public Access Internet and UNIX, NYC
Message-ID	<indi6q$r87$7@reader1.panix.com> (permalink)
References	(3 earlier) <in9erf$2d82$1@news.ett.com.ua> <dKqdnUe1du6RxwXQnZ2dnUVZ8kGdnZ2d@giganews.com> <indbul$r87$3@reader1.panix.com> <HOSdna4IXtArpQfQnZ2dnUVZ8vOdnZ2d@giganews.com> <indh8e$6mc$1@reader1.panix.com>

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On Mon, 04 Apr 2011 22:38:06 +0000, Ruben Safir wrote:

> On Mon, 04 Apr 2011 22:31:35 +0100, Leigh Johnston wrote:
> 
>> On 04/04/2011 22:07, Ruben Safir wrote:
>>> On Sun, 03 Apr 2011 12:29:00 +0100, Leigh Johnston wrote:
>>>
>>>> On 03/04/2011 10:32, Balog Pal wrote:
>>>>> "Ruben Safir"<ruben@mrbrklyn.com>
>>>>>> I don't think I have a copy constructor, which might be the problem
>>>>>> although it never had been.
>>>>>
>>>>> Look up "rule of 3". If you have a dtor, you will need a copy cotor
>>>>> and operator= too, or must disable them.
>>>>>
>>>>>
>>>> Like most "rules" mentioned in this newsgroup the "rule of 3" is a
>>>> nonsense: you may be adding a virtual dtor simply to make a base
>>>> class polymorphic and have no need for a copy ctor and assignment
>>>> operator or you may be relying on RAII and the behaviour of the
>>>> implicitly defined special member functions.
>>>>
>>>> /Leigh
>>>
>>>
>>> Its hard for me to make that judgment though, without a more concrete
>>> understanding of how destructors are triggered with auto values.
>>>
>>> I'm not sure why any allocated memory created with new and dropped on
>>> the heap is auto destructed
>>>
>>>
>> Well the typical example of when to use the rule of 3 is if you have a
>> member variable pointer to allocated memory and it would be incorrect
>> to copy that pointer from one object to another.  If this was a pointer
>> to a dynamic array for example then I would use std::vector instead and
>> the rule of 3 requirement disappears as vectors can be safely copied
>> between objects.
>> 
>> /Leigh
> 
> 
> 
> If a function creates a new object with new
> 
> class myobj{
> 
> obj * myfunct(){
> 	obj* a = new obj;
> 	return a;
> }
> }
> myobj::~myobj(){
> 	delete a;
> }
> 
> int main(int argv, char ** argc){
> 
>     obj * out;
>     myobj * parent = new myobj;
>     out = parent->myfunct();
> 
>   return 0;
> }
> 
> 
> Why should this be a problem?  The pointer of out should be addressing
> valid memory on the heap.
> 
> Ruben

Actually - here is a more coherent example


#include<iostream>
#include<string>

class obj{
   public:
      obj():a(10), b("Hello World") {}
      int a;
      std::string b;
};

class myobj{

public:
myobj(){
   b = myfunct();
}

~myobj();
obj * b;

obj * myfunct(){
	obj* a = new obj;
	return a;
}
};
myobj::~myobj(){
	delete b;
}

int main(int argv, char ** argc){

    obj * out;
    obj * out2;

    myobj * parent = new myobj;
    out = parent->myfunct();
    out2 = parent->b;

   std::cout << out->a << " " << out->b << std::endl;

  return 0;
}

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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