Re: Destructors can copying objects

From:

Leigh Johnston <leigh@i42.co.uk>

Newsgroups:

comp.lang.c++

Date:

Mon, 04 Apr 2011 23:42:06 +0100

Message-ID:

<TpOdnVPYxu-h1AfQnZ2dnUVZ8hqdnZ2d@giganews.com>

On 04/04/2011 23:38, 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.

Sorry but I cannot make any sense of your reply; your code is ill-formed
and I don't understand the question.

/Leigh