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From: mittle@blinn.watson.ibm.com (Josh Mittleman)
Newsgroups: comp.lang.c++
Subject: Re: Want pointers on intelligent pointers
Message-ID: <1991Mar18.220013.18185@watson.ibm.com>
Date: 18 Mar 91 22:00:13 GMT
References: <838@antares.Concordia.CA>
Sender: @watson.ibm.com
Reply-To: mittle@ibm.com
Organization: IBM T. J. Watson Research
Lines: 66


In article <838@antares.Concordia.CA>, marcap@antares.concordia.ca (
MARC ANDREW PAWLOWSKY ) writes:

I think you're may be going about the reference count backward.  The
reference count is controlled by the smart pointer, not the object.
Consider this approach.

class Object {
friend class ObjectP;
protected:
  int refcount;
  Object() 			{ refcount = 1; }
public:
  ~Object();
};

class ObjectP {
protected:
  Object* myOb;
public:
  ObjectP()	  		{ myOb = new Object(); }
  ObjectP(const ObjectP& ObP);	{ myOb = ObP.myOb; myOb->refcount++; }
  ~ObjectP();			{ if (myOb->refcount == 1)
				     delete myOb;
				  else
				     myOb->refcount--;
				}
};

ObjectP::ObjectP() {
  myOb = new Object();
}

When you need an Object, you actually declare an ObjectP.

> In my creation routines can I refer the the object being created?
> I have a class Box which has a global variable called "default_box".
> I also have a function "operator =" that will copy a box into
> this.  can I do something like
> 
> Box::Box(void) {
>     this = default_box; // does not work with TC++ 1.01
>     return this; // return not allowed
> }

The first line is an anachronism (see ARM, p.406, Assignment to this).  The
type of this in the class Box is Box *const (ARM, p.177).

The C++ approach would be to include a field-by-field assignment in Box,
like this:

Box::Box() {
  width = 50;
  height = 20;
}

This code does refer to the object being constructed, by referencing its
members.  Constructors do not return anything in C++.  On the other hand,
the operator new does.  This is correct:

Box *B = new Box;

===========================================================================
Josh Mittleman (mittle@ibm.com or joshua@paul.rutgers.edu)
J2-C28 T.J. Watson Research Center, PO Box 704, Yorktown Heights, NY  10598