Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!ames!uakari.primate.wisc.edu!polyslo!ttwang
From: ttwang@polyslo.CalPoly.EDU (Thomas Wang)
Newsgroups: comp.lang.c++
Subject: Re: Support for movable objects in C++
Message-ID: <1989Oct4.161939.12442@polyslo.CalPoly.EDU>
Date: 4 Oct 89 16:19:39 GMT
References: <1989Sep30.051559.20644@polyslo.CalPoly.EDU> <6590275@hplsla.HP.COM>
Reply-To: ttwang@polyslo.CalPoly.EDU (Thomas Wang)
Distribution: usa
Organization: Cal Poly State University -- San Luis Obispo
Lines: 75

I have come up with what I think is a better alternative than
'volatile classes'.  Instead of using master pointers, each member
function is required to declare a local variable of 'watcher_t'.
The 'watcher_t' class keep track of the address of 'this' on the stack.
When the objects are moved, a post watcher list traversal will make all
the values of 'this' point to the right location.

This means that I can use modern GC methods with real copying.


#ifndef WATCHER_T
#define WATCHER_T

// File:    watcher_t.h
// Author:  Thomas Wang
// Purpose: watch the value of 'this' on the stack & fix them when necessary
// Date:    Oct 3, 1989
// Usage:   put watcher_t watcher(&this); at start of every member function

class watcher_t
{
  friend class gc_root_t;

  int* this_location;
  watcher_t* prev_link;
  watcher_t* next_link;

  void update_this(); // make 'this' point to the right location

public:
  watcher_t(void*);
  // constructor telling the address of 'this' for gc objects

  ~watcher_t()
  {
    prev_link->next_link = next_link;
    next_link->prev_link = prev_link;
  }

};


#endif


// File:    watcher_t.cc
// Author:  Thomas Wang
// Purpose: watch the value of 'this' on the stack & fix them when necessary
// Date:    Oct 3, 1989
// Usage:   put watcher_t watcher(&this); at start of every member function

#include "watcher_t.h"

// make 'this' point to the right location
void watcher_t::update_this()
{
  static int* forward_ptr;
  forward_ptr = this_location - 1;
  if (*forward_ptr)
    *this_location = *forward_ptr; // fix the value of 'this'
}

// constructor telling the address of 'this' for gc objects
watcher_t::watcher_t(void* val)
{
  this_location = (int*) val;
  the_root.insert_watcher(this);
}


 -Thomas Wang ("This is a fantastic comedy that Ataru and his wife Lum, an
                invader from space, cause excitement involving their neighbors."
                  - from a badly translated Urusei Yatsura poster)

                                                     ttwang@polyslo.calpoly.edu