Re: [boost] Problem with shared_ptr

From: Beman Dawes (bdawes_at_[hidden])
Date: 2001-05-16 06:56:53

At 03:29 AM 5/16/2001, Thomas Maeder wrote:

 >This problem report is a result of a discussion in
 >news://newsgroups.borland.com/borland.public.cppbuilder.language
 >
 >The following program demonstrates it, using boost 1.21.1 and gcc 2.95.3:
 >
 >#include <iostream>
 >
 >#include "boost/smart_ptr.hpp"
 >
 >struct S
 >{
 >    ~S()
 >    {
 >      std::cout << "~S()\n";
 >    }
 >
 >    boost::shared_ptr<S> next;
 >};
 >
 >int main()
 >{
 >  boost::shared_ptr<S> head(new S);
 >  head->next.reset(new S);
 >  std::cout << "before assignment\n";
 >  head = head->next;
 >  std::cout << "after assignment\n";
 >}
 >
 >The output is
 >
 >before assginment
 >~S()
 >~S()
 >after assginment

Ouch!

 >while one would expect it to be
 >
 >before assignment
 >~S()
 >after assignment
 >~S()
 >
 >The reason is that the shared_ptr copy-assignment operator first reduces
 >the reference count of the previously owned object and only then 
increases
 >that of the newly owned object; if both objects are the same, strange
 >things happen.

It isn't really a case of both objects being the same, but that the right 
hand object is transitively dependent on the left hand object.

The code causing the problem is:

    void share(T* rpx, long* rpn) {
       if (pn != rpn) { // Q: why not px != rpx? A: fails when both == 0
          dispose();
          px = rpx;
          ++*(pn = rpn);
       }

My first reaction is to change it to:

    void share(T* rpx, long* rpn) {
       if (pn != rpn) { // Q: why not px != rpx? A: fails when both == 0
          ++*rpn; // done before dispose() in case rpn transitively
                  // dependent on *this (bug reported by Thomas Maeder)
          dispose();
          px = rpx;
          pn = rpn;
       }

I'll have give it a bit more thought.  Greg?  Also do some testing.  The 
test program needs updating anyhow.

Thanks for the bug report.

--Beman