Re: [Boost-Users] Why not "if (px != rpx) " ?

From: Louis Lavery (Louis_at_[hidden])
Date: 2001-12-14 16:40:34

----- Original Message ----- 
From: Peter Dimov <pdimov_at_[hidden]>
To: <Boost-Users_at_[hidden]>
Sent: Friday, 14 December, 2001 2:33 PM
Subject: Re: [Boost-Users] Why not "if (px != rpx) " ?

> From: "Louis Lavery" <Louis_at_[hidden]>
> > Hello,
> > 
> > The following is use by the boost shared_ptr assignment operators
> > (see boost\smart_ptr.hpp)...
> > 
> >    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 Ken Johnson)
> >          dispose();
> >          px = rpx;
> >          pn = rpn;
> >       }
> >    } // share
> > 
> > ...I curious to know what it means by "fails when both == 0".
> 
> It's possible that px == rpx == 0, yet pn != rpn.
> 
> --
> Peter Dimov
> Multi Media Ltd.

Hello,

    Yes, I see that, but I can't see what's wrong with using the
test "if ( px != rpx )". 

Or, to put it another way, if px == rpx == 0 who cares whether
pn equals rpn or not? 

Louis.