[boost] Re: new approach to smart ptrs -- no new()s is goodnews

From: Greg Colvin (gcolvin_at_[hidden])
Date: 2000-01-29 00:13:39

From: Dave Abrahams <abrahams_at_[hidden]>
> > The copy code for the current implementation expands to this:
> > 
> >       if (pn != rpn) {
> >          if (--*pn == 0) { delete px; delete pn; }
> >          px = rpx;
> >          ++*(pn = rpn);
> >       }
> >
> > The copy code for Mark's implementation expands to this:
> >
> >       lastref = !left && !right;
> >       if (left || right)
> >       {
> >          if (left)
> >             left->right = right;
> >          if (right)
> >             right->left = left;
> >          right = left = 0;
> >       }
> >       if (lastref && ptr)
> >          delete ptr;
> >       ptr = pNewLeft->ptr;
> >       left = pNewLeft;
> >       right = pNewLeft->right;
> >       pNewLeft->right = this;
> >       if (right)
> >          right->left = this;
> 
> Hmm. Never bet against the man with 6+ years' experience in one topic ;)

Not only that topic, thank God.

> 1. There's no need to check for ptr being 0 before deleting it
> 2. and of course, using a circular list could cut down the code somewhat:
> 
> if (left == this)
>     delete ptr;
> left->right = right; // unlink
> right->left = left;
> ptr = pNewLeft->ptr;       // adopt
> left = pNewLeft;    // link
> right = pNewLeft->right;
> right->left = this;
> pNewLeft->right = this;
> 
> I notice this has the advantage of containing more straight-line code than
> the current implementation does (one branch vs. two). I'd rather see a test
> of the circular case ;)

This looks much better, and may indeed be much faster.  I'll leave it
to Mark to tweak his code to get this expansion.

I still count 6 loads/stores versus 12 loads/stores.

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