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

From: Dave Abrahams (abrahams_at_[hidden])
Date: 2000-01-29 00:08:03

> 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 ;)

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 ;)