Re: [boost] Self-assignment Guards No Longer Necessary in shared_ptr

From: Greg Colvin (gcolvin_at_[hidden])
Date: 2001-06-05 21:30:29

I'm convinced.  Thanks.  If passes our tests I think
it should go in.

From: Joe Gottman <joegottman_at_[hidden]>
> With the new versions of shared_ptr and shared_array, it is no longer
> necessary to guard against self-assignment.  Here is the current version of
> the share() method, which is used to implement assignment (I added the
> numbers to the left)
> 
> void share(T* rpx, long* rpn) {
>       if (pn != rpn) { // Q: why not px != rpx? A: fails when both == 0
> 1)         ++*rpn; // done before dispose() in case rpn transitively
>                  // dependent on *this (bug reported by Ken Johnson)
> 2)        dispose();
> 3)         px = rpx;
> 4)         pn = rpn;
>       }
> 
> 
> Consider what would happen if the "if (pn != rpn) clause didn't appear and
> pn were equal to rpn.  In step 1),  *rpn would be incremented by 1.  Since
> *rpn was originally at least 1, after this step it would be at least 2.
> Then in step 2), dispose() would decrement *rp.  Since (pn == rpn) and by
> the above *rpn is at least 2, after this step *pn is at least 1.  Thus,
> dispose() will not delete pn and px, and step 1) followed by step 2) results
> in a no-op.  Then step 3) and step 4) assign rpn to pn and rpx to px.  Since
> they were equal to begin with, these are also no-ops.  Thus, the
> self-assignment guards are not necessary.  Their only effect is to speed up
> the code in the case of self-assignments at the expense of slowing it down
> in when there is no self-assignment.  Since self-assignment is rare, the if
> clause should be eliminated.
> 
> Joe Gottman
> 
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