Re: References to hash elements?

[ apologies for the duplicate email message from my last post--mail 
client problems... ]

On Jan 6, 2004, at 11:50 AM, Simon Cozens wrote:

> Jeff Clites:
>> $a = $hash{bar};
>
> Here you used the copy constructor before taking the reference. It 
> might look
> like an assignment operator, but it isn't. You're better off thinking 
> that
> assignment doesn't exist. It's a copy constructor. It makes the PMC 
> referred
> to by $a a copy of the PMC in $hash{bar}. Their values may be "equal" 
> but
> they're two different PMCs.

But here what I'm copying is the _contents_ of the hash slot--so I'm 
copying a PerlString PMC, for instance.

>> $b = \$hash{bar};
>
> Here you didn't make a copy before taking the reference. No copy, only 
> one
> PMC. It all works.

And here I'm not making a copy, but also the thing I'm taking a 
reference to is not the same thing I copied above. Here, it's a 
reference to a hash slot.


So maybe the way to think about it is:

$a = $hash{bar};
#This finds the hash slot, pulls out its contents, and copies that 
contents into $a.

$b = \$hash{bar};
#This finds the hash slot, takes a reference to it, and places that 
reference in $b.

So the first case involves copying a string, and the second involves 
creating a reference to a hash slot. In particular, if the hash is 
initially empty, the first case should end up with undef in $a (so 
nothing actually copied, if undef is a singleton), but $b would 
(presumably) still contain a real reference (to a hash slot which 
doesn't contain anything yet), which would create a hash entry if you 
"$$b = 'goo'". And that reference-to-a-hash-slot is probably a separate 
special PMC (it needs to keep track of the hash and key, and work even 
if the hash is empty).

JEff