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Date: Wed, 19 Jan 2011 18:01:44 +1100
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To: Larry Garfield <larry@garfieldtech.com>
CC: "internals@lists.php.net" <internals@lists.php.net>
References: <201101190045.31781.larry@garfieldtech.com>
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Subject: Re: [PHP-DEV] How deep is copy on write?
From: mail_ben_schmidt@yahoo.com.au (Ben Schmidt)

It does the whole of $b. It has to, because when you change 'baz', a reference in 
'bar' needs to change to point to the newly copied 'baz', so 'bar' is 
written...and likewise 'foo' is written.

Ben.



On 19/01/11 5:45 PM, Larry Garfield wrote:
> Hi folks.  I have a question about the PHP runtime that I hope is appropriate
> for this list.  (If not, please thwap me gently; I bruise easily.)
>
> I know PHP does copy-on-write.  However, how "deeply" does it copy when
> dealing with nested arrays?
>
> This is probably easiest to explain with an example...
>
> $a['foo']['bar']['baz'] = 1;
> $a['foo']['bar']['bob'] = 1;
> $a['foo']['bar']['narf'] = 1;
> $a['foo']['poink']['narf'] = 1;
>
> function test($b) {
>    // Assume each of the following lines in isolation...
>
>    // Does this copy just the one variable baz, or the full array?
>    $b['foo']['bar']['baz'] = 2;
>
>    // Does this copy $b, or just $b['foo']['poink']?
>    $b['foo']['poink']['stuff'] = 3;
>
>    return $b;
> }
>
> // I know this is wasteful; I'm trying to figure out just how wasteful.
> $a = test($a);
>
> test() in this case should take $b by reference, but I'm trying to determine
> how much of a difference it is.  (In practice my use case has a vastly larger
> array, so any inefficiencies are multiplied.)
>
> --Larry Garfield
>