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Date: Wed, 19 Jan 2011 17:27:48 -0000
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Subject: Re: [PHP-DEV] How deep is copy on write?
From: glopes@nebm.ist.utl.pt ("Gustavo Lopes")

On Wed, 19 Jan 2011 14:23:49 -0000, Martin Scotta <martinscotta@gmail.com>  
wrote:

> What about objects?

With objects less copying occurs because the object value (zval) data is  
actually just a pointer and an id that for most purposes works as a  
pointer.

However, it should be said that while a copy of an array forces more  
memory to be copied, the inner zvals are not actually copied. In this  
snippet:

$a = array(1, 2, array(3));
$b = $a;
function separate(&$dummy) { }
separate($a);

the copy that occurs when you force the separation of the zval that is  
shared by $a and $b ($b = $a doesn't copy the array in $a to $b, it merely  
copies the zval pointer of $a to $b and increments its reference count) is  
just a shallow copy of hash table and a increment of the first level  
zvals' refcounts. This means the zvals that have their pointers stored in  
the array $a's HashTable are not themselves copied.

Interestingly (or should I say, unfortunately), this happens even if the  
inner zvals are references. See  
http://php.net/manual/en/language.references.whatdo.php the part on arrays.

>
> class Foo {
>     public $foo;
> }
>
> function test($o) {
>     $o->foo->foo->foo = 2;
> }
>
> $bar = new Foo;
> $bar->foo = new Foo;
> $bar->foo->foo = new Foo;
>
> test( $bar );

This example shows no copying (in the sense of "new zval allocation on  
passing or assignment") at all.

>
> ---
> Also... is it better to pass an object as a parameter rather than many
> values?
>
> function withValues($anInteger, $aBool, $aString) {
>    var_dump($anInteger, $aBool, $aString);
> }
>
> function withObject(ParamOject $o) {
>    var_dump( $o->theInteger(), $o->theBool(), $o->theString() );
> }
>

It should be indifferent. In normal circumstances, there is no zval  
copying at all (only the pointers of arguments' symbols are copied). Only  
when you start throwing references into the mix will you start forcing  
copied.


-- 
Gustavo Lopes