Smarty (and other tpl ngins): assign and assign_by

问题:

This is not just about Smarty, but I guess most template engines that have variables assigned. It's more a theoretical question, than a practical. I have no use case.

What happens in PHP when you assign a big array $a to another variable $b? PHP copies the array? Maybe, just maybe, internally it creates a pointer. Then what happens when you alter $a slightly? $b shouldn't be changed, because no & was used to create $b. Did PHP just double the memory usage??

More specifically: What happens when you assign a big array from you Controller ($a) to your template engine ($tpl->vars['a']) and to use in the view (extract to $a)? Did PHP's memory just triple??

Now what happens if I assign all my variables by reference? I'm cool with my view being able to alter the array back into the Controller (I won't be coming back there anyway). It's also fine if the variable changes within the templat engine ($tpl->vars['a']).

Is assigning all vars by reference better for memory? Better for performance? If so: any chances of strange, unwanted side effects?

Because people like code and not stories:

// copies
$a = array( ... );
$tpl->assign('a', $a); // creates a copy (?) in $tpl->vars['a']

// pointer / by ref
$a = array( ... );
$tpl->assign_by_ref('a', $a); // creates a pointer in $tpl->vars['a'] because:

function assign_by_ref( $name, &$var ) {
  $this->vars[$name] = $var; // voila pointer?
}

I'm pretty sure PHP doesn't mind big arrays and copies and clones, but performance and memory wise: which's 'better'?

edit
For objects, all of this doesn't matter. Objects are always, automatically assigned by reference. And since objects are hot, maybe this is an outdated question, but I am very curious.

UPDATE
So PHP uses copy on write... Love it. And objects are always pointers. What happens when you:

$a = new BigObject;
$b = $a; // pointer, right?
$b->updateSomethingInternally(); // $b is now changed > what about $a?

Did this trigger the copy-on-write? Or are $a and $b still identical (like in ===)?

edit
Could I conclude that assigning by ref is really not worth it just to spare memory? PHP in itself is smart enough?

edit
Interesting visualization of copy, clone, by-ref etc: http://www.phpinsider.com/download/PHP5RefsExplained.pdf

回答1:

PHP uses a concept called copy on write. I.e. if you just do a $a = $b PHP will not copy the whole value of $b to $a. It will just create some kind of a pointer. (To be more precise both $a and $b will point to the same zval and it's refcount will be increased.)

Now, if either $a or $b were modified the value obviously can't be shared anymore and must be copied.

So, unless you aren't modifying the array in your template code, no copying will be done.

Some further notes:

• Beware of trying to optimize your code by blindly inserting references. Often they will have an effect contrary to what you expect. Example to explain why:

  $a = SOMETHING_BIG; // $a points to a zval with refcount 1 and is_ref 0
  $b = $a;            // $a and $b both point to a zval with refcount 2 and is_ref 0
  $c =& $a;           // Now we have a problem: $c can't just point to the same zval
                      // anymore, because that zval has is_ref to 0, but we need one
                      // with is_ref 1. So The zval gets copied. You now have $b
                      // pointing to one zval with refcount 1 and is_ref 0 and $a and
                      // $c pointing to another one with refcount 2 and is_ref 1
  

  So the contrary to what you wanted actually happened. Instead of saving memory you are actually allocating additional. It's often hard to judge whether adding a reference will make it better or worse because it's often hard to trace all different variables pointing to one zval (it's often not as easy as it looks, just have a look at the examples of the debug_zval_dump function. So, really, the only safe way to know, whether a reference is good for performance or not, is to actually profile both variants.

• Objects are, just like everything else, passed by value in PHP. Still you are right that they behave reference-like, because with objects this value that get's passed around is just a pointer to some other data structure. In most cases the distinction between passing by reference and reference-like behavior isn't of importance, but there still is a difference.

This was just a short introduction to the topic. You can find a more in-depth analysis of the topic in a blog post by Sara Golemon with the porvoking title "You're being lied to".

回答2:

As mentioned in other answers, PHP employs copy-on-write. However I do wish to answer this part of your post:

Objects are always, automatically assigned by reference.

This is not entirely true. They are assigned an identifier which points to an object.

$a = new stdClass();
$b = $a; // $a and $b now share same identifier
$b = 0; // $b no longer contains identifier
var_dump($a); // outputs object

Contrast this with assigning by reference:

$a = new stdClass();
$b =& $a; // $a and $b now share same reference
$b = 0; //
var_dump($a); // outputs int(0)

Update

In your edit you ask:

$a = new BigObject;
$b = $a; // pointer, right?
$b->updateSomethingInternally(); // $b is now changed > what about $a?

Did this trigger the copy-on-write? Or are $a and $b still identical (like in ===)?

Because $b now contains an identifier, i.e. now "points" to same object as $a, $a is also affected. There was never any copying of objects involved.

回答3:

PHP uses copy-on-write when passing arrays around, so no extra memory is used until you modify the array. Sorry, no link to back up this claim.