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php

Destructors

php.internals

Jason Garber

22 years ago

Hello,

This is a question relating to the new PHP5 Object Destructor
functionality.  Consider the following:

<?php

class DB
{
function Query() { ... }
}

class Session
{
function SetVar() {
//sets a variable to be saved in the session
}

function __destruct() {
//Save session variables in database using global $oDB object.
global $oDB;
$oDB->Query(...);
}
}

$oDB = new DB();

$oSession = new Session();
$oSession->SetVar('foo', 'New Value');

//End of script
?>


Based on the above example, the $oSession->__destruct() method relies on
the $oDB object still being usable.  How can this be structured to ensure
that the DB object does not get released first?

Thanks,
Jason



__________________________________________________________
Jason Garber
President & Chief Technology Officer
IonZoft, Inc.
814.742.8030 :: jason@ionzoft.com :: http://IonZoft.com
__________________________________________________________

Christian Schneider

22 years ago

Jason Garber wrote:

> Based on the above example, the $oSession->__destruct() method relies on
> the $oDB object still being usable.  How can this be structured to
> ensure that the DB object does not get released first?

Adding something like $oSession->db = $oDB; should work in my opinion.
Didn't test it and I have no real experience with destructors.

- Chris

Adam Maccabee Trachtenberg

22 years ago

On Mon, 24 May 2004, Christian Schneider wrote:

> Jason Garber wrote:
> > Based on the above example, the $oSession->__destruct() method relies on
> > the $oDB object still being usable.  How can this be structured to
> > ensure that the DB object does not get released first?

It can't.

> Adding something like $oSession->db = $oDB; should work in my opinion.
> Didn't test it and I have no real experience with destructors.

That's not guaranteed to work. PHP 5 does not guarantee that object
destructors will be called in any particular order or that the order
will remain constant from one invocation to another.

Before asking for changes to this behavior, please check the mailing
list archives for a long discussion on the topic. :)

-adam

--
adam@trachtenberg.com
author of o'reilly's php cookbook
avoid the holiday rush, buy your copy today!

Christian Schneider

22 years ago

Adam Maccabee Trachtenberg wrote:

> That's not guaranteed to work. PHP 5 does not guarantee that object
> destructors will be called in any particular order or that the order
> will remain constant from one invocation to another.

Ok, I was under the impression that PHP won't call any destructors of
objects still in use but from your comment I guess it does two passes:
First collect a list of objects to destruct (which will be everything at
the end of the script) and then call all the destructors (in random order).

Ignore my first comment, it was posted without any insight in the
internals of the GC mechanism :-)

- Chris

Lukas Smith

22 years ago

Christian Schneider wrote:

> Adam Maccabee Trachtenberg wrote:
>
>> That's not guaranteed to work. PHP 5 does not guarantee that object
>> destructors will be called in any particular order or that the order
>> will remain constant from one invocation to another.
>
>
> Ok, I was under the impression that PHP won't call any destructors of
> objects still in use but from your comment I guess it does two passes:
> First collect a list of objects to destruct (which will be everything at
> the end of the script) and then call all the destructors (in random order).

unless you manually unset the objects in the order in which you want
them to be destructed.

regards,
Lukas Smith

Adam Maccabee Trachtenberg

22 years ago

On Mon, 24 May 2004, Lukas Smith wrote:

> unless you manually unset the objects in the order in which you want
> them to be destructed.

Right. That'll also work. :)

-adam

--
adam@trachtenberg.com
author of o'reilly's php cookbook
avoid the holiday rush, buy your copy today!

John Coggeshall

22 years ago

On Mon, 2004-05-24 at 10:40, Adam Maccabee Trachtenberg wrote:

> Right. That'll also work. :)

I suppose you could push object references on a stack and register a
shutdown function which pops / unset()s if you really wanted a order to
it.. (that's an untested theory though)

John

--
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John Coggeshall                   http://www.coggeshall.org/
The PHP Developer's Handbook    http://www.php-handbook.com/
-=~=--=~=--=~=--=~=--=~=--=~=--=~=--=~=--=~=--=~=--=~=--=~=-

Lukas Smith

22 years ago

John Coggeshall wrote:

> On Mon, 2004-05-24 at 10:40, Adam Maccabee Trachtenberg wrote:
>
>>Right. That'll also work. :)
>
>
> I suppose you could push object references on a stack and register a
> shutdown function which pops / unset()s if you really wanted a order to
> it.. (that's an untested theory though)

Which is what PEAR does to "emulate" destructors. But its doesnt really
give you much control over the order of the stack. Its just the order in
which you have created your instances. Anyways this is more and more
turning into php-general@ material :-)

regards,
Lukas Smith