Object suicude

**Andy Hassall** · Jul 17 '05, 02:58 AM

Re: Object suicude

On Thu, 5 Feb 2004 17:25:28 +0100, "Berislav Lopac" <berislav.lopac @dimedia.hr>
wrote:
[color=blue]
>Is there a way for an object to unset itself in PHP4?
>
>Of course I could unset it from outside, but I would like to keep the code
>clean. The object has a method which deletes it's "footprint" in the
>database, and as the idea is that we don't need the object once itsin't in
>the DB it would be nice if the same method could get rid of the PHP object
>iself as well.[/color]

The obvious idea of using unset($this) doesn't appear to work:

<?php
class Test
{
var $x = 'x';
function pressBigRedButt on() {
unset($this);
}
}

$test = new Test;
$test->pressBigRedBut ton();

echo $test->x;
?>

(outputs 'x', rather than a warning).

--
Andy Hassall <andy@andyh.co. uk> / Space: disk usage analysis tool
<http://www.andyh.co.uk > / <http://www.andyhsoftwa re.co.uk/space>

**Chung Leong** · Jul 17 '05, 02:58 AM

Re: Object suicude

unset($this) only remove the variable "this" from the current namespace.
What you want to do is $this = null. That sets what $this was pointing to to
null.

Uzytkownik "Andy Hassall" <andy@andyh.co. uk> napisal w wiadomosci
news:c5i5205qo4 knujmnral6bu4lq 252ncgecd@4ax.c om...[color=blue]
> On Thu, 5 Feb 2004 17:25:28 +0100, "Berislav Lopac"[/color]
<berislav.lopac @dimedia.hr>[color=blue]
> wrote:
>[color=green]
> >Is there a way for an object to unset itself in PHP4?
> >
> >Of course I could unset it from outside, but I would like to keep the[/color][/color]
code[color=blue][color=green]
> >clean. The object has a method which deletes it's "footprint" in the
> >database, and as the idea is that we don't need the object once itsin't[/color][/color]
in[color=blue][color=green]
> >the DB it would be nice if the same method could get rid of the PHP[/color][/color]
object[color=blue][color=green]
> >iself as well.[/color]
>
> The obvious idea of using unset($this) doesn't appear to work:
>
> <?php
> class Test
> {
> var $x = 'x';
> function pressBigRedButt on() {
> unset($this);
> }
> }
>
> $test = new Test;
> $test->pressBigRedBut ton();
>
> echo $test->x;
> ?>
>
> (outputs 'x', rather than a warning).
>
> --
> Andy Hassall <andy@andyh.co. uk> / Space: disk usage analysis tool
> <http://www.andyh.co.uk > / <http://www.andyhsoftwa re.co.uk/space>[/color]

**Phil Powell** · Jul 17 '05, 02:59 AM

Re: Object suicude

Have a question about $this = null:

In PHP is this its equivalent of destroying an object, say, if you
will, Garbage Collection? I am working on a project that has anywhere
between 10 - 50 different instances occurring at one time (I kid you
not!) and realize this will turn into a horrific bottleneck unless
there is some means of garbage collection.

Thanx
Phil

"Chung Leong" <chernyshevsky@ hotmail.com> wrote in message news:<vMGdnQV_P eytTb_dRVn-uA@comcast.com> ...[color=blue]
> unset($this) only remove the variable "this" from the current namespace.
> What you want to do is $this = null. That sets what $this was pointing to to
> null.
>
> Uzytkownik "Andy Hassall" <andy@andyh.co. uk> napisal w wiadomosci
> news:c5i5205qo4 knujmnral6bu4lq 252ncgecd@4ax.c om...[color=green]
> > On Thu, 5 Feb 2004 17:25:28 +0100, "Berislav Lopac"[/color]
> <berislav.lopac @dimedia.hr>[color=green]
> > wrote:
> >[color=darkred]
> > >Is there a way for an object to unset itself in PHP4?
> > >
> > >Of course I could unset it from outside, but I would like to keep the[/color][/color]
> code[color=green][color=darkred]
> > >clean. The object has a method which deletes it's "footprint" in the
> > >database, and as the idea is that we don't need the object once itsin't[/color][/color]
> in[color=green][color=darkred]
> > >the DB it would be nice if the same method could get rid of the PHP[/color][/color]
> object[color=green][color=darkred]
> > >iself as well.[/color]
> >
> > The obvious idea of using unset($this) doesn't appear to work:
> >
> > <?php
> > class Test
> > {
> > var $x = 'x';
> > function pressBigRedButt on() {
> > unset($this);
> > }
> > }
> >
> > $test = new Test;
> > $test->pressBigRedBut ton();
> >
> > echo $test->x;
> > ?>
> >
> > (outputs 'x', rather than a warning).
> >
> > --
> > Andy Hassall <andy@andyh.co. uk> / Space: disk usage analysis tool
> > <http://www.andyh.co.uk > / <http://www.andyhsoftwa re.co.uk/space>[/color][/color]

**Andy Hassall** · Jul 17 '05, 02:59 AM

Re: Object suicude

On Thu, 5 Feb 2004 18:16:47 -0500, "Chung Leong" <chernyshevsky@ hotmail.com>
wrote:
[color=blue]
>unset($this) only remove the variable "this" from the current namespace.
>What you want to do is $this = null. That sets what $this was pointing to to
>null.[/color]

Ah - thanks, I wasn't aware of the distinction. Looks like this is what Phil
is looking for, as a quick test appears to show it does destroy the object.

--
Andy Hassall <andy@andyh.co. uk> / Space: disk usage analysis tool
<http://www.andyh.co.uk > / <http://www.andyhsoftwa re.co.uk/space>

**Chung Leong** · Jul 17 '05, 02:59 AM

Re: Object suicude

In theory, yes. In practice, I have not a clue. On my server the Apache
process regularly chew up half a gig of memory.

Uzytkownik "Phil Powell" <soazine@erols. com> napisal w wiadomosci
news:1cdca2a7.0 402061306.26974 f67@posting.goo gle.com...[color=blue]
> Have a question about $this = null:
>
> In PHP is this its equivalent of destroying an object, say, if you
> will, Garbage Collection? I am working on a project that has anywhere
> between 10 - 50 different instances occurring at one time (I kid you
> not!) and realize this will turn into a horrific bottleneck unless
> there is some means of garbage collection.
>
> Thanx
> Phil
>
> "Chung Leong" <chernyshevsky@ hotmail.com> wrote in message[/color]
news:<vMGdnQV_P eytTb_dRVn-uA@comcast.com> ...[color=blue][color=green]
> > unset($this) only remove the variable "this" from the current namespace.
> > What you want to do is $this = null. That sets what $this was pointing[/color][/color]
to to[color=blue][color=green]
> > null.
> >
> > Uzytkownik "Andy Hassall" <andy@andyh.co. uk> napisal w wiadomosci
> > news:c5i5205qo4 knujmnral6bu4lq 252ncgecd@4ax.c om...[color=darkred]
> > > On Thu, 5 Feb 2004 17:25:28 +0100, "Berislav Lopac"[/color]
> > <berislav.lopac @dimedia.hr>[color=darkred]
> > > wrote:
> > >
> > > >Is there a way for an object to unset itself in PHP4?
> > > >
> > > >Of course I could unset it from outside, but I would like to keep the[/color]
> > code[color=darkred]
> > > >clean. The object has a method which deletes it's "footprint" in the
> > > >database, and as the idea is that we don't need the object once[/color][/color][/color]
itsin't[color=blue][color=green]
> > in[color=darkred]
> > > >the DB it would be nice if the same method could get rid of the PHP[/color]
> > object[color=darkred]
> > > >iself as well.
> > >
> > > The obvious idea of using unset($this) doesn't appear to work:
> > >
> > > <?php
> > > class Test
> > > {
> > > var $x = 'x';
> > > function pressBigRedButt on() {
> > > unset($this);
> > > }
> > > }
> > >
> > > $test = new Test;
> > > $test->pressBigRedBut ton();
> > >
> > > echo $test->x;
> > > ?>
> > >
> > > (outputs 'x', rather than a warning).
> > >
> > > --
> > > Andy Hassall <andy@andyh.co. uk> / Space: disk usage analysis tool
> > > <http://www.andyh.co.uk > / <http://www.andyhsoftwa re.co.uk/space>[/color][/color][/color]

**Alex Farran** · Jul 17 '05, 03:00 AM

Re: Object suicude

Andy Hassall writes:
[color=blue]
> On Thu, 5 Feb 2004 18:16:47 -0500, "Chung Leong" <chernyshevsky@ hotmail.com>
> wrote:[/color]
[color=blue][color=green]
>> unset($this) only remove the variable "this" from the current namespace.
>> What you want to do is $this = null. That sets what $this was pointing to to
>> null.[/color][/color]
[color=blue]
> Ah - thanks, I wasn't aware of the distinction. Looks like this is what Phil
> is looking for, as a quick test appears to show it does destroy the object.[/color]

Well that is interesting. I had to try it myself. Conceptually $this
is just another parameter passed by reference, and you can do anything
you want to it, including assigning it a completely different object.

I adapted the suicide test to do this -

<?php
class Transmute {
var $y = 'y';
}

class Test {
var $x = 'x';
function pressBigRedButt on() {
$this= new Transmute();
}
}
$test = new Test;
echo $test->x;
$test->pressBigRedBut ton();
echo $test->x;
echo $test->y;
?>

Returns

x
Notice: Undefined property: x in suicide.php on line 16
y

--

__o Alex Farran
_`\<,_ Analyst / Programmer
(_)/ (_) www.alexfarran.com

**Zurab Davitiani** · Jul 17 '05, 03:01 AM

Re: Object suicude

Phil Powell wrote:
[color=blue]
> Have a question about $this = null:
>
> In PHP is this its equivalent of destroying an object, say, if you
> will, Garbage Collection? I am working on a project that has anywhere
> between 10 - 50 different instances occurring at one time (I kid you
> not!) and realize this will turn into a horrific bottleneck unless
> there is some means of garbage collection.[/color]

There's none in PHP4. Even if you do $someObject = null or unset() or
anything else it will not free up memory used by that object. Memory is
only freed up when the script execution ends.

This only applies to objects and not other types like arrays, strings, etc.

**Alex Farran** · Jul 17 '05, 03:01 AM

Re: Object suicude

Zurab Davitiani writes:
[color=blue]
> Phil Powell wrote:[color=green]
>> Have a question about $this = null:
>>
>> In PHP is this its equivalent of destroying an object, say, if you
>> will, Garbage Collection? I am working on a project that has anywhere
>> between 10 - 50 different instances occurring at one time (I kid you
>> not!) and realize this will turn into a horrific bottleneck unless
>> there is some means of garbage collection.[/color][/color]
[color=blue]
> There's none in PHP4. Even if you do $someObject = null or unset() or
> anything else it will not free up memory used by that object. Memory is
> only freed up when the script execution ends.[/color]

What makes you think that? PHP4's garbage collection algorithm is an
unsophisticated reference counter, so it may not be the most efficient
in the world, but I'd be very surprised if objects weren't GC'd.
--

__o Alex Farran
_`\<,_ Analyst / Programmer
(_)/ (_) www.alexfarran.com

**Zurab Davitiani** · Jul 17 '05, 03:02 AM

Re: Object suicude

Alex Farran wrote:
[color=blue][color=green]
>> There's none in PHP4. Even if you do $someObject = null or unset() or
>> anything else it will not free up memory used by that object. Memory is
>> only freed up when the script execution ends.[/color]
>
> What makes you think that? PHP4's garbage collection algorithm is an
> unsophisticated reference counter, so it may not be the most efficient
> in the world, but I'd be very surprised if objects weren't GC'd.[/color]

I can't see how they are. I've tried few scripts quite a few times and
different ways that create new objects in a for-loop. Memory consumption
never goes down, but only increases as the script continues to run.
Eventually, the available memory is used up and you either get a fatal
error (unable to allocate memory), or the process simply crashes, depending
on your version of PHP4.

This only happens with objects, but not with other types, as I said; even if
you try to unset or =null or reassign the same variable name to another
object - doesn't matter how you do it.

I'd be interested to see what method you are suggesting to use that will
properly destroy the object, freeing the associated memory. It just doesn't
happen automatically with the garbage collection like with the other types
of variables.

**Alex Farran** · Jul 17 '05, 03:02 AM

Re: Object suicude

Zurab Davitiani writes:
[color=blue]
> I'd be interested to see what method you are suggesting to use that will
> properly destroy the object, freeing the associated memory. It just doesn't
> happen automatically with the garbage collection like with the other types
> of variables.[/color]

Well I was arguing from my understanding of the garbage collection
mechanism rather than practical experience, but I ran simple test that
proves that objects are garbage collected just like any other block of
memory. It wouldn't be a very practical object system if they
weren't.

The PHP4 garbage collector uses a reference counting algorithm. What
this means is that any block of memory, be it an array, object, string
or integer, is freed when there are no references to it. This works
well for simple cases, but can be fooled by cyclic references. If two
objects reference each other, or one object references itself, then
the reference count will always be more than 0 so the object(s) will
never be GC'd.

Here are my tests -

This script will run to completion, since each time a new object is
created the reference to the previous object is destroyed.

echo "start\n";
for ($i = 1; $i<100000; $i++ ) {
$mem = new memory();
}
echo "end\n";

This script will run out of memory because the reference to each
object is retained.

echo "start\n";
for ($i = 1; $i<100000; $i++ ) {
$mem[$i] = new memory();
}
echo "end\n";

This will also fail because each object contains a reference to
itself.

echo "start\n";
for ($i = 1; $i<100000; $i++ ) {
$mem =& new memory();
$mem->a =& $mem;
}
echo "end\n";

--

__o Alex Farran
_`\<,_ Analyst / Programmer
(_)/ (_) www.alexfarran.com

**Jochen Daum** · Jul 17 '05, 03:02 AM

Re: Object suicude

Hi Berislav!

On Thu, 5 Feb 2004 17:25:28 +0100, "Berislav Lopac"
<berislav.lopac @dimedia.hr> wrote:
[color=blue]
>Is there a way for an object to unset itself in PHP4?
>
>Of course I could unset it from outside, but I would like to keep the code
>clean. The object has a method which deletes it's "footprint" in the
>database, and as the idea is that we don't need the object once itsin't in
>the DB it would be nice if the same method could get rid of the PHP object
>iself as well.
>
>Berislav[/color]

I have been loking at PostNuke recently and they include everything in
a function. Though they don't use objects, everything is gone and out
of scope, once the function ends.

HTH, Jochen

--
Jochen Daum - Cabletalk Group Ltd.
PHP DB Edit Toolkit -- PHP scripts for building
database editing interfaces.

PHP DB Edit Toolkit

http://sourceforge.net/projects/phpdbedittk/

Download PHP DB Edit Toolkit for free. PHP DB Edit Toolkit is a set of PHP classes makes the generation of database edit interfaces easier and faster. The main class builds tabular and form views based on a data dictionary and takes over handling of insert/update/delete and user input.

**Zurab Davitiani** · Jul 17 '05, 03:18 AM

Re: Object suicude

Alex Farran wrote:
[color=blue]
> Well I was arguing from my understanding of the garbage collection
> mechanism rather than practical experience, but I ran simple test that
> proves that objects are garbage collected just like any other block of
> memory. It wouldn't be a very practical object system if they
> weren't.
>
> The PHP4 garbage collector uses a reference counting algorithm. What
> this means is that any block of memory, be it an array, object, string
> or integer, is freed when there are no references to it. This works
> well for simple cases, but can be fooled by cyclic references. If two
> objects reference each other, or one object references itself, then
> the reference count will always be more than 0 so the object(s) will
> never be GC'd.
>
> Here are my tests -
>
> This script will run to completion, since each time a new object is
> created the reference to the previous object is destroyed.
>
> echo "start\n";
> for ($i = 1; $i<100000; $i++ ) {
> $mem = new memory();
> }
> echo "end\n";
>
> This script will run out of memory because the reference to each
> object is retained.
>
> echo "start\n";
> for ($i = 1; $i<100000; $i++ ) {
> $mem[$i] = new memory();
> }
> echo "end\n";
>
> This will also fail because each object contains a reference to
> itself.
>
> echo "start\n";
> for ($i = 1; $i<100000; $i++ ) {
> $mem =& new memory();
> $mem->a =& $mem;
> }
> echo "end\n";
>[/color]

This doesn't seem to work on all objects; and I am confused that it works on
some but not others. I was trying to determine exactly what objects have to
contain for PHP GC not to be able to release the memory, but I was unable
to create a simple test case scenario in a limited free time I had. I will
research it more, and hopefully have more details, and probably start
another thread when I do. Thanks for the info!

Object suicude

Object suicude

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