RE: [PHP] Static Class Member References
-Original Message-
From: Daniel Kolbo [mailto:kolb0...@umn.edu]
Sent: 11 July 2010 23:19
Hello PHPers,
I'm having some trouble understanding some PHP behaviour. The
following
example script exhibits the behaviour which I cannot understand.
I'm pretty sure that this is *not* a bug. I'll answer your last
question first, and then demonstrate with your code.
I'm nervous to use self::$a = $this; because I don't want to be
copying the whole object. However, isn't $this just a reference to
the
object, so self::$a = $this; is just copying the reference and not
the
actual object, right?
Not exactly, although everybody seems to refer to it as a reference
for convenience. Most of the time it doesn't matter, but when you
start introducing references to objects it can - it's better to think
of an object variable as holding the object's *handle*
(see http://php.net/anguage.oop5.references.php for more on this), so
it's clear exactly what a reference is referencing.
Now for your code:
[code]
?php
class A
{
public static $a = 3;
function __construct()
{
//self::$a = $this; //[i]
self::$a = $this; //[ii]
}
}
class B extends A
{
function __construct()
{
parent::__construct();
}
}
class C {
var $c;
function __construct()
{
$this-c = A::$a;
}
}
$c = new C;
[i] [ii] in C::__construct(): $c-c = reference to same value as
A::$a (currently (int)3)
NOTE: because of how references work, A::$a is now also a
reference to (int)3.
$b = new B;
[i] in A::__construct(): A::$a = handle of object B(1) (also assigned to
global $b)
NOTE: $c-c, as reference to $A::a, now also references handle of
object B(1)
[ii] in A::__construct(): A::$a = reference to handle of object B(1)
NOTE: since we are assigning a new reference to a variable which is
already a reference, ONLY this reference changes -- so $c-c
is still a reference to (int)3...!
$cee = new C;
Irrelevant -- the damage has already been done!
var_dump($c-c); // [i] prints object(B), but [ii] prints int 3
var_dump($cee-c); // [i] prints object(B), and [ii] prints
object(B)
... which is correct according to my interpretation.
This has been such a regular confusion, that some time ago I wrote
this test script:
?php
class test {
public $me;
}
$t = new test;
$t-me = 'Original';
$copy_t = $t;
$ref_t = $t;
$copy_t = new test;
$copy_t-me = 'Altered Copy';
echo RESULT1
Original: $t-mebr /
Copy: $copy_t-mebr /
Reference: $ref_t-mebr /
RESULT1;
$ref_t = new test;
$ref_t-me = 'Altered Reference';
echo RESULT2
br /
Original: $t-mebr /
Copy: $copy_t-mebr /
Reference: $ref_t-mebr /
RESULT2;
$s = 'String';
$copy_s = $s;
$ref_s = $s;
$copy_s = 'String Copy';
echo RESULT3
br /
Original: $sbr /
Copy: $copy_sbr /
Reference: $ref_sbr /
RESULT3;
$ref_s = 'String Reference';
echo RESULT4
br /
Original: $sbr /
Copy: $copy_sbr /
Reference: $ref_sbr /
RESULT4;
?
Which gives this output:
Original: Original
Copy: Altered Copy
Reference: Original
Original: Altered Reference
Copy: Altered Copy
Reference: Altered Reference
Original: String
Copy: String Copy
Reference: String
Original: String Reference
Copy: String Copy
Reference: String Reference
Which demonstrates how exactly the behaviour of objects correlates to
scalars with regards to copying and referencing -- but may not be
exactly what you expect if you think of object variables as always
holding a reference to the object. I would heartily recommend always
to think of an object variable as holding the object's *handle*, and
*not* a reference - this may croggle your brain a bit, but makes it a
Lot clearer what's happening in edge cases like this.
Cheers!
Mike
--
Mike Ford,
Electronic Information Developer, Libraries and Learning Innovation,
Leeds Metropolitan University, C507, Civic Quarter Campus,
Woodhouse Lane, LEEDS, LS1 3HE, United Kingdom
Email: m.f...@leedsmet.ac.uk
Tel: +44 113 812 4730
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Re: [PHP] Static Class Member References
Ah, so assigning a reference to a variable already holding a reference
changes that variable's reference only in the same way that unsetting a
reference doesn't unset the other variables referencing the same thing, yes?
$a = 5;
$b = $a;
print $a;
5
unset($b); // does not affect $a
print $a;
5
// and according to Mike's previous message
$b = $a;
$c = 10;
$b = $c; // does not affect $a
print $a
5
That makes a lot of sense. If it didn't work this way there would be no easy
way to untangle references. In the case of
foreach($array as $key = $value) { ... }
the first value in the array would continuously be overwritten by the next
value.
1. $value gets reference to first array value
2. on each step through the loop, the first array value would be overwritten
by the next value in the loop since $value is forever tied to it by the
initial reference assignment.
That would be a Bad Thing (tm).
Thanks for the clarification, Mike.
David
Re: [PHP] Static Class Member References
David Harkness wrote:
Ah, so assigning a reference to a variable already holding a reference
changes that variable's reference only in the same way that unsetting a
reference doesn't unset the other variables referencing the same thing, yes?
$a = 5;
$b = $a;
print $a;
5
unset($b); // does not affect $a
print $a;
5
// and according to Mike's previous message
$b = $a;
$c = 10;
$b = $c; // does not affect $a
print $a
5
That makes a lot of sense. If it didn't work this way there would be no easy
way to untangle references. In the case of
foreach($array as $key = $value) { ... }
the first value in the array would continuously be overwritten by the next
value.
1. $value gets reference to first array value
2. on each step through the loop, the first array value would be overwritten
by the next value in the loop since $value is forever tied to it by the
initial reference assignment.
That would be a Bad Thing (tm).
Thanks for the clarification, Mike.
David
The big aha moment for was when realizing that when assigning a
reference to a variable you only change its reference and not any other
variable that may also have shared the same reference. Yuck that's a
mouth full. This happened when Mike said, NOTE: since we are
assigning a new reference to a variable which is already a reference,
ONLY this reference changes.
Yes, i agree with you David (on both of your points). Thanks for the
example using the unset. This further clarified/solidified my
understanding.
Now, with this new understanding, I also wish to comment that if i
assign (without reference) $this, i don't have to be too worried about
bloating the memory, b/c i'm only assigning/copying the identifer or
*handle* and not the actual object itself.
In case, someone reads this in the archive the link is:
http://php.net/manual/en/language.oop5.references.php
Mike, thank you a ton.
Regards.
`
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Re: [PHP] Static Class Member References
On 12 July 2010 22:54, Daniel Kolbo kolb0...@umn.edu wrote:
Richard Quadling wrote:
On 11 July 2010 23:19, Daniel Kolbo kolb0...@umn.edu wrote:
Hello PHPers,
I'm having some trouble understanding some PHP behaviour. The following
example script exhibits the behaviour which I cannot understand.
[code]
?php
class A
{
public static $a = 3;
function __construct()
{
//self::$a = $this; //[i]
self::$a = $this; //[ii]
}
}
class B extends A
{
function __construct()
{
parent::__construct();
}
}
class C {
var $c;
function __construct()
{
$this-c = A::$a;
}
}
$c = new C;
$b = new B;
$cee = new C;
var_dump($c-c); // [i] prints object(B), but [ii] prints int 3
var_dump($cee-c); // [i] prints object(B), and [ii] prints object(B)
?
[/code]
Why does $c-c print 'int 3' ?
I'm nervous to use self::$a = $this; because I don't want to be
copying the whole object. However, isn't $this just a reference to the
object, so self::$a = $this; is just copying the reference and not the
actual object, right?
Thanks in advance
What do you think the value should be?
A static property is bound to the class and not to an instance of the class.
So, A::$a is a reference to the static value. If you alter the value,
it will be altered for a subclasses of A and for any other reference
to it.
I think
var_dump($c-c); would print object(B), but it's printing int 3.
The reference is *not* being updated. I think this is a bug. What do
you think?
Thanks
`
What version of PHP are you using?
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Re: [PHP] Static Class Member References
On 13 July 2010 09:46, Richard Quadling rquadl...@gmail.com wrote:
On 12 July 2010 22:54, Daniel Kolbo kolb0...@umn.edu wrote:
Richard Quadling wrote:
On 11 July 2010 23:19, Daniel Kolbo kolb0...@umn.edu wrote:
Hello PHPers,
I'm having some trouble understanding some PHP behaviour. The following
example script exhibits the behaviour which I cannot understand.
[code]
?php
class A
{
public static $a = 3;
function __construct()
{
//self::$a = $this; //[i]
self::$a = $this; //[ii]
}
}
class B extends A
{
function __construct()
{
parent::__construct();
}
}
class C {
var $c;
function __construct()
{
$this-c = A::$a;
}
}
$c = new C;
$b = new B;
$cee = new C;
var_dump($c-c); // [i] prints object(B), but [ii] prints int 3
var_dump($cee-c); // [i] prints object(B), and [ii] prints object(B)
?
[/code]
Why does $c-c print 'int 3' ?
I'm nervous to use self::$a = $this; because I don't want to be
copying the whole object. However, isn't $this just a reference to the
object, so self::$a = $this; is just copying the reference and not the
actual object, right?
Thanks in advance
What do you think the value should be?
A static property is bound to the class and not to an instance of the class.
So, A::$a is a reference to the static value. If you alter the value,
it will be altered for a subclasses of A and for any other reference
to it.
I think
var_dump($c-c); would print object(B), but it's printing int 3.
The reference is *not* being updated. I think this is a bug. What do
you think?
Thanks
Aha!
$c = new C;
At this stage $c-c will be a reference to the static A::$a = 3.
$b = new B;
Now, as B's constructor calls A's constructor which replaces the
static A::$a with a reference to the instance $b, the static A::$a
should now be $b
$cee = new C;
At this stage $cee-c will be a reference to the static A::$a = $b.
But, when var_dump()'d, $c-c !== $cee-c, and I think they should as
both have been assigned to a reference of a static.
It would seem to be a bug.
I get the same output for V5.0.0 to V5.3.3RC2
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Re: [PHP] Static Class Member References
Richard Quadling wrote:
On 12 July 2010 22:54, Daniel Kolbo kolb0...@umn.edu wrote:
Richard Quadling wrote:
On 11 July 2010 23:19, Daniel Kolbo kolb0...@umn.edu wrote:
Hello PHPers,
I'm having some trouble understanding some PHP behaviour. The following
example script exhibits the behaviour which I cannot understand.
[code]
?php
class A
{
public static $a = 3;
function __construct()
{
//self::$a = $this; //[i]
self::$a = $this; //[ii]
}
}
class B extends A
{
function __construct()
{
parent::__construct();
}
}
class C {
var $c;
function __construct()
{
$this-c = A::$a;
}
}
$c = new C;
$b = new B;
$cee = new C;
var_dump($c-c); // [i] prints object(B), but [ii] prints int 3
var_dump($cee-c); // [i] prints object(B), and [ii] prints object(B)
?
[/code]
Why does $c-c print 'int 3' ?
I'm nervous to use self::$a = $this; because I don't want to be
copying the whole object. However, isn't $this just a reference to the
object, so self::$a = $this; is just copying the reference and not the
actual object, right?
Thanks in advance
What do you think the value should be?
A static property is bound to the class and not to an instance of the class.
So, A::$a is a reference to the static value. If you alter the value,
it will be altered for a subclasses of A and for any other reference
to it.
I think
var_dump($c-c); would print object(B), but it's printing int 3.
The reference is *not* being updated. I think this is a bug. What do
you think?
Thanks
`
What version of PHP are you using?
I'm using:
PHP Version 5.2.13
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Re: [PHP] Static Class Member References
Richard Quadling wrote:
On 13 July 2010 09:46, Richard Quadling rquadl...@gmail.com wrote:
On 12 July 2010 22:54, Daniel Kolbo kolb0...@umn.edu wrote:
Richard Quadling wrote:
On 11 July 2010 23:19, Daniel Kolbo kolb0...@umn.edu wrote:
Hello PHPers,
I'm having some trouble understanding some PHP behaviour. The following
example script exhibits the behaviour which I cannot understand.
[code]
?php
class A
{
public static $a = 3;
function __construct()
{
//self::$a = $this; //[i]
self::$a = $this; //[ii]
}
}
class B extends A
{
function __construct()
{
parent::__construct();
}
}
class C {
var $c;
function __construct()
{
$this-c = A::$a;
}
}
$c = new C;
$b = new B;
$cee = new C;
var_dump($c-c); // [i] prints object(B), but [ii] prints int 3
var_dump($cee-c); // [i] prints object(B), and [ii] prints object(B)
?
[/code]
Why does $c-c print 'int 3' ?
I'm nervous to use self::$a = $this; because I don't want to be
copying the whole object. However, isn't $this just a reference to the
object, so self::$a = $this; is just copying the reference and not the
actual object, right?
Thanks in advance
What do you think the value should be?
A static property is bound to the class and not to an instance of the
class.
So, A::$a is a reference to the static value. If you alter the value,
it will be altered for a subclasses of A and for any other reference
to it.
I think
var_dump($c-c); would print object(B), but it's printing int 3.
The reference is *not* being updated. I think this is a bug. What do
you think?
Thanks
Aha!
$c = new C;
At this stage $c-c will be a reference to the static A::$a = 3.
$b = new B;
Now, as B's constructor calls A's constructor which replaces the
static A::$a with a reference to the instance $b, the static A::$a
should now be $b
$cee = new C;
At this stage $cee-c will be a reference to the static A::$a = $b.
But, when var_dump()'d, $c-c !== $cee-c, and I think they should as
both have been assigned to a reference of a static.
It would seem to be a bug.
I get the same output for V5.0.0 to V5.3.3RC2
Thanks for confirming. I reported the bug. I shortened up the test
script quite a bit. Please see: Bug #52332
http://bugs.php.net/bug.php?id=52332
`
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Re: [PHP] Static Class Member References
On 11 July 2010 23:19, Daniel Kolbo kolb0...@umn.edu wrote:
Hello PHPers,
I'm having some trouble understanding some PHP behaviour. The following
example script exhibits the behaviour which I cannot understand.
[code]
?php
class A
{
public static $a = 3;
function __construct()
{
//self::$a = $this; //[i]
self::$a = $this; //[ii]
}
}
class B extends A
{
function __construct()
{
parent::__construct();
}
}
class C {
var $c;
function __construct()
{
$this-c = A::$a;
}
}
$c = new C;
$b = new B;
$cee = new C;
var_dump($c-c); // [i] prints object(B), but [ii] prints int 3
var_dump($cee-c); // [i] prints object(B), and [ii] prints object(B)
?
[/code]
Why does $c-c print 'int 3' ?
I'm nervous to use self::$a = $this; because I don't want to be
copying the whole object. However, isn't $this just a reference to the
object, so self::$a = $this; is just copying the reference and not the
actual object, right?
Thanks in advance
What do you think the value should be?
A static property is bound to the class and not to an instance of the class.
So, A::$a is a reference to the static value. If you alter the value,
it will be altered for a subclasses of A and for any other reference
to it.
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Re: [PHP] Static Class Member References
Richard Quadling wrote:
On 11 July 2010 23:19, Daniel Kolbo kolb0...@umn.edu wrote:
Hello PHPers,
I'm having some trouble understanding some PHP behaviour. The following
example script exhibits the behaviour which I cannot understand.
[code]
?php
class A
{
public static $a = 3;
function __construct()
{
//self::$a = $this; //[i]
self::$a = $this; //[ii]
}
}
class B extends A
{
function __construct()
{
parent::__construct();
}
}
class C {
var $c;
function __construct()
{
$this-c = A::$a;
}
}
$c = new C;
$b = new B;
$cee = new C;
var_dump($c-c); // [i] prints object(B), but [ii] prints int 3
var_dump($cee-c); // [i] prints object(B), and [ii] prints object(B)
?
[/code]
Why does $c-c print 'int 3' ?
I'm nervous to use self::$a = $this; because I don't want to be
copying the whole object. However, isn't $this just a reference to the
object, so self::$a = $this; is just copying the reference and not the
actual object, right?
Thanks in advance
What do you think the value should be?
A static property is bound to the class and not to an instance of the class.
So, A::$a is a reference to the static value. If you alter the value,
it will be altered for a subclasses of A and for any other reference
to it.
I think
var_dump($c-c); would print object(B), but it's printing int 3.
The reference is *not* being updated. I think this is a bug. What do
you think?
Thanks
`
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[PHP] Static Class Member References
Hello PHPers,
I'm having some trouble understanding some PHP behaviour. The following
example script exhibits the behaviour which I cannot understand.
[code]
?php
class A
{
public static $a = 3;
function __construct()
{
//self::$a = $this; //[i]
self::$a = $this; //[ii]
}
}
class B extends A
{
function __construct()
{
parent::__construct();
}
}
class C {
var $c;
function __construct()
{
$this-c = A::$a;
}
}
$c = new C;
$b = new B;
$cee = new C;
var_dump($c-c); // [i] prints object(B), but [ii] prints int 3
var_dump($cee-c); // [i] prints object(B), and [ii] prints object(B)
?
[/code]
Why does $c-c print 'int 3' ?
I'm nervous to use self::$a = $this; because I don't want to be
copying the whole object. However, isn't $this just a reference to the
object, so self::$a = $this; is just copying the reference and not the
actual object, right?
Thanks in advance
`
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