Re: Copying an Object

[Nathan Nobbe <quickshiftin@xxxxxxxxx>]

On Wed, Sep 22, 2010 at 5:35 AM, Daniel Kolbo <kolb0057@xxxxxxx> wrote:

> Hello PHPers,
>
> I have:
>
> class A {
>        ...code
> }
>
> class B extends A {
>        ...code
> }
>
> $a = new A();
>
> $b = new B();
>
> I would like to get all of the properties of $a into $b by value.


first off you have to be careful here since there is no real explicit copy
by value in php since internally the engine uses copy on write.  there is a
caveat in __clone however, where you can force new instances of objects to
be created in cloned instance variables rather than a reference to the
original object in member variables of the cloned instance.


>  Class
> A extends 3 other classes.  I would like a way to not have to manage a
> 'copy' method in B if A or one of the subclasses of A change.
>

well since B extends A directly here, obviously theres no need to worry
about other subclasses of A changing when considering the copy constructor
(__clone in php) of B.  anyway if A changes you need only potentially revise
the __clone() implementation of A.


> I was reading about clone, but this doesn't really seem to help me in
> this situation.
>

egh?  define __clone() in A to be responsible for any member variables
defined in A then define __clone() in B to be responsible for any additional
member variables defined therein, making sure to invoke parent::__clone()
from B.  naturally if you add or remove member variables from A you revise
the __clone() implementation of A, the same is true for B.

this isnt a maintenance nightmare and is also a sound oop practice.  the
same idea would be applied to a copy constructor in java or c++.

note also that php internally manages a shallow copy out of the box, meaning
you wont have to implement any code to 'copy' non-object member variables in
cloned objects.  you will however have to determine if a reference to the
same objects in instance variables of cloned objects is appropriate or if
each instance of the class you are cloning needs to have their own instance
of member variables referencing objects.

here is a somewhat contrived example which illustrates everything.  all you
have to do is run the script and inspect the id of each object stored in
member variables.  what youll find is that in each instance of B shallowObj
is referring to the same instance of stdClass, yet anotherObj from B and
someObj from A are each explicitly 'copied' for each instance of B.  also
youll notice that A manages cloning of someObj and B manages cloning of
anotherObj, keeping in line with encapsulation.  also notice the scalar
variables are 'copied' by the engine w/ no work in userspace.

<?php
class A
{
    public $blah = 0;
    protected $meh = 1;
    private $care = 2;
    private $someObj = null;

    public function __construct()
    {
        $this->someObj = new stdClass();
    }

    public function __clone()
    {
        // note only if we want to force a new instance of
        // stdClass into $this->someObj in cloned instance of A
        $this->someObj = clone $this->someObj;
    }
}

class B extends A
{
    protected $what = 5;
    protected $anotherObj = null;
    protected $shallowObj = null;

    public function __construct()
    {
        parent::__construct();
        $this->anotherObj = new stdClass();
        $this->shallowObj = new stdClass();
    }

    public function __clone()
    {
        // make sure to invoke parent __clone()
        parent::__clone();

        // note as in A::__clone() above, forcing new instance
        // of $this->anotherObj in cloned instance of B
        $this->anotherObj = clone $this->anotherObj;

        // note also that we do not clone $this->shallowObj here,
        // meaning each instance of B will reference the same instance of
$this->shallowObj
    }
}

$b1 = new B();
$b2 = clone $b1;

var_dump(array(
    $b1, $b2
));
?>

/// output
array(2) {
  [0]=>
  object(B)#1 (7) {
    ["what":protected]=>
    int(5)
    ["anotherObj":protected]=>
    object(stdClass)#3 (0) {
    }
    ["shallowObj":protected]=>
    object(stdClass)#4 (0) {
    }
    ["blah"]=>
    int(0)
    ["meh":protected]=>
    int(1)
    ["care":"A":private]=>
    int(2)
    ["someObj":"A":private]=>
    object(stdClass)#2 (0) {
    }
  }
  [1]=>
  object(B)#5 (7) {
    ["what":protected]=>
    int(5)
    ["anotherObj":protected]=>
    object(stdClass)#7 (0) {
    }
    ["shallowObj":protected]=>
    object(stdClass)#4 (0) {
    }
    ["blah"]=>
    int(0)
    ["meh":protected]=>
    int(1)
    ["care":"A":private]=>
    int(2)
    ["someObj":"A":private]=>
    object(stdClass)#6 (0) {
    }
  }
}

hope it helps!

-nathan