Php passing object by value

Question

I am playing a bit with recursion in php. Unfortunately php value smart copying idea and passing objects by reference by default is not making it easier. Problem occur because modifications made by iteration number X are visible in iteration ex X-2

For example:

/**
 * Silly function to find last element in array
 * @param ArrayObject $input  
 */
function process(ArrayObject $input) {
    if ($input->count() == 1) {
        return $input->getIterator()->current();
    }
    $ar = $input->getArrayCopy();
    array_shift($ar);
    $input->exchangeArray($ar);
    return process($input);
}

$in = new ArrayObject(range('a', 'd'));
echo 'Before ' . PHP_EOL;
var_dump($in);
echo PHP_EOL . 'Process - last element is: ' . process($in) . PHP_EOL;
echo 'After ' . PHP_EOL;
var_dump($in);

output is

Before 
object(ArrayObject)#1 (1) {
  ["storage":"ArrayObject":private]=>
  array(4) {
    [0]=>
    string(1) "a"
    [1]=>
    string(1) "b"
    [2]=>
    string(1) "c"
    [3]=>
    string(1) "d"
  }
}

Process - last element is: d
After 
object(ArrayObject)#1 (1) {
  ["storage":"ArrayObject":private]=>
  array(1) {
    [0]=>
    string(1) "d"
  }
}

as you see recursion modified also original $in variable, what I expect is that every new iteration will operate on copy of value. In this example it is not needed, but what in case that recursion function is more complicated.

Simple solution - clone

function process(ArrayObject $input) {
    $input = clone ($input);
    if ($input->count() == 1) {
        return $input->getIterator()->current();
    }
    $ar = $input->getArrayCopy();
    array_shift($ar);
    $input->exchangeArray($ar);
    return process($input);
}

ok, works but what if input is very complicated multi-object nested recurred structure? Ok then I can serialize and unserialze value as a substitute for deep copy

function process(ArrayObject $input) {
    $input = unserialize(serialize($input));
    if ($input->count() == 1) {
        return $input->getIterator()->current();
    }
    $ar = $input->getArrayCopy();
    array_shift($ar);
    $input->exchangeArray($ar);
    return process($input);
}

perfect - it works, but it is a bit waste of time and cpu to un/serialize in every iteration (especially with huge $input).

Is there any other way to do it, something what lets me not worry that I need to 'hack a bit' to use php for standard usage.

//EDIT example with Tigrang suggestion

Tigrang your suggestion was very interesting, but

function process(ArrayObject $input) {
    $input = new ArrayObject(($input));
    $input->offsetSet(null, 'f');
}

$in = new ArrayObject(range('a', 'd'));
echo 'Before ' . PHP_EOL;
var_dump($in);
process($in);
echo 'After ' . PHP_EOL;
var_dump($in);

 After 
object(ArrayObject)#1 (1) {
  ["storage":"ArrayObject":private]=>
  array(5) {
    [0]=>
    string(1) "a"
    [1]=>
    string(1) "b"
    [2]=>
    string(1) "c"
    [3]=>
    string(1) "d"
    [4]=>
    string(1) "f"
  }
}

it still keeps reference to external variable

//edit 2

class Engine {

    public $power = 999;

}

class Car {

    public $name = '';
    public $engine = '';

    public function __construct($name, $power) {
        $this->name = $name;
        $this->engine = new Engine();
        $this->engine->power = $power;
    }

}

function process(ArrayObject $input) {
    $input = new ArrayObject(($input->getArrayCopy()));
    $ford = $input[0];
    $ford->name = 'Audi';
    $ford->engine->power  = 1500;
}

$ar = array(new Car('Ford',  130));
$in = new ArrayObject($ar);
echo 'Before ' . PHP_EOL;
var_dump($in);
process($in);
echo 'After ' . PHP_EOL;
var_dump($in);

@zerkms - please read description under third grey box, copy is not a perfect solution (unfortunately) — mrok
– mrok, Commented Aug 22, 2012 at 22:27
@mrok: Why? There's no advantage whatsoever in iterating objects instead of arrays. In fact, the opposite is true, arrays have better performance, and are easier to traverse and manipulate using array functions. — Madara's Ghost
– Madara's Ghost, Commented Aug 22, 2012 at 22:31

raidenace · Accepted Answer · 2012-08-22 22:55:38Z

3

AFAIK, this is a limitation in PHP. And as you pointed out this happens because of two things in PHP 5x which are not complimentary.

Objects are always passed as references
clone() only does shallow copy and there is no deep copy mechanism inherent in the language

unserialize(serialize($obj) is the only (highly inefficient) work-around currently available. Lets hope the Gods-that-be sort out some of these issues in the upcoming versions, instead of only breaking their head about how to implement unicode in the much-hyped (and delayed) version 6!

edited Aug 22, 2012 at 22:55

answered Aug 22, 2012 at 22:26

raidenace

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Comments

tigrang · Accepted Answer · 2012-08-22 22:59:26Z

0

Why not $input = new ArrayObject($input);

As far as I can tell it copies deeply nested and is faster than serialize: http://viper-7.com/kHLCCP

answered Aug 22, 2012 at 22:59

tigrang

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4 Comments

mrok Over a year ago

Interesting, but still does not solve the problem, please look at question update.

tigrang Over a year ago

Then try $input = new ArrayObject($input->getArrayCopy()); Still faster than serialize

mrok Over a year ago

a bit better - but still does not work with objects, please check 2nd update

tigrang Over a year ago

Ah I see the issue with it now. I guess serializing is the only option.

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