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clone versus no clone

Posted on 2013-12-07
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Last Modified: 2013-12-13
<?php
class CopyClass{
  public $name;
}

$first= new CopyClass();
$first->name="Number 1";
$second= clone $first;
echo "first = ".$first->name."<br>";
$second->name="Number 2";
echo "first = " . $first->name . "<br>";
echo "second = " . $second->name . "<br>";

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I know that when the word 'clone' is used
Changing second, does not change first

But is there a real world example of clone versus no clone
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Question by:rgb192
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10 Comments
 
LVL 111

Expert Comment

by:Ray Paseur
ID: 39703437
Have a read of this article:
http://www.experts-exchange.com/Web_Development/Web_Languages-Standards/PHP/A_12310-PHP-Variables-and-References.html

The real-world example might exist in an environment where an object gets mutated and you need a second copy of the object.  Perhaps you call a method after property  #1 is added to the object, then another method after property #2, etc.  If you want a record of the state of the object after each property is added you can clone the object after each addition.

You can also nullify the clone() method, as in a Singleton design pattern.

<?php // RAY_database_singleton.php
error_reporting(E_ALL);


// SINGLETON DATA BASE CONNECTION CLASS
class Database
{
    // CLASS PROPERTIES ARE ALL PRIVATE
    private static $connection;
    private static $instance;

    // CONNECTION VALUES
    const DB_HOST = 'localhost';
    const DB_USER = '?';
    const DB_PASS = '?';
    const DB_NAME = '?';

    // NULLIFY THE CLONE
    final private function __clone() {}

    // OUR ONLY PUBLIC METHOD RETURNS THE CONNECTION
    public static function getConnection()
    {
        if (!self::$instance) self::$instance = new self();
        return self::$connection;
    }

    // CONSTRUCTOR CREATES THE CONNECTION
    private function __construct()
    {
        self::$connection
        = new mysqli
        ( self::DB_HOST
        , self::DB_USER
        , self::DB_PASS
        , self::DB_NAME
        )
        ;
        if (self::$connection->connect_error)
        {
            trigger_error(self::$connection->connect_error, E_USER_ERROR);
        }
    }
}

$mysql1 = database::getConnection();
$mysql2 = database::getConnection();


// PROVE THAT THESE ARE THE SAME OBJECT http://php.net/manual/en/language.oop5.object-comparison.php
if ($mysql1 === $mysql2) echo 'EQUAL';

// SHOW THE OBJECT
var_dump($mysql1);

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Author Comment

by:rgb192
ID: 39705095
Running in a debugger
name--------------- value------------------ type
$mysql1-----------none(empty)----------object(myqli)

and this was output
EQUALobject(mysqli)#2 (0) { }


If you want a record of the state of the object after each property is added you can clone the object after each addition.


Okay I understand clone alittle better when you have example (if coder needs a snapshot) so maybe real world is windows os system restore

Or maybe a boss/client told you to make a snapshot, if so why?
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LVL 111

Expert Comment

by:Ray Paseur
ID: 39707453
Why did a boss tell you to make a snapshot?   I dunno.  Ask the boss!

:-)

~Ray
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Author Comment

by:rgb192
ID: 39707596
I meant to ask. Why would you use clone? In what type of project is clone used?
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Accepted Solution

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Ray Paseur earned 2000 total points
ID: 39708912
Here is an example.  I needed an array of similar but distinct "mock objects" for test data.  In real life, these objects would probably come from rows of a query results set.  Because objects are passed by reference, each mutation of $object would change the original object.  By using clone, I am able to get distinct objects with different properties.

<?php // RAY_temp_neonate.php
ini_set('display_errors', TRUE);
error_reporting(E_ALL);
echo '<pre>';

// SORT AN ARRAY OF OBJECTS BY LASTNAME, FIRSTNAME
// SEE http://www.experts-exchange.com/Web_Development/Web_Languages-Standards/PHP/Q_28312470.html#a39708223

// SETUP SOME TEST DATA
$object = new stdClass;

$object->id    = "1";
$object->fname = "Ray";
$object->lname = "Paseur";
$object->work  = "Programmer";
$object->func  = "testData";
$arr[] = clone $object;

$object->id    = "2";
$object->fname = "John";
$object->lname = "James";
$object->work  = "Student";
$arr[] = clone $object;

$object->id    = "3";
$object->fname = "Dave";
$object->lname = "Baldwin";
$object->work  = "Expert";
$arr[] = clone $object;

// A FUNCTION TO COMPARE BY NAMES
function cmp_name($a, $b)
{
    $a_name = $a->lname . $a->fname;
    $b_name = $b->lname . $b->fname;
    if ($a_name == $b_name) return 0;
    return ($a_name < $b_name) ? -1 : 1;
}

// SORT THE ARRAY
usort($arr, 'cmp_name');

// SHOW THE WORK PRODUCT
print_r($arr);

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Author Comment

by:rgb192
ID: 39712457
Does arr[] hold 3 objects. Would $arr[1] be ray
0
 
LVL 111

Expert Comment

by:Ray Paseur
ID: 39712711
Arrays are numbered starting at zero.  Numeric array keys are frequently subject to being reset.  The array keys are shown in the print_r() output.  Just install the script and run it to see what it does with the data.  Easy!
0
 
LVL 1

Author Comment

by:rgb192
ID: 39718011
$object->id    = "1";
$object->fname = "Ray";
$object->lname = "Paseur";
$object->work  = "Programmer";
$object->func  = "testData";
$arr[] = /*clone*/ $object;

$object->id    = "2";
$object->fname = "John";
$object->lname = "James";
$object->work  = "Student";
$arr[] = /*clone*/ $object;

$object->id    = "3";
$object->fname = "Dave";
$object->lname = "Baldwin";
$object->work  = "Expert";
$arr[] = /*clone*/ $object;

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only had the last object repeated 3 times
so now I understand clone better
0
 
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Author Closing Comment

by:rgb192
ID: 39718014
thanks.
0
 
LVL 111

Expert Comment

by:Ray Paseur
ID: 39718025
Thanks for the points!
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