[RESOLVED] Object in Singleton design pattern

[ewawong]

Hi Guys,

i am a newbie of php programming, i just started php programming for a week a go.

i tried to create a singleton object in php, but i am facing some problem.

here is my code

<?php

require_once '../util/DbConn.php';


class SearchEngine
{

private static $m_pInstance;
private $titleArray;
private $metaKeyArray;


private function __construct() 
{ 
	echo "Constructor called<br />\n"; 

	$DbConn = new DbConn;

	$DbConn->connect();



	$result = $DbConn->getResult("SELECT itemid, title, metakey FROM jos_sobi2_item j");

	while($row = mysql_fetch_array($result))
	{
		//echo $row["itemid"]."<br>";
		//echo $row["title"]."<br>";
		//echo $row["metakey"]."<br>";

		$titleArray[$row["itemid"]] = $row["title"];

		$metaKeyArray[$row["itemid"]] = $row["metakey"];
	}

	echo $titleArray['5']."<br/>";
	echo $metaKeyArray['5']."<br/>";

	$DbConn->disconnect();

	echo "end of constructor";

}


public static function getInstance()
{
	if (!self::$m_pInstance)
	{
		self::$m_pInstance = new SearchEngine();
	}

	return self::$m_pInstance;
}


public function query()
{
	echo "message from query method<br />\n";
}

}


function testFunction()
{

$se = SearchEngine::getInstance();
$se->query();
echo "end of test method";
}

// Start the test
testFunction();
testFunction();
testFunction();
echo "after test method";



?>

after i run this file, all the code inside the constructor is able to be execute(select data from db, put in to array, print out from array, and echo the word of "end of constructor"), but after the the constructor is executed, other code is not running, it seems it stuck inside the constructor.........

so do you guys have any idea on what went wrong inside my code?

Hope to hear from you guys soon, Thanks alot in advance

[NogDog]

The idea behind a singleton is that only one instance of the object gets created, so by definition the constructor is only executed the first time you instantiate it (via the getInstance() method in this case). Every other call to getInstance will return a reference to the initially generated object and will not run the __construct() method.

So, if in fact it makes sense for this class to be a singleton and you only want that code in the constructor to be executed on the initial instantiation of the class as an object, then you probably want to save the data it generates in a class variable rather than simply echoing it out, so that other instances (references, really) to the object can access that data as needed.

[NogDog]

Oh, and before you get too enamored with the singleton pattern, be sure to read up on http://www.google.com/search?q=why+singletons+are+evil to make sure you really want to use it in this case. (I'm not saying to never use it, just to make an educated choice.)

[ewawong]

Hi Nogdog,

Thank you very much for your reply and information.

the code above is actually not the complete one. i just echo the generate value for testing purpose only, see whether the value can be insert in to the array successfully.

here is my problem :

output from the above coding

Constructor called
Cafe 57
cafe 57, fusion, western cuisine, local delights, bayan lepas, penang, malaysia
end of constructor

output after i remarked the code for all the database connection code inside the constructor and left only two echos (Constructor called, end of constructor)

Constructor called
end of constructor
message from query method
after test method
message from query method
after test method
message from query method
after test method

from the above testing shows that the code with the database connection code stop at the "end of constructor", after i remarked the database connection coding, it seem fine.

So what do you think?

[NogDog]

Nothing jumps out at me. All I can suggest is to turn on display_errors and set error_reporting to E_ALL, then see if PHP has anything "interesting" to tell us about what's happening.

[ewawong]

Hi NogDog,

i found the problem, after i turn on display_errors. it is caused by my other php file. There is nothing wrong with the above code.

now i only know how to turn on the display_errors 😛

Thanks for your help and sorry for wasting you time 😃

[NogDog]

No problem. Don't forget to mark this thread "resolved" via the Thread Tools link at the top.