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Design a class that should always return a single object ?
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Answer Posted / webguy
The answer is to use : The singleton pattern
Description:
Some application resources are exclusive in that there is
one and only one of this type of resource. For example, the
connection to a database through the database handle is
exclusive. You want to share the database handle in an
application because it's an overhead to keep opening and
closing connections, particularly during a single page fetch.
The singleton pattern covers this need. An object is a
singleton if the application can include one and only one of
that object at a time. The code in Listing 3 shows a
database connection singleton in PHP V5.
Ex: Singleton.php
<?php
require_once("DB.php");
class DatabaseConnection
{
public static function get()
{
static $db = null;
if ( $db == null )
$db = new DatabaseConnection();
return $db;
}
private $_handle = null;
private function __construct()
{
$dsn = 'mysql://root:password@localhost/photos';
$this->_handle =& DB::Connect( $dsn, array() );
}
public function handle()
{
return $this->_handle;
}
}
print( "Handle = ".DatabaseConnection::get()->handle()."\n" );
print( "Handle = ".DatabaseConnection::get()->handle()."\n" );
?>
This code shows a single class called DatabaseConnection.
You can't create your own DatabaseConnection because the
constructor is private. But you can get the one and only one
DatabaseConnection object using the static get method. The
UML for this code is shown in Figure 3.
Figure 3. The database connection singleton
The database connection singleton
The proof in the pudding is that the database handle
returned by the handle method is the same between two calls.
You can see this by running the code on the command line.
% php singleton.php
Handle = Object id #3
Handle = Object id #3
%
The two handles returned are the same object. If you use the
database connection singleton across the application, you
reuse the same handle everywhere.
You could use a global variable to store the database
handle, but that approach only works for small applications.
In larger applications, avoid globals, and go with objects
and methods to get access to resources.
| Is This Answer Correct ? | 1 Yes | 0 No |
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