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Creating Core System Classes
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<h1>Creating Core System Classes</h1>
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<p>Every time CodeIgniter runs there are several base classes that are initialized automatically as part of the core framework.
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It is possible, however, to swap any of the core system classes with your own versions or even extend the core versions.</p>
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<p><strong>Most users will never have any need to do this,
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but the option to replace or extend them does exist for those who would like to significantly alter the CodeIgniter core.</strong>
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<p class="important"><strong>Note:</strong> Messing with a core system class has a lot of implications, so make sure you
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know what you are doing before attempting it.</p>
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<h2>System Class List</h2>
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<p>The following is a list of the core system files that are invoked every time CodeIgniter runs:</p>
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<h2>Replacing Core Classes</h2>
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<p>To use one of your own system classes instead of a default one simply place your version inside your local <dfn>application/core</dfn> directory:</p>
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<code>application/core/<dfn>some-class.php</dfn></code>
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<p>If this directory does not exist you can create it.</p>
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<p>Any file named identically to one from the list above will be used instead of the one normally used.</p>
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<p>Please note that your class must use <kbd>CI</kbd> as a prefix. For example, if your file is named <kbd>Input.php</kbd> the class will be named:</p>
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class CI_Input {<br /><br />
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<h2>Extending Core Class</h2>
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<p>If all you need to do is add some functionality to an existing library - perhaps add a function or two - then
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it's overkill to replace the entire library with your version. In this case it's better to simply extend the class.
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Extending a class is nearly identical to replacing a class with a couple exceptions:</p>
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<li>The class declaration must extend the parent class.</li>
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<li>Your new class name and filename must be prefixed with <kbd>MY_</kbd> (this item is configurable. See below.).</li>
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<p>For example, to extend the native <kbd>Input</kbd> class you'll create a file named <dfn>application/core/</dfn><kbd>MY_Input.php</kbd>, and declare your class with:</p>
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class MY_Input extends CI_Input {<br /><br />
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<p>Note: If you need to use a constructor in your class make sure you extend the parent constructor:</p>
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class MY_Input extends CI_Input {<br />
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function __construct()<br />
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{<br />
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parent::__construct();<br />
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}<br />
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<p class="important"><strong>Tip:</strong> Any functions in your class that are named identically to the functions in the parent class will be used instead of the native ones
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(this is known as "method overriding").
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This allows you to substantially alter the CodeIgniter core.</p>
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<p>If you are extending the Controller core class, then be sure to extend your new class in your application controller's constructors.</p>
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<code>class Welcome extends MY_Controller {<br />
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function __construct()<br />
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{<br />
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parent::__construct();<br />
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}<br />
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function index()<br />
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{<br />
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$this->load->view('welcome_message');<br />
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}<br />
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<h3>Setting Your Own Prefix</h3>
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<p>To set your own sub-class prefix, open your <dfn>application/config/config.php</dfn> file and look for this item:</p>
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<code>$config['subclass_prefix'] = 'MY_';</code>
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<p>Please note that all native CodeIgniter libraries are prefixed with <kbd>CI_</kbd> so DO NOT use that as your prefix.</p>
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