CI框架的load_class函数

if ( ! function_exists('load_class'))
{
    /**
     * Class registry
     *
     * This function acts as a singleton. If the requested class does not
     * exist it is instantiated and set to a static variable. If it has
     * previously been instantiated the variable is returned.
     *
     * @param    string    the class name being requested
     * @param    string    the directory where the class should be found
     * @param    string    an optional argument to pass to the class constructor
     * @return    object
     */
    function &load_class($class, $directory = 'libraries', $param = NULL)
    {
        static $_classes = array();

        // Does the class exist? If so, we're done...
        if (isset($_classes[$class]))
        {
            return $_classes[$class];
        }

        $name = FALSE;

        // Look for the class first in the local application/libraries folder
        // then in the native system/libraries folder
        foreach (array(APPPATH, BASEPATH) as $path) //核心代码的路径和项目代码的路径
        {
            if (file_exists($path.$directory.'/'.$class.'.php'))
            {
                $name = 'CI_'.$class;

                if (class_exists($name, FALSE) === FALSE)
                {
                    require_once($path.$directory.'/'.$class.'.php');
                }

                break;
            }
        }

        // Is the request a class extension? If so we load it too
        if (file_exists(APPPATH.$directory.'/'.config_item('subclass_prefix').$class.'.php'))
        {
            $name = config_item('subclass_prefix').$class;

            if (class_exists($name, FALSE) === FALSE)
            {
                require_once(APPPATH.$directory.'/'.$name.'.php');
            }
        }

        // Did we find the class?
        if ($name === FALSE)
        {
            // Note: We use exit() rather than show_error() in order to avoid a
            // self-referencing loop with the Exceptions class
            set_status_header(503);
            echo 'Unable to locate the specified class: '.$class.'.php';
            exit(5); // EXIT_UNK_CLASS
        }

        // Keep track of what we just loaded
        is_loaded($class);

        $_classes[$class] = isset($param)
            ? new $name($param)
            : new $name();
        return $_classes[$class];
    }
}