php类中引用smarty的有关问题?

php类中引用smarty的问题???
问题概述：
想在类中调用smarty方法，却出现如下错误：
PHP Fatal error: Using $this when not in object context in G:\SVN\YCMS\localhost\core\smarty\sysplugins\smarty_internal_data.php on line 38 
下面贴代码，希望各位能帮我下[app/model/home.php已经通过其他页面引入了_Model类]：
app/model/home.php

PHP code

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<?php /**
 * Name: app-model-home
 * Author: yinhexi
 */
if(!defined('IN_MODEL')) die('Access Violation Error:app-model-home!');

class home extends _Model {

    public static function index() {
        parent::assign('title', 'YCMS');
    }

}

?>

core/model/index.php

PHP code

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<?php /**
 * Name: core-model-index
 * Author: yinhexi
 */
define('IN_MODEL', 1);
include_once(Y_CORE.DS.'config'.DS.'smarty.php'); //引入smarty类

class _Model extends Smarty {

    public function __construct() {

    }

    //检测相关类中是否存在对应方法
    protected function instance($c_f,$c_c_m) {
        if(!in_array($c_f,get_class_methods($c_c_m))) {
            die("Model: Class $c_c_m does not exist in Method $c_f!");
        }
    }

}


?>

core/config/smarty

PHP code

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<?php /**
 * Name: core-config-smarty
 * Author: yinhexi
 */
if(!defined('IN_CORE')) die('Access Violation Error:core-config-smarty!');

include_once(Y_CORE.DS.'smarty'.DS.'Smarty.class.php');

$smarty = new Smarty();
$smarty->template_dir = Y_APP.DS.'view';//设置模版目录
$smarty->compile_dir = Y_CACHE.DS."view_c";//设置编译目录
$smarty->config_dir = Y_APP.DS.'config'.DS.'smarty';//配置目录变量
//$smarty->php_handling = SMARTY_PHP_ALLOW; //不赞成设置为SMARTY_PHP_ALLOW
$smarty->left_delimiter = "{";
$smarty->right_delimiter = "}";

//compile
#$smarty->compile_check = false;//编译检查变量

//Cache
$smarty->caching = true; //缓存模版的输出
$smarty->cache_dir = Y_CACHE; //存放模版缓存的目录名
#$smarty->cache_lifetime = 1; //缓存生命周期
?>

文件关联都没有问题,有些在这没有显示。

提示的错误所指向的是smarty源代码，在此也贴出：

PHP code

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 public function assign($tpl_var, $value = null, $nocache = false)
    {
        if (is_array($tpl_var)) {
            foreach ($tpl_var as $_key => $_val) {
                if ($_key != '') {
                    $this->tpl_vars[$_key] = new Smarty_variable($_val, $nocache);
                }
            }
        } else {
            if ($tpl_var != '') {
                $this->tpl_vars[$tpl_var] = new Smarty_variable($value, $nocache);
            }
        }
    }

望各位不吝指教！

------解决方案--------------------
assign需要实例化后才能引用，你的类home中有一句，parent::assign()这句其父类根本没有实例化