codeigniter cache not working

codeigniter cache not working

I'm using memcached in codeigniter framework. But it not working :( In my
controller:
$this->load->driver('cache');
$cachedata= $this->cache->memcached->get('cachedata');
if ($cachedata) {
exit($cachedata);
}
$this->cache->memcached->save('cachedata',
$data['status'], 0);
But when i'm run it not working
<h4>A PHP Error was encountered</h4>
<p>Severity: Notice</p>
<p>Message: Undefined property: Offers::$cache</p>
<p>Filename: controllers/data.php</p>
<p>Line Number: 55</p>
</div><div style="border:1px solid #990000;padding-left:20px;margin:0 0
10px 0;">
<h4>A PHP Error was encountered</h4>
<p>Severity: Notice</p>
<p>Message: Trying to get property of non-object</p>
<p>Filename: controllers/data.php</p>
<p>Line Number: 5</p>
</div><br />
<b>Fatal error</b>: Call to a member function get() on a non-object<br />