如何在我们的自定义模块magento中覆盖购物车控制器优惠券操作

I'm having a module which needs to override coupon code functionality

 <frontend>
    <routers>
        <customcoupons>
            <use>standard</use>
            <args>
                <module>Mycompany_Customcoupons</module>
                <frontName>customcoupons</frontName>
            </args>
        </customcoupons>
        <checkout><!-- Override code starts-->
             <args>
                 <modules>
                    <Mycompany_Customcoupons before="Mage_Checkout">Mycompany_Customcoupons</Mycompany_Customcoupons>
                </modules>
             </args>
        </checkout><!-- Override code ends-->
    </routers>
    <layout>
        <updates>
            <customcoupons>
                <file>customcoupons.xml</file>
            </customcoupons>
        </updates>
    </layout>
</frontend>

My custom module admin page working fine if i didn't add override code but if i add <checkout> block it show 404 page. What could be the problem? how to add override block in this?

Please write this in your config.XML

<global>
    <rewrite>
         <mycompany_customcoupons>
               <from><![CDATA[#^/checkout/cart/#]]></from>
               <to>/customcoupons/cart/</to>
         </mycompany_customcoupons>
    </rewrite>

</global>

Now Create File : app/code/local/Mycompany/Customcoupons/CartController.php

In CartController.php, write bellow code.

<?PHP

require_once 'Mage/Checkout/controllers/CartController.php';

class Mycompany_Customcoupons_CartController extends Mage_Checkout_CartController
{

   public function couponPostAction()
    {
         // Start You code here.
    }
}

?>

You need to add block override code into this XML. THis link will be helpful: http://magentocustom.blogspot.in/2011/05/how-to-override-magento-block-classes.html