Magento 1.9 overwrite core

Question

Is it possible to overwrite Magento core functionality in this way? -

Mage_Sales_Block_Order_Total {

    protected function _initTotals(){
    //this is core 
    }
}

and overwrite it with this -

//This Totals class path would be Mage_Sales_Block_Order_Totals
//but in local codePool
//now is there a need to create a rewrite in config.xml and register module 
//in app/etc/ if I would do it this way? Since local codePool is loaded 
//first

Totals extends Mage_Sales_Block_Order_Totals {

    protected function _initTotals(){
    //changed core functionality
    }
}

Answer 1

This has been asked before and even something I had to iron out.

This link should help: Override Magento Class Community Code Pool.

This is a nice tutorial that covers what to do as well: https://sherocommerce.com/how-to-override-rewrite-classes-in-magento/