overriding Cart.php class doesn't work in prestashop

overriding Cart.php class doesn't work in prestashop

I am try to override the default Cart.php class. I put my Cart.php in
override/classes/Cart.php. but It doesn't make any changes in the output.
<?php
class Cart extends CartCore
{
print_r("sample test override");
die();
}
?>
my cart.php code is like above. Can anyone help me, is there any other
additional steps needed to override?