How to make login mandatory before adding products into cart?

We are observing that visitors come and add products into cart as "Guest" and than left the site, and we have no info like theirs email addresses, contact numbers to provide them some discount to complete their orders. so we want to make the login mendatory for visitors as if they left and there are products in cart, we can make a contact with them easily. so please if anybody done it beofre guide me...

Hi,

Try disable Public store. Enable shopping cart for guests in configuration > access control list. But in that case guests won`t see "add to cart"

dunaenko.m wrote:

In this case I think it's good that they don't see the 'add to cart' until they've logged in. If they did, the alternative will be that when the customer clicks the 'add to cart' button that they will be taken to the authentication page. Once they've logged in they will then be asking 'did my product get added to the cart?' and will have to perform this extra step.