Solidity - Pay with one token for another one

Question

I would like to create two types of tokens:

1. Let's call them ND, about 10 contracts with non-divisible tokens (so far) in the mapping uint => address
2. Let's call it DV, one contract with divisible ether-like tokens

In my contract users can buy DV for ether, it's working. But the problem is that I want to make an automatic way to buy ND with DV, for example with function.

For example I set price 0.1 of ND[3] (3rd from the array of 10 total), and when user want to buy ND[3] and send 0.1 of DV, will recive 1 ND[3].

I would need to implement cross trade between contracts somehow. Does anybody have any kind of help how can I make such thing?

For now two tokens are pretty basic from tutorial with simple buy for ether function and decimals difference, as said above, so I don't think there is any need to share that extremely basic code.

Answer 1

As token contract itself usually doesn't include any purchasing mechanisms you can probably have quite standard token contracts. Especially if transferring DV tokens in normal way is ok - if it's not then you need to disallow transfers where the transferrer (msg.sender) is not ND.

To allow purchasing/trading of DV tokens from ND, you need an extra purchase/trade function. This function should only allow calls from the ND contract, so something like this:

address ndAddress;

    modifier onlyND() {
        require(msg.sender == ndAddress);
        _;
    }

    function purchase() onlyND public {
        // purchase process
    }

As you create both tokens you can just add whatever purchase logic you wish - decrease/increase balances as needed. Remember that you need special implementation in both token contracts to modify its internal token balances - you can't modify another token's internal balances directly from a different token contract.