2 way smart contract exchange

Question

Need a smart contract to exchange ERC20 for Ether and Ether for ERC20.

The goal is, you send ether you get erc20, you send erc20 you get ether back. Without having to use web3 tools.

The only way I can think for implementation is to use the ERC20 itself as exchange contract.

Then the token address will have to hold both ether and its own tokens.

This sounds like bad practice, or is it? Any better idea to achieve that?

Answer 1

You can certainly combine them, but I think it makes more sense to have a second contract. That way the ERC20 token contract stays simple and focused on just the token interface.

The contract doing the selling just needs two functions that deal with the token contract. In pseudocode, ignoring error checking, etc.

buyTokens():
    howMany = msg.value / pricePerToken * 10**decimals
    tokenContract.transfer(msg.sender, howMany)

sellTokens(howMany):
    // Caller will need to first approve() that many tokens
    tokenContract.transferFrom(msg.sender, howMany * 10**decimals)

    msg.sender.transfer(howMany * pricePerToken)