Interface function in Ethereum contract - solidity

Question

I was looking at the code on the Ethereum website, what does this code do?
interface tokenRecipient { function receiveApproval(address _from, uint256 _value, address _token, bytes _extraData); }
Available at: https://ethereum.org/token#full-coin-code
Thanks!

Answer 1

It does nothing as it's an interface. An interface is a way to define constraints so that you can communicate with any object that implements this and know that the functions defined in the interface will existing in the implementation.
interface tokenRecipient {
function receiveApproval(address _from, uint256 _value, address _token, bytes _extraData);
}
One example of this would be a way from one contract to communicate to another and looks like an ERC223 implementation for a recipient
In contract you want to communicate to you have to have implemented the tokenRecipient e.g.
pragma solidity ^0.4.0;
interface tokenRecipient {
function receiveApproval(address _from, uint256 _value, address _token, bytes _extraData);
}
contract MyContract is tokenRecipient {
function receiveApproval(address _from, uint256 _value, address _token, bytes _extraData) {
// functionality
}
}
So if we now implement a contract me know that we can interact with any contract that has tokenRecipient implemented e.g.
pragma solidity ^0.4.0;
interface tokenRecipient {
function receiveApproval(address _from, uint256 _value, address _token, bytes _extraData);
}
contract RemoteContract {
function func(address _addr, uint _value) {
tokenRecipient _tokenRecipient = tokenRecipient(_addr);
_tokenRecipient.receiveApproval(msg.sender, _value, address(this), empty);
}
}
It will error if the call fails which is what we need to happen to prevent things such as a token being sent to a contract which cannot do anything with it. As Solidity is atom and works in a transactional way if the contract cannot receive it then it will rollback all functionality already executed.