Checking balance of msg.sender before 'approve'

Question

I analyzed this token contract on open-zepplin here : https://github.com/OpenZeppelin/zeppelin-solidity/blob/master/contracts/token/ERC20/StandardToken.sol

I wonder why there is not a statement in approve function which should check whether owner holds those tokens.

Answer 1

Because the balance is checked at transferFrom anyway. In between the calls, the balance could have been updated; the user could have transfered their tokens just before transferFrom is called. That means that any security has to be implemented on transferFrom, and when that is done, approve does not need to do the checks