Solidity Error Handling Example

pragma solidity ^0.4.0;

contract TheSharer {
    function sendHalfSum(address addrTo) payable returns (uint balance) {
        require(msg.value % 2 == 0); // Allow even numbers only
        uint balanceBefore = this.balance;
        addrTo.transfer(msg.value / 2);
        // Because transfer will throw an exception if it fails and
        // is unable to call back here, we should have no way to
        // = have half of all the money still.
        assert(this.balance == balanceBefore - msg.value / 2);
        return this.balance;
    }
}