pragma solidity ^0.4.25;

contract BalanceHolder {

    mapping(address => uint256) public balanceOf;

    event LogWithdraw(
        address indexed user,
        uint256 amount
    );

    function withdraw() 
    public {
        uint256 bal = balanceOf[msg.sender];
        balanceOf[msg.sender] = 0;
        msg.sender.transfer(bal);
        emit LogWithdraw(msg.sender, bal);
    }

}