why could I not transfer by using msg.sender instead of passing address as an argument to the function?

Question

The function withdraw() is not working, it can not transfer. But the function withdraw2() is working. What's wrong with my code?

pragma solidity ^0.4.18;

contract SupportPay4 {

    function() public payable {

    }

    function deposit() public payable {

    }

    function withdraw() public {
        msg.sender.transfer(getBalance());
    }

    function withdraw2(address addr) public {
        addr.transfer(getBalance());
    }

    function getBalance() public constant returns(uint256) {
        return address(this).balance;
    }
}

Answer 1

If you are using v0.8 you need to change this to

payable(msg.sender).transfer(address(this).balance);

Hope this helps

Answer 2

Try like this:

pragma solidity ^0.4.18;

contract SupportPay4 {

    function() public payable {

    }

    function deposit() public payable {

    }

    function withdraw() public {
        msg.sender.transfer(getBalance());
    }

    function withdraw2() public {
        msg.sender.transfer(address(this).balance);
    }

    function getBalance() public constant returns(uint256) {
        return address(this).balance;
    }
}

Hope it helps.