表示一个留白，等待填充代码， 他在修改器（Modifier）中使用。 举个例子：

contract owned {
    function owned() public { owner = msg.sender; }
    address owner;

    modifier onlyOwner {
        require(
            msg.sender == owner,
            "Only owner can call this function."
        );
        _;    // 使用了_
    }

   // 使用了onlyOwner 修饰
    function withdraw() public onlyOwner {
        msg.sender.transfer(10 ether);
    }
}

withdraw() 展开之后：

    function withdraw() public onlyOwner {
     require(
            msg.sender == owner,
            "Only owner can call this function."
        );        
        msg.sender.transfer(10 ether);    // 填补了 _
    }