简单一点就是定义一个接口，然后合约（其它）地址+调用方法

接口形式

interface IRC20 {
    function withdraw(uint256 amount) external; 
}

contract Attack {

    address constant private addr = 0x78E74b14512f2f9d3C26aeE24d902Fef10F46d73;

    IRC20 private tract;

    event withdrawEth(address sender,uint256 value);

    constructor(){
        tract = IRC20(addr);
    }

    function despoit() public payable{
        tract.withdraw(1 ether);
    }
    }