如何在c ++中清除istringstream对象的缓冲区？

I am trying to validate the input in a way that the user always has to enter kg or lb, if it doesn't do it will ask the user to reenter. It works until that point but it doesn't read the new input. I know that I need to clear the buffer but iss.clear() is not working. I am trying to validate the input in a way that