为派生类覆盖>>运算符

Question

I have a program I am working on that requires us to overload the >> for the Employee class so that when it's used it reads in the right type of employee so I have something like this in my main program:

 Employee *emp;
 empIn >> emp;

I figured the base class is where I would want to do it, because it is the only one that applies to all of the derived classes. The type is determined by an integer at the start of the line so I figured that something like this might work (as I don't know the type until I read it):

istream &operator >> (istream &stream, Employee &emp)
{
   int type;
   stream >> type;
   switch(type){
      case 1:
         *emp = new Hourly;
         break;
           ...
   }
   return stream;
}

But, it doesn't work. I'm I going about this the right way? And if not, please point me in the right direction.

Answer 1

The problem, it seems, is that you mix references and pointer. In the operator>> function, you receive emp as a reference, but access it as a pointer, which is wrong.

For the whole thing to work, the emp object needs to be allocated before you try to input to it. You can not allocate it inside the function.

Employe emp;
empIn >> emp;

Or

Employe *emp = new Employe;
empIn >> *emp;

And of course, don't use new inside the function.

The compiler should have given you quite a lot of errors for this, you should check out what they say first.

Answer 2

The second argument to your istream function should be Employee* &emp if you want to be able to assign to the pointer as in your example usage. The type would then be a reference to a pointer, which lets you assign it as you want to do instead the method.