Pointer to base class-argument type in friend function

Question

I'm writing a C++ program involving polymorphism. I need to overload the operators "<<" and ">>" as friend functions. I have the base class base and 3 derived classes: der1,der2,der3 and a pointer to the base class * p. If I declare the functions as

istream &operator>>(istream &i,der1 &d1) {...}
istream &operator>>(istream &i,der2 &d2) {...}
istream &operator>>(istream &i,der3 &d3) {...}

and call them like

base *p;
p=new der1;
cin>>p;

the program doesn't compile as there's no function with a base parameter. But if I declare it so,it doesn't make any difference if the actual object is of type der1,der2 or der3,although I want it to behave differently depending on the type of the object. All I could think of was to create a virtual function in base and override it the inherited classes. But thus,I'll have to call the functions as

*p>>cin

which doesn't look so natural. How can I solve the problem?

Answer 1

You can create an operator>> for base class:

istream &operator>>(istream &i, base *b) {
    b->ReadFromIstream(i);
    return i;
}

Then you make the ReadFromIstream a virtual function, and overload it from the derived classes. This way you have the same syntax cin >> p, and they behave differently according to the type of p.