<< operator overloading

lazyboy0001

I have a base class:

class base;

And some derived classes from base:

class derived1 : public base {
double a;
int        b;
};   // << should output both a and b

class derived2 : public base {
string   c;
};  // << should output only c

How could I implement a << operator overloading that can work on
both derived1 and derived2? What's more, it can determine the type
that pointed to by a base pointer, like,

base *pbase;
derived1  d1;
derived2 d2;

cout<<d1;  
cout<<d2;
pbase = &d1;
cout<<*pbase;  // should give output the same as d1
pbase = &d2;
cout<<*pbase;  // should give output the same as d2

How can I implement such an operator overloading? virtual seems won't
work with << overloading.