Virtual Base Classes in C++

An element of ambiguity can be introduced into a C++ program when multiple base classes are inherited. For example, consider this incorrect program:
#include <iostream.h>
using namespace std;
class base {
public:
virtual void vfunc() {
cout << "This is base's vfunc().\n";
}
};

class derived1 : public base {
public:
void vfunc() {
cout << "This is derived1's vfunc().\n";
}
};

class derived2 : public base {
public:
void vfunc() {
cout << "This is derived2's vfunc().\n";
}
};

void f(base &r) {
r.vfunc();
}

int main()
{
base b;
derived1 d1;
derived2 d2;
f(b); // pass a base object to f()
f(d1); // pass a derived1 object to f()
f(d2); // pass a derived2 object to f()
return 0;
}