Item 21. Overriding Virtual Functions

Difficulty: 6

Virtual functions are a pretty basic feature, but they occasionally harbor subtleties that trap the unwary. If you can answer questions like this one, then you know virtual functions cold, and you're less likely to waste a lot of time debugging problems like the ones illustrated below.

In your travels through the dusty corners of your company's code archives, you come across the following program fragment written by an unknown programmer. The programmer seems to have been experimenting to see how some C++ features work. What did the programmer probably expect the program to print, and what is the actual result?

#include <iostream> 
#include <complex>
using namespace std;
class Base
{
public:
  virtual void f( int );
  virtual void f( double );
  virtual void g( int i = 10 );
};
void Base::f( int )
{
  cout << "Base::f(int)" << endl;
}
void Base::f( double )
{
  cout << "Base::f(double)" << endl;
}
void Base::g( int i )
{
  cout << i << endl;
}
class Derived: public Base
{
public:
  void f( complex<double> );
  void g( int i = 20 );
};
void Derived::f( complex<double> )
{
  cout << "Derived::f(complex)" << endl;
}
void Derived::g( int i )
{
  cout << "Derived::g() " << i << endl;
}
void main()
{
  Base    b;
  Derived d;
  Base*   pb = new Derived;
  b.f(1.0);
  d.f(1.0);
  pb->f(1.0);
  b.g();
  d.g();
  pb->g();
  delete pb;
}