This is a misconception that a lot of people understandably have when they first hear about the C++ Object Oriented Template Library ( OOTL ). In the September 2004 issue of the C/C++ Users Journal I outline a technique in C++ for implementing interface reference types without requrining changes to existing objects. This technique is used in the upcoming Boost Interfaces Library (BIL) by Jonathan Turkanis. The BIL is not yet ready for a public beta release, but a pre-release version has been included with the OOTL version 0.1.

Given two existing class such as:

  class Faz {
  public:
    int Fu() { return 1; }
    void Bar(int x) { printf("faz %d\n", x); }
  }

  class Baz {
  public:
    int Fu() { return 2; }
    void Bar(int x) { printf("baz %d\n", x); }
  }

  BOOST_IDL_BEGIN(IFuBar)
    BOOST_IDL_FN0(Fu, int)
    BOOST_IDL_FN1(Bar, void, (int, x))
  BOOST_IDL_END(IFuBar)

Once we define this interface, after the fact, we can use it to make Faz and Baz, polymorphic. Consider the following:

  int main{
    faz f;
    baz b;  
    IFuBar i;
    if ((rnd() % 2) == 0) {
      i = f;
    } else {
      i = b;
    }
    i.Bar(i.Fu());
  }

BOOST_IDL(IFuBar,
   (Fu, (int))
   (Bar, (void)(int, x))
)