/* The following code example is taken from the book * "C++ Templates - The Complete Guide" * by David Vandevoorde and Nicolai M. Josuttis, Addison-Wesley, 2002 * * (C) Copyright David Vandevoorde and Nicolai M. Josuttis 2002. * Permission to copy, use, modify, sell and distribute this software * is granted provided this copyright notice appears in all copies. * This software is provided "as is" without express or implied * warranty, and with no claim as to its suitability for any purpose. */// primary template for type of Nth field of (duo) Ttemplate <int N, typename T>class DuoT {  public:    typedef void ResultT;    // in general, the result type is void};// specialization for 1st field of a plain duotemplate <typename A, typename B>class DuoT<1, Duo<A,B> > {  public:    typedef A ResultT;};// specialization for 2nd field of a plain duotemplate <typename A, typename B>class DuoT<2, Duo<A,B> > {  public:    typedef B ResultT;};// specialization for Nth field of a recursive duotemplate <int N, typename A, typename B, typename C>class DuoT<N, Duo<A, Duo<B,C> > > {  public:    typedef typename DuoT<N-1, Duo<B,C> >::ResultT ResultT;};// specialization for 1st field of a recursive duotemplate <typename A, typename B, typename C>class DuoT<1, Duo<A, Duo<B,C> > > {  public:    typedef A ResultT;};// specialization for 2nd field of a recursive duotemplate <typename A, typename B, typename C>class DuoT<2, Duo<A, Duo<B,C> > > {  public:    typedef B ResultT;};