/* 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. */#include "typeop.hpp"// primary template for value of Nth field of (duo) Ttemplate <int N, typename T>class DuoValue {  public:    static void get(T&) {        // in general, we have no value    }    static void get(T const&) {    }};// specialization for 1st field of a plain duotemplate <typename A, typename B>class DuoValue<1, Duo<A, B> > {  public:    static A& get(Duo<A, B> &d) {         return d.v1();    }    static A const& get(Duo<A, B> const &d) {        return d.v1();    }};// specialization for 2nd field of a plain duotemplate <typename A, typename B>class DuoValue<2, Duo<A, B> > {  public:    static B& get(Duo<A, B> &d) {         return d.v2();     }    static B const& get(Duo<A, B> const &d) {         return d.v2();    }};// specialization for Nth field of recursive duotemplate <int N, typename A, typename B, typename C>struct DuoValue<N, Duo<A, Duo<B,C> > > {    static    typename TypeOp<typename DuoT<N-1, Duo<B,C> >::ResultT>::RefT    get(Duo<A, Duo<B,C> > &d) {        return DuoValue<N-1, Duo<B,C> >::get(d.v2());    }    static typename TypeOp<typename DuoT<N-1, Duo<B,C>                          >::ResultT>::RefConstT    get(Duo<A, Duo<B,C> > const &d) {        return DuoValue<N-1, Duo<B,C> >::get(d.v2());    }};// specialization for 1st field of recursive duotemplate <typename A, typename B, typename C>class DuoValue<1, Duo<A, Duo<B,C> > > {  public:    static A& get(Duo<A, Duo<B,C> > &d) {         return d.v1();     }    static A const& get(Duo<A, Duo<B,C> > const &d) {         return d.v1();     }};// specialization for 2nd field of recursive duotemplate <typename A, typename B, typename C>class DuoValue<2, Duo<A, Duo<B,C> > > {  public:    static B& get(Duo<A, Duo<B,C> > &d) {         return d.v2().v1();     }    static B const& get(Duo<A, Duo<B,C> > const &d) {        return d.v2().v1();    }};