/* 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. */template <typename A, typename B, typename C>class Duo<A, Duo<B,C> > {  public:    typedef A        T1;           // type of first field    typedef Duo<B,C> T2;           // type of second field    enum { N = Duo<B,C>::N + 1 };  // number of fields    private:    T1 value1;         // value of first field    T2 value2;         // value of second field    public:    // constructors    Duo() : value1(), value2() {    }    Duo (T1 const & a, T2 const & b)     : value1(a), value2(b) {    }    // for implicit type conversion during construction    template <typename U1, typename U2>    Duo (Duo<U1,U2> const & d)     : value1(d.v1()), value2(d.v2()) {    }    // for implicit type conversion during assignments    template <typename U1, typename U2>    Duo<T1, T2>& operator = (Duo<U1,U2> const & d) {        value1 = d.value1;        value2 = d.value2;        return *this;    }    // field access    T1& v1() {         return value1;    }    T1 const& v1() const {        return value1;    }        T2& v2() {         return value2;    }    T2 const& v2() const {        return value2;    }};