/* 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. */#ifndef DUO_HPP#define DUO_HPPtemplate <typename T1, typename T2>class Duo {  public:    typedef T1 Type1;  // type of first field    typedef T2 Type2;  // type of second field    enum { N = 2 };    // 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;    }};// comparison operators (allow mixed types):template <typename T1, typename T2,          typename U1, typename U2>inlinebool operator == (Duo<T1,T2> const& d1, Duo<U1,U2> const& d2){    return d1.v1()==d2.v1() && d1.v2()==d2.v2();}template <typename T1, typename T2,          typename U1, typename U2>inlinebool operator != (Duo<T1,T2> const& d1, Duo<U1,U2> const& d2){    return !(d1==d2);}// convenience function for creation and initializationtemplate <typename T1, typename T2> inlineDuo<T1,T2> make_duo (T1 const & a, T2 const & b){    return Duo<T1,T2>(a,b);}#endif // DUO_HPP