/* 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. */// partial specialization for Duo<> with only one fieldtemplate <typename A>struct Duo<A,void> {  public:    typedef A    T1;  // type of first field    typedef void T2;  // type of second field    enum { N = 1 };   // number of fields  private:    T1 value1;        // value of first field  public:    // constructors    Duo() : value1() {    }    Duo (T1 const & a)     : value1(a) {    }    // field access    T1& v1() {         return value1;    }    T1 const& v1() const {        return value1;    }        void v2() {     }    void v2() const {    }    //...};