/* 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. *//* helper traits class to select how to refer to an ``expression template node'' * - in general: by reference * - for scalars: by value */template <typename T> class A_Scalar;// primary templatetemplate <typename T>class A_Traits {  public:    typedef T const& ExprRef;     // type to refer to is constant reference};// partial specialization for scalarstemplate <typename T>class A_Traits<A_Scalar<T> > {  public:    typedef A_Scalar<T> ExprRef;  // type to refer to is ordinary value};