The following code example is taken from the book
C++ Templates - The Complete Guide
by David Vandevoorde and Nicolai M. Josuttis, Addison-Wesley, 2002
© Copyright David Vandevoorde and Nicolai M. Josuttis 2002
/* 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 template
template <typename T>
class A_Traits {
public:
typedef T const& ExprRef; // type to refer to is constant reference
};
// partial specialization for scalars
template <typename T>
class A_Traits<A_Scalar<T> > {
public:
typedef A_Scalar<T> ExprRef; // type to refer to is ordinary value
};