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
#define MK_PROMOTION(T1,T2,Tr) \
template<> class Promotion<T1, T2> { \
public: \
typedef Tr ResultT; \
}; \
\
template<> class Promotion<T2, T1> { \
public: \
typedef Tr ResultT; \
};