Question

Is there a way to have a template specialization based on a range of values instead of just one? I know the following code is not valid C++ code but it shows what I would like to do. I’m writing code for a 8-bit machine, so there is a difference in speed for using ints and chars.

template<unsigned SIZE>
class circular_buffer {
   unsigned char buffer[SIZE];
   unsigned int head; // index
   unsigned int tail; // index
};

template<unsigned SIZE <= 256>
class circular_buffer {
   unsigned char buffer[SIZE];
   unsigned char head; // index
   unsigned char tail; // index
};

Answer 1

Try std::conditional:

#include <type_traits>

template<unsigned SIZE>
class circular_buffer {

    typedef typename
        std::conditional< SIZE < 256,
                          unsigned char,
                          unsigned int
                        >::type
        index_type;

    unsigned char buffer[SIZE];
    index_type head;
    index_type tail;
};

If your compiler doesn’t yet support this part of C++11, there’s equivalent in boost libraries.

Then again, it’s easy to roll your own (credit goes to KerrekSB):

template <bool, typename T, typename F>
struct conditional {
    typedef T type;
};

template <typename T, typename F>  // partial specialization on first argument
struct conditional<false, T, F> {
    typedef F type;
};