How can I specialize a C++ template for a range of integer values?

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 li

Answer 1

Use an extra defaulted bool parameter:

// primary template handles false
template<unsigned SIZE, bool IsSmall = SIZE <= 256>
class circular_buffer {
   unsigned char buffer[SIZE];
   unsigned int head; // index
   unsigned int tail; // index
};

// specialization for true
template<unsigned SIZE>
class circular_buffer<SIZE, true> {
   unsigned char buffer[SIZE];
   unsigned char head; // index
   unsigned char tail; // index
};

Answer 2

I hate how messy dealing with types can be so I propose something a bit simpler, leveraging constexpr. This variant allows for differing behavior when a varying type is not required and addresses the need to fit within a range and not just one side of a value:

template<bool> struct If;

constexpr bool InClosedRange(std::size_t Value, std::size_t Lower, std::size_t Upper)
{
    return (Lower <= Value) && (Value <= Upper);
}

// Usage:
template<size_t Width, If<true>>
class Foo;

template<size_t Width, If<InClosedRange(Width, 1, 41)>>
class Foo { /* ... */ };

template<size_t Width, If<InClosedRange(Width, 42, 142)>>
class Foo { /* ... */ };

Inspired by: https://stackoverflow.com/a/9516959/929315

Answer 3

Another possible option:

template <unsigned SIZE>
struct offset_size {
    typedef typename offset_size<SIZE - 1>::type type;
};

template <>
struct offset_size<0> {
    typedef unsigned char type;
};

template <>
struct offset_size<257> {
    typedef unsigned int type;
};

template<unsigned SIZE>
class circular_buffer {
   unsigned char buffer[SIZE];
   typename offset_size<SIZE>::type head; // index
   typename offset_size<SIZE>::type tail; // index
};

(Ideone example)

Answer 4

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;
};