如何为一系列整数值专门化C++模板？

Question

有没有办法根据一系列值而不是一个值进行模板专业化？我知道以下代码不是有效的C++代码,但它显示了我想要做的事情.我正在为8位机器编写代码,因此使用整数和字符的速度有所不同.

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

尝试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;
};

如果你的编译器还不支持C++ 11的这一部分,那么在boost库中就有了相同的功能.

然后,你可以很容易地推出自己的东西(功劳归于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;
}; 

Answer 2

使用额外的默认bool参数:

// 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 3

另一种可能选择:

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示例)