模板 constexpr endian 转换器（无 UB）

Question

我看到其他一些答案建议使用联合进行字节交换（这是 UB 或无法在编译时完成）。

我已经写了我的，它一直有效，直到我遇到一些表明我的实施无效的案例。我找不到错误，你能帮我吗？

namespace impl
    {
                // ENDIAN is defined via CMake TestBigEndian
        constexpr bool native_is_big_endian()
        {
#ifdef ENDIAN
            return true;
#else
            return false;
#endif
        }
    }

    /*!
     * \brief std compliant type for endianness
     * \details
     * If all scalar types are little-endian, endian::native equals endian::little
     * If all scalar types are big-endian, endian::native equals endian::big
     */
    enum class endian
    {
        little,
        big,
        native = impl::native_is_big_endian() ? big : little
    };

    template<typename T>
    class swap_endian
    {
        constexpr static size_t sz_minus_one = sizeof(T) - 1;
        template<size_t> struct tag_s
        {
        };

        constexpr static T bitwise_or(tag_s<0>, T original, T res)
        {
            return res | (original >> sz_minus_one * 8);
        }

        template<size_t i>
        constexpr static T bitwise_or(tag_s<i>, T original, T res)
        {
            return bitwise_or(tag_s<i - 1>(), original, original << i * 8 >> sz_minus_one * 8 << i * 8);
        }

    public:
        constexpr static T swap(T u)
        {
            return bitwise_or(tag_s<sz_minus_one>(), u, 0);
        }
    };

    template<typename T>
    constexpr T swap_endian_v(T u)
    {
        return swap_endian<T>::swap(u);
    }

    template<endian From, typename T>
    constexpr T to_native_endian(T u)
    {
        return From == endian::native ? u : swap_endian_v(u);
    }

int main()
{
    static_assert(uint8_t(0xFA) == swap_endian_v(uint8_t(0xFA)), "Invalid result for endian swapping");
    static_assert(uint16_t(0x00AA) == swap_endian_v(uint16_t(0xAA00)), "Invalid result for endian swapping");
    static_assert(uint16_t(0xF0AA) == swap_endian_v(uint16_t(0xAAF0)), "Invalid result for endian swapping");
    static_assert(uint32_t(0x00'00'CC'00) == swap_endian_v(uint32_t(0x00'CC'00'00)),
                  "Invalid result for endian swapping");

// this fails
//    static_assert(uint32_t(0x6A'25'65'75) == swap_endian_v(uint32_t(0x75'65'25'6A)),
//                  "Invalid result for endian swapping");
    return 0;
}

请不要建议使用BOOST。在这一点上，我非常想知道我在算法中犯了什么样的错误。

Answer 1

您忽略了通过 res 参数传递给 bitwise_or 的循环重载的第三个参数。 if

似乎有效

return bitwise_or(tag_s<i - 1>(), original,
  original << i * 8 >> sz_minus_one * 8 << i * 8);

更改为：

return bitwise_or(tag_s<i - 1>(), original,
  res | original << i * 8 >> sz_minus_one * 8 << i * 8);

现场演示：https://godbolt.org/z/xW81z4

模板 constexpr endian 转换器（无 UB）

template constexpr endian converter (without UB)

c++

endianness

constexpr

c++11