模板constexpr-endian转换器(不带UB)

Question

我看到了其他一些建议使用并集进行字节交换的答案(这是UB，或者在编译时无法完成(。

我已经写了我的，它一直有效，直到我遇到一些案例，这些案例表明我的实现是无效的。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，我非常感兴趣的是找出我在算法中犯了什么样的错误。

Answer 1

您忽略了通过res参数传递到bitwise_or的循环重载中的第三个参数。如果

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