make_guid.cpp

#include <string_view>
#include <exception>
#include <assert.h>

using namespace std::literals;

struct guid
{
    uint32_t Data1;
    uint16_t Data2;
    uint16_t Data3;
    uint8_t  Data4[8];
};

constexpr bool operator==(guid const& left, guid const& right) noexcept
{
    return left.Data1 == right.Data1 &&
        left.Data2 == right.Data2 &&
        left.Data3 == right.Data3 &&
        left.Data4[0] == right.Data4[0] &&
        left.Data4[1] == right.Data4[1] &&
        left.Data4[2] == right.Data4[2] &&
        left.Data4[3] == right.Data4[3] &&
        left.Data4[4] == right.Data4[4] &&
        left.Data4[5] == right.Data4[5] &&
        left.Data4[6] == right.Data4[6] &&
        left.Data4[7] == right.Data4[7];
}


constexpr uint32_t to_uint(char const value) noexcept
{
    if (value >= '0' && value <= '9')
    {
        return value - '0';
    }

    if (value >= 'A' && value <= 'F')
    {
        return 10 + value - 'A';
    }

    if (value >= 'a' && value <= 'f')
    {
        return 10 + value - 'a';
    }

    std::terminate();
}

constexpr guid make_guid(std::string_view const& value) noexcept
{
    if (value.size() != 36 || value[8] != '-' || value[13] != '-' || value[18] != '-' || value[23] != '-')
    {
        std::terminate();
    }

    return
    {
        ((to_uint(value[0]) * 16 + to_uint(value[1])) << 24) +
        ((to_uint(value[2]) * 16 + to_uint(value[3])) << 16) +
        ((to_uint(value[4]) * 16 + to_uint(value[5])) << 8) +
         (to_uint(value[6]) * 16 + to_uint(value[7])),

        static_cast<uint16_t>(((to_uint(value[9]) * 16 + to_uint(value[10])) << 8) +
        (to_uint(value[11]) * 16 + to_uint(value[12]))),

        static_cast<uint16_t>(((to_uint(value[14]) * 16 + to_uint(value[15])) << 8) +
         (to_uint(value[16]) * 16 + to_uint(value[17]))),

        {
            static_cast<uint8_t>(to_uint(value[19]) * 16 + to_uint(value[20])),
            static_cast<uint8_t>(to_uint(value[21]) * 16 + to_uint(value[22])),

            static_cast<uint8_t>(to_uint(value[24]) * 16 + to_uint(value[25])),
            static_cast<uint8_t>(to_uint(value[26]) * 16 + to_uint(value[27])),
            static_cast<uint8_t>(to_uint(value[28]) * 16 + to_uint(value[29])),
            static_cast<uint8_t>(to_uint(value[30]) * 16 + to_uint(value[31])),
            static_cast<uint8_t>(to_uint(value[32]) * 16 + to_uint(value[33])),
            static_cast<uint8_t>(to_uint(value[34]) * 16 + to_uint(value[35])),
        }
    };
}

static constexpr guid a
{ 0x8aa90cad, 0xfed1, 0x4c54, { 0x89, 0xdb, 0x9b, 0x75, 0x22, 0xd8, 0xaa, 0x92 } };

static constexpr guid b
{ make_guid("8AA90CAD-fed1-4c54-89db-9B7522D8AA92"sv) };

static_assert(a == b);

int main()
{
    assert(a == b);
}

Question: Based on the Wikipedia description here:

https://en.wikipedia.org/wiki/Universally_unique_identifier

in the test case above, the first 2 bits of 0x89 (10001001) identify this as "variant 1", which means that the integers should be stored in big-endian order, yes? The MSVC compiler on Intel will assemble the integers in little-endian order. The endianness doesn't matter if the UUIDs are all generated, stored, compared, etc., within a homogeneous MSVC environment, but might matter in some kind of heterogeneous environment -- maybe sending and receiving UUIDs generated on various platforms across the network. If I understand the Wikipedia article correctly, and if the article is correct, either:

• that 0x89 should start with 0xc_ or 0xd_ (1100hhhh or 1101hhhh) to mark this is "variant 2", or
• the integers should be swapped to big-endian order.

Yes? No?

The binary encoding of UUIDs varies between systems. Variant 1 UUIDs, nowadays the most common variant, are encoded in a big-endian format. For example, 00112233-4455-6677-8899-aabbccddeeff is encoded as the bytes 00 11 22 33 44 55 66 77 88 99 aa bb cc dd ee ff

Variant 2 UUIDs, historically used in Microsoft's COM/OLE libraries, use a mixed-endian format, whereby the first three components of the UUID are little-endian, and the last two are big-endian. For example, 00112233-4455-6677-8899-aabbccddeeff is encoded as the bytes 33 22 11 00 55 44 77 66 88 99 aa bb cc dd ee ff