| ___ | ___ | | --- | --- | | Doc. No.: | P0959R2 | | Date: | 2019-01-08 | | Reply to: | Marius Bancila (marius.bancila@gmail.com), Tony van Eerd (tvaneerd@gmail.com) | | Audience: | Library WG | | Title: | A Proposal for a Universally Unique Identifier Library | # A Proposal for a Universally Unique Identifier Library ## I. Revision History ### 1.1 P0959R0 Original UUID library paper with motivation, specification, and examples. ### 1.2 P0959R1 Revised with feedback from the LWG and the community. * Removed string constructors and replaced with an overloaded static member function `from_string()`. * Parsing strings to `uuid` throws exception `uuid_error` instead of creating a nil uuid when the operation fails. * {} included in the supported format for string parsing, i.e. `"{xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}"`. * Removed `state_size`. * Rename member function `nil()` to `is_nil()`. * The default constructor is defaulted. * Added a conversion construct from `std::span`. * Added the member function `as_bytes()` to convert the `uuid` into a view of its underlying bytes. * Constructing a `uuid` from a range with a size other than 16 is undefined behaviour. * Replaced operators `==`, `!=` and `<` with the three-way operator `<=>` * Removed mutable iterators (but preserved the constant iterators). * Removed typedefs and others container-like parts. * Defined the correlation between the internal UUID bytes and the string representation. * Added UUID layout and byte order specification from the RFC 4122 document. * Added section on alternative ordering design * Most functions are constexpr. * Replaced typedefs with using statements. ### 1.3 P0959R2 P0959R1 was discussed in San Diego LEWGI (attendance 13 people) with the following feedback in polls: * `std::uuid`'s byte ordering should be fixed (implying we have the option to make it memcpy and there is no need for a container-like interface) | SF | F | N | A | SA | |---|---|---|---|---| | 4 | 3 | 1 | 0 | 0 | * `std::uuid` should have iterators (e.g. begin/end methods) | SF | F | N | A | SA | |---|---|---|---|---| | 2 | 3 | 2 | 1 | 1 | *Comment*: interpreted as "author's discretion". * `std::uuid` should be able to generate uuids from names | SF | F | N | A | SA | |---|---|---|---|---| | 4 | 2 | 3 | 0 | 0 | * explore having `std::uuid` be able to generate from times | SF | F | N | A | SA | |---|---|---|---|---| | 4 | 3 | 2 | 0 | 0 | * We want `std::basic_uuid_random_generator` | SF | F | N | A | SA | |---|---|---|---|---| | 0 | 6 | 2 | 0 | 1 | *Comments*: Author is instructed to investigate less hazardous API, and not just fix the examples. * Explore non-exception based approach to error handling in `std::uuid::from_string` | SF | F | N | A | SA | |---|---|---|---|---| | 9 | 0 | 0 | 0 | 0 | Based on this feedback the following changes have been done in this version: * `std::uuid` byte order is fixed * removed container interface (iterators and `size()`) because, fundamentally, a uuid is a single entity, an identifier, and not a container of other things * removed `basic_uuid_random_generator` default constructor * added `uuid_time_generator` functor to generate variant 1 time-based uuids * proper initialization for the pseudo-random generator engines in all examples * removed `to_wstring()` and made `to_string()` a function template * made `from_string()` a non-throwing function template returning `std::optional` * added `is_valid_uuid()` a non-throwing function template that checks if a string contains a valid uuid * removed the `std::wstring` overloaded call operator for `uuid_name_generator` and replaced with with function templates * `uuid`s produced from names in different character sets or encodings are different (i.e. "jane" and L"jane") * removed the class `uuid_error` * footnote on the use of the name Microsoft ## II. Motivation Universally unique identifiers (*uuid*), also known as Globally Unique Identifiers (*guid*s), are commonly used in many types of applications to uniquely identify data. A standard uuid library would benefit developers that currently have to either use operating system specific APIs for creating new uuids or resort to 3rd party libraries, such as *boost::uuid*. UUIDs are 128-bit numbers that are for most practical purposes unique, without depending on a central registration authority for ensuring their uniqueness. Although the probability of UUID duplication exists, it is negligible. According to Wikipedia, "*for there to be a one in a billion chance of duplication, 103 trillion version 4 UUIDs must be generated.*" UUID is an Internet Engineering Task Force standard described by RFC 4122. The library proposed on this paper is a light one: it enables developers to generate random and name-based UUIDs, serialize and deserialize UUIDs to and from strings, validate UUIDs and other common operations. ## III. Impact On the Standard This proposal is a pure library extension. It does not require changes to any standard classes, functions or headers. It does not require any changes in the core language, and it has been implemented in standard C++ as per ISO/IEC 14882:2017. The implementation is available at https://github.com/mariusbancila/stduuid. ## IV. Design Decisions The proposed library, that should be available in a new header called `` in the namespace `std`, provides: * a class called `uuid` that represents a universally unique identifier * strongly type enums `uuid_variant` and `uuid_version` to represent the possible variant and version types of a UUID * function objects that generate UUIDs, called generators: `basic_uuid_random_generator`, `uuid_random_generator`, `uuid_name_generator`, `uuid_time_generator` * conversion functions from strings `from_string()` and to strings `std::to_string()`, as well as an overloaded `operator<<` for `std::basic_ostream` * `std::swap()` overload for `uuid` * `std::hash<>` specialization for `uuid` ### Default constructor Creates a nil UUID that has all the bits set to 0 (i.e. `00000000-0000-0000-0000-000000000000`). ```cpp uuid empty; auto empty = uuid{}; ``` ### Iterators constructors The conversion constructor that takes two forward iterators constructs an `uuid` with the content of the range \[first, last). It requires the range to have exactly 16 elements, otherwise the behaviour is undefined. This constructor follows the conventions of other containers in the standard library. ```cpp std::array arr{{ 0x47, 0x18, 0x38, 0x23, 0x25, 0x74, 0x4b, 0xfd, 0xb4, 0x11, 0x99, 0xed, 0x17, 0x7d, 0x3e, 0x43 }}; uuid id(std::begin(arr), std::end(arr)); ``` ```cpp uuid::value_type arr[16] = { 0x47, 0x18, 0x38, 0x23, 0x25, 0x74, 0x4b, 0xfd, 0xb4, 0x11, 0x99, 0xed, 0x17, 0x7d, 0x3e, 0x43 }; uuid id(std::begin(arr), std::end(arr)); ``` ### Span constructor The conversion constructor that takes a `std::span` and constructs a `uuid` from a contiguous sequence of 16 bytes. ```cpp std::array arr{ { 0x47, 0x18, 0x38, 0x23, 0x25, 0x74, 0x4b, 0xfd, 0xb4, 0x11, 0x99, 0xed, 0x17, 0x7d, 0x3e, 0x43 } }; std::span data(arr); uuid id{ data }; assert(to_string(id) == "47183823-2574-4bfd-b411-99ed177d3e43"); ``` ### Nil A nil UUID is a special UUID that has all the bits set to 0. Its canonical textual representation is `00000000-0000-0000-0000-000000000000`. Member function `is_nil()` indicates whether the `uuid` has all the bits set to 0. A nil UUID is created by the default constructor or by parsing the strings `00000000-0000-0000-0000-000000000000` or `{00000000-0000-0000-0000-000000000000}`. ```cpp uuid id; assert(id.is_nil()); std::optional id = uuid::from_string("00000000-0000-0000-0000-000000000000"); assert(id.has_value() && id.value().is_nil()); ``` ### `variant` and `version` Member functions `variant()` and `version()` allow checking the variant type of the uuid and, respectively, the version type. These are defined by two strongly typed enums called `uuid_variant` and `uuid_version`. ```cpp uuid id = uuid::from_string("47183823-2574-4bfd-b411-99ed177d3e43").value(); assert(id.version() == uuid_version::random_number_based); assert(id.variant() == uuid_variant::rfc); ``` ### Comparisons Although it does not make sense to check whether a UUID is less (or less or equal) then another UUID, the overloading of this operator for `uuid` is necessary in order to be able to store `uuid` values in containers such as `std::set` that by default use `operator <` to compare keys. Because operators `==` and `!=` are needed for checking whether two UUIDs are the same (a basic operation for an identifier type) the three-way comparison operator `<=>` is defined so that all comparison operators are provided. ```cpp std::array arr{ { 0x47, 0x18, 0x38, 0x23, 0x25, 0x74, 0x4b, 0xfd, 0xb4, 0x11, 0x99, 0xed, 0x17, 0x7d, 0x3e, 0x43 } }; uuid id1{ std::span{arr} }; uuid id2 = uuid::from_string("47183823-2574-4bfd-b411-99ed177d3e43").value(); assert(id1 == id2); std::set ids{ uuid{}, uuid::from_string("47183823-2574-4bfd-b411-99ed177d3e43").value() }; assert(ids.size() == 2); assert(ids.find(uuid{}) != ids.end()); ``` ### Span view Member function `as_bytes()` converts the `uuid` into a view of its underlying bytes. ```cpp std::array arr{ { 0x47, 0x18, 0x38, 0x23, 0x25, 0x74, 0x4b, 0xfd, 0xb4, 0x11, 0x99, 0xed, 0x17, 0x7d, 0x3e, 0x43 } }; uuid const id{ arr }; assert(!id.is_nil()); auto view = id.as_bytes(); assert(memcmp(view.data(), arr.data(), arr.size()) == 0); ``` ### Swapping Both member and non-member `swap()` functions are available to perform the swapping of `uuid` values. ```cpp uuid empty; uuid id = uuid::from_string("47183823-2574-4bfd-b411-99ed177d3e43").value(); assert(empty.is_nil()); assert(!id.is_nil()); std::swap(empty, id); assert(!empty.is_nil()); assert(id.is_nil()); empty.swap(id); assert(empty.is_nil()); assert(!id.is_nil()); ``` ### String parsing Static overloaded member function template `from_string()` allows to create `uuid` instances from various strings. The input argument must have the form `xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx` or `{xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}` where `x` is a hexadecimal digit. The return value is an `std::optional` that contains a valid `uuid` if the parsing completed successful. ```cpp auto id1 = uuid::from_string("47183823-2574-4bfd-b411-99ed177d3e43"); assert(id1.has_value()); auto id2 = uuid::from_string(L"{47183823-2574-4bfd-b411-99ed177d3e43}"); assert(id2.has_value()); ``` The order of the bytes in the input string reflects directly into the internal representation of the UUID. That is, for an input string in the form `"aabbccdd-eeff-gghh-iijj-kkllmmnnoopp"` or `"{aabbccdd-eeff-gghh-iijj-kkllmmnnoopp}"` the internal byte order of the resulted UUID is `aa,bb,cc,dd,ee,ff,gg,hh,ii,jj,kk,ll,mm,nn,oo,pp`. ### String conversion Non-member template function `to_string()` returns a string with the UUID formatted to the canonical textual representation `xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx`, where `x` is a lower case hexadecimal digit. ```cpp auto id = uuid::from_string("47183823-2574-4bfd-b411-99ed177d3e43"); assert(id.has_value()); assert(to_string(id.value()) == "47183823-2574-4bfd-b411-99ed177d3e43"); assert(to_string(id.value()) == L"47183823-2574-4bfd-b411-99ed177d3e43"); ``` The order of the internal UUID bytes reflects directly into the string bytes order. That is, for a UUID with the internal bytes in the form `aa,bb,cc,dd,ee,ff,gg,hh,ii,jj,kk,ll,mm,nn,oo,pp` the resulted string has the form `"aabbccdd-eeff-gghh-iijj-kkllmmnnoopp"`. ### Hashing A `std::hash<>` specialization for `uuid` is provided in order to enable the use of `uuid`s in associative unordered containers such as `std::unordered_set`. ```cpp std::unordered_set ids{ uuid{}, uuid::from_string("47183823-2574-4bfd-b411-99ed177d3e43").value(), }; assert(ids.size() == 2); assert(ids.find(uuid{}) != ids.end()); ``` ### Generating new uuids Several function objects, called generators, are provided in order to create different versions of UUIDs. Examples for generating new UUIDs with the `basic_uuid_random_generator` class: ```cpp { std::random_device rd; auto seed_data = std::array {}; std::generate(std::begin(seed_data), std::end(seed_data), std::ref(rd)); std::seed_seq seq(std::begin(seed_data), std::end(seed_data)); std::mt19937 engine(seq); basic_uuid_random_generator dgen{engine}; auto id1 = dgen(); assert(!id1.is_nil()); assert(id1.size() == 16); assert(id1.version() == uuid_version::random_number_based); assert(id1.variant() == uuid_variant::rfc); } { std::random_device rd; auto seed_data = std::array {}; std::generate(std::begin(seed_data), std::end(seed_data), std::ref(rd)); std::seed_seq seq(std::begin(seed_data), std::end(seed_data)); std::ranlux48_base engine(seq); basic_uuid_random_generator dgen{engine}; auto id1 = dgen(); assert(!id1.is_nil()); assert(id1.size() == 16); assert(id1.version() == uuid_version::random_number_based); assert(id1.variant() == uuid_variant::rfc); } { std::random_device rd; auto seed_data = std::array {}; std::generate(std::begin(seed_data), std::end(seed_data), std::ref(rd)); std::seed_seq seq(std::begin(seed_data), std::end(seed_data)); auto engine = std::make_unique(seq); basic_uuid_random_generator dgen(engine.get()); auto id1 = dgen(); assert(!id1.is_nil()); assert(id1.size() == 16); assert(id1.version() == uuid_version::random_number_based); assert(id1.variant() == uuid_variant::rfc); } ``` Examples for generating new UUIDs with the `uuid_random_generator` type alias: ```cpp std::random_device rd; auto seed_data = std::array {}; std::generate(std::begin(seed_data), std::end(seed_data), std::ref(rd)); std::seed_seq seq(std::begin(seed_data), std::end(seed_data)); std::mt19937 engine(seq); uuid const guid = uuid_random_generator{engine}(); auto id1 = dgen(); assert(!id1.is_nil()); assert(id1.size() == 16); assert(id1.version() == uuid_version::random_number_based); assert(id1.variant() == uuid_variant::rfc); ``` Examples for genearting new UUIDs with the `uuid_name_generator` class: ```cpp uuid_name_generator dgen(uuid::from_string("415ccc2b-f5cf-4ec1-b544-45132a518cc8").value()); auto id1 = dgen("john"); assert(!id1.is_nil()); assert(id1.size() == 16); assert(id1.version() == uuid_version::name_based_sha1); assert(id1.variant() == uuid_variant::rfc); auto id2 = dgen("jane"); assert(!id2.is_nil()); assert(id2.size() == 16); assert(id2.version() == uuid_version::name_based_sha1); assert(id2.variant() == uuid_variant::rfc); auto id3 = dgen("jane"); assert(!id3.is_nil()); assert(id3.size() == 16); assert(id3.version() == uuid_version::name_based_sha1); assert(id3.variant() == uuid_variant::rfc); auto id4 = dgen(L"jane"); assert(!id4.is_nil()); assert(id4.size() == 16); assert(id4.version() == uuid_version::name_based_sha1); assert(id4.variant() == uuid_variant::rfc); assert(id1 != id2); assert(id2 == id3); assert(id3 != id4); ``` ## V. Technical Specifications ### Header Add a new header called ``. ### `uuid_variant` enum ```cpp namespace std { enum class uuid_variant { ncs, rfc, microsoft, future }; } ``` **Footnote**: Microsoft is a registered trademark of Microsoft Corporation. This information is given for the convenience of users of this document and does not constitute an endorsement by ISO or IEC of these products. ### `uuid_version` enum ```cpp namespace std { enum class uuid_version { none = 0, time_based = 1, dce_security = 2, name_based_md5 = 3, random_number_based = 4, name_based_sha1 = 5 }; } ``` ### `uuid` class ```cpp namespace std { struct uuid { using value_type = uint8_t; constexpr uuid() noexcept = default; constexpr explicit uuid(std::span bytes); template constexpr explicit uuid(ForwardIterator first, ForwardIterator last); constexpr uuid_variant variant() const noexcept; constexpr uuid_version version() const noexcept; constexpr bool is_nil() const noexcept; constexpr void swap(uuid & other) noexcept; constexpr std::span as_bytes() const; constexpr std::strong_ordering operator<=>(uuid const&) const noexcept = default; template static bool is_valid_uuid(CharT const * str) noexcept; template, class Allocator = std::allocator> static bool is_valid_uuid(std::basic_string const & str) noexcept; template static uuid from_string(CharT const * str); template, class Allocator = std::allocator> static uuid from_string(std::basic_string const & str); private: template friend std::basic_ostream & operator<<(std::basic_ostream &s, uuid const & id); }; } ``` ### non-member functions ```cpp namespace std { inline constexpr void swap(uuid & lhs, uuid & rhs); template std::basic_ostream & operator<<(std::basic_ostream &s, uuid const & id); template, class Allocator = std::allocator> inline std::string to_string(uuid const & id); } ``` ### Generators `basic_uuid_random_generator` is a class template for generating random or pseudo-random UUIDs (version 4, i.e. `uuid_version::random_number_based`). The type template parameter represents a function object that implements both the [`RandomNumberEngine`](http://en.cppreference.com/w/cpp/concept/UniformRandomBitGenerator) and [`UniformRandomBitGenerator`](http://en.cppreference.com/w/cpp/concept/RandomNumberEngine) concepts. `basic_uuid_random_generator` can be constructed with a reference or pointer to a an objects that satisfies the `UniformRandomNumberGenerator` requirements. ```cpp namespace std { template class basic_uuid_random_generator { public: using engine_type = UniformRandomNumberGenerator; explicit basic_uuid_random_generator(engine_type& gen); explicit basic_uuid_random_generator(engine_type* gen); uuid operator()(); }; } ``` A type alias `uuid_random_generator` is provided for convenience as `std::mt19937` is probably the preferred choice of a pseudo-random number generator engine in most cases. ```cpp namespace std { using uuid_random_generator = basic_uuid_random_generator; } ``` `uuid_name_generator` is a function object that generates new UUIDs from a name and has to be initialized with another UUID. This generator produces different uuids for the same text represented in different character sets or encodings. In order words, the uuids generated from "jane" and L"jane" are different. ```cpp namespace std { class uuid_name_generator { public: explicit uuid_name_generator(uuid const& namespace_uuid) noexcept; template uuid operator()(CharT const * name); template, class Allocator = std::allocator> uuid operator()(std::basic_string const & name); }; } ``` `uuid_time_generator` is a function object that generates a time-based UUID as described in the RFC4122 document. The generated uuid's parts are as follows: * the timestamp is a 60-bit value, representing the number of 100 nanosecond intervals since 15 October 1582 00:00:000000000. * the clock sequence is a 14-bit value, that is initially a high-quality pseudo-random value; when the previous value is known, it is simply incremented by one. * the node identifier is an IEEE 802 MAC address (when multiple are available any could be used); if no such address is available, a pseudo-randomly generated value may be used, in which case the multicast bit (least significant bit of the first byte) is set to 1, this to avoid clashes with legitimate IEEE 802 addresses. ```cpp namespace std { class uuid_time_generator { public: uuid_time_generator() noexcept; uuid operator()(); }; } ``` ### Specialization The template specializations of `std::hash` for the `uuid` class allow users to obtain hashes of UUIDs. ```cpp namespace std { template <> struct hash { using argument_type = uuid; using result_type = std::size_t; result_type operator()(argument_type const &uuid) const; }; } ``` ## VI. Alternative ordering design The comparison of UUIDs is not an operation that makes logical sense but it is required for using UUIDs as keys for containers such as `std::set` or `std::map`. The technical specification of the `uuid` class given in the previous section features the three-way operator `<=>` as member of the class. The C++ community, however, is divided on this particular aspect (with long discussions happening in the [ISO C++ proposals forum](https://groups.google.com/a/isocpp.org/forum/#!forum/std-proposals)) between those that want convenience and those that want rigour. Although the RFC 4122 document, as quoted in the next section, does specify rules for lexical equivalence, not everybody is happy with the definition of comparison operator `<`. The alternative put forward is to define non-member ordering functors. In this case, the library can define the following functors: * `uuid_lexicographical_order` performs a lexicographic comparison (can provide compatibility with other systems) * `uuid_fast_order` performs a memberwise comparison for efficiency. These could be used as the comparison function for containers such as `std::set` or `std::map`: ```cpp std::map map; ``` ```cpp template> using uuid_map = std::map; ``` In this case, the `uuid` class should be defined as follows: ```cpp namespace std { struct uuid { using value_type = uint8_t; constexpr uuid() noexcept = default; constexpr explicit uuid(std::span bytes); template constexpr explicit uuid(ForwardIterator first, ForwardIterator last); constexpr uuid_variant variant() const noexcept; constexpr uuid_version version() const noexcept; constexpr bool is_nil() const noexcept; constexpr void swap(uuid & other) noexcept; constexpr std::span as_bytes() const; template static bool is_valid_uuid(CharT const * str) noexcept; template, class Allocator = std::allocator> static bool is_valid_uuid(std::basic_string const & str) noexcept; template static uuid from_string(CharT const * str); template, class Allocator = std::allocator> static uuid from_string(std::basic_string const & str); private: template friend std::basic_ostream & operator<<(std::basic_ostream &s, uuid const & id); }; } ``` In addition, the following must be added to the `` header: ```cpp namespace std { struct uuid_lexicographical_order { constexpr bool operator()(uuid const&, uuid const&) const; }; struct uuid_fast_order { constexpr bool operator()(uuid const&, uuid const&) const; }; } ``` Should users require other types of comparison they can provide their own implementation and use them instead of the standard ones. ## VII. UUID format specification The content of this section is copied from the RFC 4122 document that describes the UUID standard. The UUID format is 16 octets; some bits of the eight octet variant field determine the layout of the UUID. That is, the interpretation of all other bits in the UUID depends on the setting of the bits in the variant field. The variant field consists of a variable number of the most significant bits of octet 8 of the UUID. The following table lists the contents of the variant field, where the letter "x" indicates a "don't-care" value. | Msb0 | Msb1 | Msb2 | Description | | ---- | ---- | ---- | ----------- | | 0 | x | x | Reserved, NCS backward compatibility. | | 1 | 0 | x | The variant specified in this document. | | 1 | 1 | 0 | Reserved, Microsoft Corporation backward compatibility. | | 1 | 1 | 1 | Reserved for future definition. | Only UUIDs of the variant with the bit pattern 10x are considered in this document. All the other references to UUIDs in this document refer to this variant. For simplicity, we will call this the RFC variant UUID. The UUID record definition is defined only in terms of fields that are integral numbers of octets. The fields are presented with the most significant one first. | Field | Data Type | Octet number | Note | | ----- | --------- | ------------ | ---- | | `time_low` | unsigned 32 bit integer | 0-3 | The low field of the timestamp | | `time_mid` | unsigned 16 bit integer | 4-5 | The middle field of the timestamp | | `time_hi_and_version` | unsigned 16 bit integer | 6-7 | The high field of the timestamp multiplexed with the version number | | `clock_seq_hi_and_reserved` | unsigned 8 bit integer | 8 | The high field of the clock sequence multiplexed with the variant | | `clock_seq_low` | unsigned 8 bit integer | 9 | The low field of the clock sequence | | `node` | unsigned 48 bit integer | 10-15 | The spatially unique node identifier | In the absence of explicit application or presentation protocol specification to the contrary, a UUID is encoded as a 128-bit object, as follows: The fields are encoded as 16 octets, with the sizes and order of the fields defined above, and with each field encoded with the Most Significant Byte first (known as network byte order). ``` 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | time_low | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | time_mid | time_hi_and_version | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |clk_seq_hi_res | clk_seq_low | node (0-1) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | node (2-5) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ``` The version number is in the most significant 4 bits of the timestamp (bits 4 through 7 of the `time_hi_and_version` field). The following table lists the currently-defined versions for the RFC variant. | Msb0 | Msb1 | Msb2 | Msb3 | Version | Description | | ---- | ---- | ---- | ---- | ------- | ----------- | | 0 | 0 | 0 | 1 | 1 | The time-based version specified in this document. | | 0 | 0 | 1 | 0 | 2 | DCE Security version, with embedded POSIX UIDs. | | 0 | 0 | 1 | 1 | 3 | The name-based version specified in this document that uses MD5 hashing. | | 0 | 1 | 0 | 0 | 4 | The randomly or pseudo-randomly generated version specified in this document. | | 0 | 1 | 0 | 1 | 5 | The name-based version specified in this document that uses SHA-1 hashing. | ### Rules for Lexical Equivalence Consider each field of the UUID to be an unsigned integer as shown in the table in the section above. Then, to compare a pair of UUIDs, arithmetically compare the corresponding fields from each UUID in order of significance and according to their data type. Two UUIDs are equal if and only if all the corresponding fields are equal. As an implementation note, equality comparison can be performed on many systems by doing the appropriate byte-order canonicalization, and then treating the two UUIDs as 128-bit unsigned integers. UUIDs, as defined in this document, can also be ordered lexicographically. For a pair of UUIDs, the first one follows the second if the most significant field in which the UUIDs differ is greater for the first UUID. The second precedes the first if the most significant field in which the UUIDs differ is greater for the second UUID. ## VIII. References * [1] Universally unique identifier, https://en.wikipedia.org/wiki/Universally_unique_identifier * [2] A Universally Unique IDentifier (UUID) URN Namespace, https://tools.ietf.org/html/rfc4122 * [3] Boost Uuid Library http://www.boost.org/doc/libs/1_65_1/libs/uuid/uuid.html * [4] crossguid - Lightweight cross platform C++ GUID/UUID library, https://github.com/graeme-hill/crossguid