stduuid/P0959.md

___	___
Doc. No.:	P0959R3
Date:	2020-09-09
Reply to:	Marius Bancila, Tony van Eerd
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<std::byte, 16>.
• 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<std::uuid>
• 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
• uuids 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

1.4 P0959R3

P0959R2 was discussed by LEWGI in Kona with the following feedback:

• The random number generator is not random enough; may want to consult W23 - or not provide a default.
• Why string and char* instead of string_view?
• Some methods that can be constexpr/noexcept are not.
• Need more explanations about the choices of ordering and how the RFC works in that regard.
• Need to specify which algorithm the name generator uses.
• The uuid should be a class not a struct.
• The three-way comparison operator cannot be defaulted if the class doesn't have at least exposition only members.

Based on this feedback and further considerations, the following changes have been done in this version:

• Added detailed explanation of the algorithms for generating uuids.
• from_string() and non-member swap() declared with noexcept.
• The uuid type is now defined as a class.
• Added an exposition-only member to hint and the possible internal representation of the uuid data.
• Added uuid constructors from arrays.
• Added implementation-specific constant uuids that can be used for generating name-based uuids.
• Added a new section (Open discussion) in this document to address questions/concerns from the committee.
• The global constants in the namespace std for generating name-based uuids are defined inline

II. Motivation

Universally unique identifiers (uuid), also known as Globally Unique Identifiers (guids), 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 <uuid> 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<T>, 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).

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.

std::array<uuid::value_type, 16> 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));

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));

Array constructors

The array constructors allow to create a uuid from an array or using direct initialization.

uuid id{ {0x47, 0x18, 0x38, 0x23,
          0x25, 0x74,
          0x4b, 0xfd,
          0xb4, 0x11,
          0x99, 0xed, 0x17, 0x7d, 0x3e, 0x43 } };
          
assert(to_string(id) == "47183823-2574-4bfd-b411-99ed177d3e43");

uuid::value_type arr[16] = {
   0x47, 0x18, 0x38, 0x23,
   0x25, 0x74,
   0x4b, 0xfd,
   0xb4, 0x11,
   0x99, 0xed, 0x17, 0x7d, 0x3e, 0x43 };
uuid id(arr);

assert(to_string(id) == "47183823-2574-4bfd-b411-99ed177d3e43");

Span constructor

The conversion constructor that takes a std::span and constructs a uuid from a contiguous sequence of 16 bytes.

std::array<uuid::value_type, 16> arr{ {
   0x47, 0x18, 0x38, 0x23,
   0x25, 0x74,
   0x4b, 0xfd,
   0xb4, 0x11,
   0x99, 0xed, 0x17, 0x7d, 0x3e, 0x43
} };

std::span<uuid::value_type, 16> 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}.

uuid id;
assert(id.is_nil());

std::optional<uuid> 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.

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.

std::array<uuid::value_type, 16> arr{ {
   0x47, 0x18, 0x38, 0x23,
   0x25, 0x74,
   0x4b, 0xfd,
   0xb4, 0x11,
   0x99, 0xed, 0x17, 0x7d, 0x3e, 0x43
} };

uuid id1{ std::span<uuid::value_type, 16>{arr} };
uuid id2 = uuid::from_string("47183823-2574-4bfd-b411-99ed177d3e43").value();
assert(id1 == id2);

std::set<std::uuid> 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.

std::array<uuids::uuid::value_type, 16> 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.

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<uuid> that contains a valid uuid if the parsing completed successful.

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.

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<wchar_t>(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 uuids in associative unordered containers such as std::unordered_set.

std::unordered_set<uuid> 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:

{
  std::random_device rd;
  auto seed_data = std::array<int, std::mt19937::state_size> {};
  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<std::mt19937> 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<int, 6> {};
  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<std::ranlux48_base> 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<int, std::mt19937::state_size> {};
  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<std::mt19937>(seq);

  basic_uuid_random_generator<std::mt19937> 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:

std::random_device rd;
auto seed_data = std::array<int, std::mt19937::state_size> {};
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:

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>.

uuid_variant enum

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

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

namespace std {
   class uuid
   {
   public:
      using value_type = uint8_t;
     
      constexpr uuid() noexcept = default;
      constexpr uuid(value_type(&arr)[16]) noexcept;
      constexpr uuid(std::array<value_type, 16> const & arr) noexcept;
      constexpr explicit uuid(std::span<value_type, 16> bytes);

      template<typename ForwardIterator>
      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<std::byte const, 16> as_bytes() const;

      constexpr std::strong_ordering operator<=>(uuid const&) const noexcept = default;
     
      template<class CharT = char>
      static bool is_valid_uuid(CharT const * str) noexcept;
     
      template<class CharT = char,
               class Traits = std::char_traits<CharT>,
               class Allocator = std::allocator<CharT>>
      static bool is_valid_uuid(std::basic_string<CharT, Traits, Allocator> const & str) noexcept;
     
      template<class CharT = char>
      static uuid from_string(CharT const * str) noexcept;
   
      template<class CharT = char, 
               class Traits = std::char_traits<CharT>,
               class Allocator = std::allocator<CharT>>
      static uuid from_string(std::basic_string<CharT, Traits, Allocator> const & str) noexcept;
   private:
      template <class Elem, class Traits>
      friend std::basic_ostream<Elem, Traits> & operator<<(std::basic_ostream<Elem, Traits> &s, uuid const & id);     
      
      value_type data[16];  // exposition-only
   };
}

non-member functions

namespace std {
   inline constexpr void swap(uuid & lhs, uuid & rhs) noexcept;
   
   template <class Elem, class Traits>
   std::basic_ostream<Elem, Traits> & operator<<(std::basic_ostream<Elem, Traits> &s, uuid const & id);

   template<class CharT = char,
            class Traits = std::char_traits<CharT>,
            class Allocator = std::allocator<CharT>>
   inline std::basic_string<CharT, Traits, Allocator> to_string(uuid const & id);
}

Constants

The following implementation-specific constant uuid values can be used for generating name-based uuids.

namespace std {
   inline constexpr uuid uuid_namespace_dns  = /* implementation-specific */;
   inline constexpr uuid uuid_namespace_url  = /* implementation-specific */;
   inline constexpr uuid uuid_namespace_oid  = /* implementation-specific */;
   inline constexpr uuid uuid_namespace_x500 = /* implementation-specific */;
}

Generators

Random uuid generators

basic_uuid_random_generator<T> 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 and UniformRandomBitGenerator concepts. basic_uuid_random_generator can be constructed with a reference or pointer to a an objects that satisfies the UniformRandomNumberGenerator requirements.

namespace std {
   template <typename UniformRandomNumberGenerator>
   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.

namespace std {
   using uuid_random_generator = basic_uuid_random_generator<std::mt19937>;
}

The algorithm for generating random uuids is as follows:

• Set the two most significant bits (bits 6 and 7) of the clock_seq_hi_and_reserved to zero and one, respectively.
• Set the four most significant bits (bits 12 through 15) of the time_hi_and_version to the binary value 0100 (representing version 3 as defined in the section VII. UUID format specification).
• Set all the other bits to randomly (or pseudo-randomly) chosen values.

Name-base uuid generator

uuid_name_generator is a function object that generates new UUIDs from a name and has to be initialized with another UUID.

namespace std {
   class uuid_name_generator
   {
   public:
     explicit uuid_name_generator(uuid const& namespace_uuid) noexcept;
     
     template<class CharT = char>
     uuid operator()(CharT const * name);
     
     template<class CharT = char,
              class Traits = std::char_traits<CharT>,
              class Allocator = std::allocator<CharT>>
     uuid operator()(std::basic_string<CharT, Traits, Allocator> const & name);
   };
}

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.

The algorithm for generating name-based uuids is as follows:

• Use a uuid as a namespace identifier for all uuids generated from names in that namespace
• Convert the name to a canonical sequence of octets (as defined by the standards or conventions of its name space); put the name space ID in network byte order.
• Compute the SHA-1 hash of the name space ID concatenated with the name.
• Copy the octects of the hash to the octets of the uuid as follows:
  • octets 0 to 3 of the hash to octets 0 to 3 of time_low field,
  • octets 4 and 5 of the hash to octets 0 and 1 of time_mid,
  • octets 6 and 7 of the hash to octets 0 and 1 of time_hi_and_version
  • octet 8 of the hash to clock_seq_hi_and_reserved
  • octet 9 of the hash to clock_seq_low
  • octets 10 to 15 of the hash to octets 0 to 5 of node
• Set the four most significant bits (bits 12 through 15) of the time_hi_and_version field to binary value 0101 (representing version 5 as defined in the section VII. UUID format specification).
• Set the two most significant bits (bits 6 and 7) of the clock_seq_hi_and_reserved to zero and one, respectively.
• Convert the resulting uuid to local byte order.

Time-based uuid generator

uuid_time_generator is a function object that generates a time-based UUID as described in the RFC4122 document.

namespace std {
   class uuid_time_generator
   {
   public:
     uuid_time_generator() noexcept;
     
     uuid operator()();
   };
}

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.

The algorithm for generating time-based uuids is as follows:

• Consider the timestamp to be a 60-bit unsigned integer and the clock sequence to be a 14-bit unsigned integer. Sequentially number the bits in a field, starting with zero for the least significant bit.
• Determine the values for the UTC-based timestamp and clock sequence to be used in the UUID as a 60-bit count of 100-nanosecond intervals since 00:00:00.00, 15 October 1582.
• Copy the bits of the timestamp to the uuid bits as follows, using the same order of significance:
  • bits 0 through 31 to the time_low field
  • bits 32 through 47 to the time_mid field
  • bits 48 through 59 to the 12 least significant bits (bits 0 through 11) of the time_hi_and_version field
• Copy the bits of the clock sequence to the uuid bits as follows, using the same order of significance:
  • bits 0 through 7 to the clock_seq_low field
  • bits 8 through 13 to the 6 least significant bits (bits 0 through 5) of the clock_seq_hi_and_reserved field
• Set the four most significant bits (bits 12 through 15) of the time_hi_and_version field to the binary value 0001 (representing version 1 as defined in the section VII. UUID format specification).
• Set the two most significant bits (bits 6 and 7) of the clock_seq_hi_and_reserved to zero and one, respectively.
• Set the node field to the 48-bit IEEE address in the same order of significance as the address.

Specialization

The template specializations of std::hash for the uuid class allow users to obtain hashes of UUIDs.

namespace std {
   template <>
   struct hash<uuid>
   {
      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) 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:

std::map<std::uuid, Value, std::uuid_fast_order> map;

template<typename Value, typename Allocator = std::allocator<T>>
using uuid_map = std::map<std::uuid, Value, std::uuid_lexicographical_order, Allocator>;

In this case, the uuid class should be defined as follows:

namespace std {
   class uuid
   {
   public:
      using value_type = uint8_t;
     
      constexpr uuid() noexcept = default;
      constexpr uuid(value_type(&arr)[16]) noexcept;
      constexpr uuid(std::array<value_type, 16> const & arr) noexcept;
      constexpr explicit uuid(std::span<value_type, 16> bytes);

      template<typename ForwardIterator>
      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<std::byte const, 16> as_bytes() const;
     
      template<class CharT = char>
      static bool is_valid_uuid(CharT const * str) noexcept;
     
      template<class CharT = char,
               class Traits = std::char_traits<CharT>,
               class Allocator = std::allocator<CharT>>
      static bool is_valid_uuid(std::basic_string<CharT, Traits, Allocator> const & str) noexcept;
     
      template<class CharT = char>
      static uuid from_string(CharT const * str) noexcept;
   
      template<class CharT = char, 
               class Traits = std::char_traits<CharT>,
               class Allocator = std::allocator<CharT>>
      static uuid from_string(std::basic_string<CharT, Traits, Allocator> const & str) noexcept;
   private:
      template <class Elem, class Traits>
      friend std::basic_ostream<Elem, Traits> & operator<<(std::basic_ostream<Elem, Traits> &s, uuid const & id);     
      
      value_type data[16];  // exposition-only
   };
}

In addition, the following must be added to the <uuid> header:

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. Open discussion

The LEWGI in Kona have raised the following questions or concerns, which are answered below.

Why string and char* instead of string_view?

The reason for having these two overloads is because it should be possible to create uuids both from literals and objects.

auto id1 = uuid::from_string("47183823-2574-4bfd-b411-99ed177d3e43"); // [1]

std::string str{ "47183823-2574-4bfd-b411-99ed177d3e43" };
auto id2 = uuid::from_string(str);                                    // [2]

Because template argument deduction does not work, both [1] and [2] would produce compiler errors. Instead, it should be one of the following, neither being desirable.

auto id1 = uuid::from_string(
   std::string_view {"47183823-2574-4bfd-b411-99ed177d3e43"});        // [1]

std::string str{ "47183823-2574-4bfd-b411-99ed177d3e43" };
auto id2 = uuid::from_string(std::string_view {str});                 // [2]

or

auto id1 = uuid::from_string<char, std::char_traits<char>>(
   "47183823-2574-4bfd-b411-99ed177d3e43");                           // [1]

std::string str{ "47183823-2574-4bfd-b411-99ed177d3e43" };
auto id2 = uuid::from_string<char, std::char_traits<char>>(str);      // [2]

Need more explanations about the choices of ordering and how the RFC works in that regard.

The RFC states the rules for lexical equivalence. These are mentioned in section VII, paragraph Rules for Lexical Equivalence . They say that:

• to compare two UUIDs you must compare the fields they are composed of in the order of significance; two UUIDs are equal if all the fields are equal.
• a UUID follows another one if the most significant field in which the UUIDs differ is greater for the first UUID.

IX. 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