stduuid/P0959.md

___	___
Doc. No.:	P0959R1
Date:	2018-09-05
Reply to:	Marius Bancila
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.

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
• a class called uuid_error representing an exception type for uuid operations
• function objects that generate UUIDs, called generators: basic_uuid_random_generator<T>, uuid_random_generator, uuid_name_generator
• conversion functions from strings from_string() and to strings std::to_string() \ std::to_wstring(), 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));

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");

Size

Member function size() indicates the number of bytes in the UUID. Because this is a fixed size structure this function always returns 16.

uuid id;
assert(id.size() == 16);

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());

uuid id = uuid::from_string("00000000-0000-0000-0000-000000000000");
assert(id.is_nil());

Iterators

Constant iterators allow direct access to the underlaying uuid data. This enables both direct reading of the uuid bits. The uuid class has const begin() and end() members to return constant iterators to the first and the one-past-last element of the UUID data.

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

size_t i = 0;
for (auto const & b : id)
   assert(arr[i++] == b);

Because the internal representation may not be a straightforward array of bytes and may have arbitrary endianness iterators are not defined as pointers.

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");
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");
assert(id1 == id2);

std::set<std::uuid> ids{
   uuid{},
   uuid::from_string("47183823-2574-4bfd-b411-99ed177d3e43")
};

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
   } };

const uuid 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");

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 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. Should the argument be of a different format, an uuid_error exception is thrown.

auto id1 = uuid::from_string("47183823-2574-4bfd-b411-99ed177d3e43");
auto id2 = uuid::from_string(L"{47183823-2574-4bfd-b411-99ed177d3e43}");

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 functions to_string() and to_wstring() return a string with the UUID formatted to the canonical textual representation xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx, where x is a lower case hexadecimal digit.

uuid id = uuid::from_string("47183823-2574-4bfd-b411-99ed177d3e43");
assert(to_string(id) == "47183823-2574-4bfd-b411-99ed177d3e43");
assert(to_wstring(id) == 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"),
};

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:

{
  basic_uuid_random_generator<std::mt19937> dgen;
  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);
}
{
  basic_uuid_random_generator<std::ranlux48_base> dgen;
  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;
  std::ranlux48_base generator(rd());

  basic_uuid_random_generator<std::ranlux48_base> dgen(&generator);
  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 generator = std::make_unique<std::mt19937>(rd());

  basic_uuid_random_generator<std::mt19937> dgen(generator.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:

uuid_random_generator dgen;
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"));
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
   };
}

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_error class

namespace std {
   struct uuid_error : public std::runtime_error
   {
      explicit uuid_error(std::string_view message);
      explicit uuid_error(char const * message);
   };
}

uuid class

namespace std {
   struct uuid
   {
     using value_type = uint8_t;
     using const_iterator = /*implementation-defined*/;
     
     constexpr uuid() noexcept = default;
     
     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 std::size_t size() const noexcept;
     constexpr bool is_nil() const noexcept;

     constexpr void swap(uuid & other) noexcept;

     constexpr const_iterator begin() const noexcept;
     constexpr const_iterator end() const noexcept;

     constexpr std::span<std::byte const, 16> as_bytes() const;

     constexpr std::strong_ordering operator<=>(uuid const&) const noexcept = default;   
     
     template <typename TChar>
     static uuid from_string(TChar const * const str, size_t const size);
     static uuid from_string(std::string_view str);
     static uuid from_string(std::wstring_view str);
   
   private:
     template <class Elem, class Traits>
     friend std::basic_ostream<Elem, Traits> & operator<<(std::basic_ostream<Elem, Traits> &s, uuid const & id);     
   };
}

non-member functions

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

   inline std::string to_string(uuid const & id);
   inline std::wstring to_wstring(uuid const & id);  
}

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 either default constructed or 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 result_type = uuid;

     basic_uuid_random_generator();
     explicit basic_uuid_random_generator(UniformRandomNumberGenerator& gen);
     explicit basic_uuid_random_generator(UniformRandomNumberGenerator* 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>;
}

uuid_name_generator is a function object that generates new UUIDs from a name. It has to be initialized with another UUID and has overloaded operator() for both std::string_view and std::wstring_view.

namespace std {
   class uuid_name_generator
   {
   public:
     using result_type = uuid;

     explicit uuid_name_generator(uuid const& namespace_uuid) noexcept;
     
     uuid operator()(std::string_view name);
     uuid operator()(std::wstring_view name);
   };
}

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 {
   struct uuid
   {
     using value_type = uint8_t;
     using const_iterator = /*implementation-defined*/;
     
     constexpr uuid() noexcept = default;
     
     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 std::size_t size() const noexcept;
     constexpr bool is_nil() const noexcept;

     constexpr void swap(uuid & other) noexcept;

     constexpr const_iterator begin() const noexcept;
     constexpr const_iterator end() const noexcept;

     constexpr std::span<std::byte const, 16> as_bytes() const;
     
     template <typename TChar>
     static uuid from_string(TChar const * const str, size_t const size);
     static uuid from_string(std::string_view str);
     static uuid from_string(std::wstring_view str);
   
   private:
     template <class Elem, class Traits>
     friend std::basic_ostream<Elem, Traits> & operator<<(std::basic_ostream<Elem, Traits> &s, uuid const & id);     
   };
}

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. 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