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Uuid

Contents

  1. Introduction
  2. Examples
  3. Reference
  4. Design notes
  5. History and Acknowledgements

Introduction

A UUID, or Universally unique identifier, is intended to uniquely identify information in a distributed environment without significant central coordination. It can be used to tag objects with very short lifetimes, or to reliably identify very persistent objects across a network.

UUIDs have many applications. Some examples follow: Databases may use UUIDs to identify rows or records in order to ensure that they are unique across different databases, or for publication/subscription services. Network messages may be identified with a UUID to ensure that different parts of a message are put back together again. Distributed computing may use UUIDs to identify a remote procedure call. Transactions and classes involved in serialization may be identified by UUIDs. Microsoft's component object model (COM) uses UUIDs to distinguish different software component interfaces. UUIDs are inserted into documents from Microsoft Office programs. UUIDs identify audio or video streams in the Advanced Systems Format (ASF). UUIDs are also a basis for OIDs (object identifiers), and URNs (uniform resource name).

An attractive feature of UUIDs when compared to alternatives is their relative small size, of 128-bits, or 16-bytes. Another is that the creation of UUIDs does not require a centralized authority.

When UUIDs are generated by one of the defined mechanisms, they are either guaranteed to be unique, different from all other generated UUIDs (that is, it has never been generated before and it will never be generated again), or it is extremely likely to be unique (depending on the mechanism).

Examples

Tagging

// example of tagging an object with a uuid
// see boost/libs/uuid/test/test_tagging.cpp

#include <boost/uuid/uuid.hpp>
#include <boost/uuid/uuid_generators.hpp>

class object
{
public:
    object()
        : tag(boost::uuids::random_generator()())
        , state(0)
    {}
    
    explicit object(int state)
        : tag(boost::uuids::random_generator()())
        , state(state)
    {}
    
    object(object const& rhs)
        : tag(rhs.tag)
        , state(rhs.state)
    {}
    
    bool operator==(object const& rhs) const {
        return tag == rhs.tag;
    }
    
    object& operator=(object const& rhs) {
        tag = rhs.tag;
        state = rhs.state;
    }
    
    int get_state() const { return state; }
    void set_state(int new_state) { state = new_state; }
    
private:
    boost::uuids::uuid tag;
    
    int state;
};

object o1(1);
object o2 = o1;
o2.set_state(2);
assert(o1 == o2);

object o3(3);
assert(o1 != o3);
assert(o2 != o3);

POD Efficiencies

This library implements a UUID as a POD allowing a UUID to be used in the most efficient ways, including using memcpy, and aggregate initializers. A drawback is that a POD can not have any constructors, and thus declaring a UUID will not initialize it to a value generated by one of the defined mechanisms. But a class based on a UUID can be defined that does initialize itself to a value generated by one of the defined mechanisms.

// example using memcpy and aggregate initializers
// example of a class uuid see boost/libs/uuid/test/test_uuid_class.cpp

#include <boost/uuid/uuid.hpp>
#include <boost/uuid/uuid_generators.hpp>

{ // example using memcpy
    unsigned char uuid_data[16];
    // fill uuid_data

    boost::uuids::uuid u;

    memcpy(&u, uuid_data, 16);
}

{ // example using aggregate initializers
    boost::uuids::uuid u =
    { 0x12 ,0x34, 0x56, 0x78
    , 0x90, 0xab
    , 0xcd, 0xef
    , 0x12, 0x34
    , 0x56, 0x78, 0x90, 0xab, 0xcd, 0xef
    };
}

// example of creating a uuid class that
// initializes the uuid in the constructor
// using a defined mechanism

class uuid_class : public boost::uuids::uuid
{
public:
    uuid_class()
        : boost::uuids::uuid(boost::uuids::random_generator()())
    {}
    
    explicit uuid_class(boost::uuids::uuid const& u)
        : boost::uuids::uuid(u)
    {}

    operator boost::uuids::uuid() {
        return static_cast<boost::uuids::uuid&>(*this);
    }

    operator boost::uuids::uuid() const {
        return static_cast<boost::uuids::uuid const&>(*this);
    }
};

uuid_class u1;
uuid_class u2;

assert(u1 != u2);

Byte Extraction

It is sometimes useful to get at the 16 bytes of a uuid directly. Typical use is as follows:

boost::uuids::uuid u;
std::vector<char> v(u.size());
std::copy(u.begin(), u.end(), v.begin());

Note: boost::uuids::uuid::size() always returns 16.

Reference

boost/uuid/uuid.hpp

Synopsis

namespace boost {
namespace uuids {

class uuid {
public:
    typedef uint8_t value_type;
    typedef uint8_t& reference;
    typedef uint8_t const& const_reference;
    typedef uint8_t* iterator;
    typedef uint8_t const* const_iterator;
    typedef std::size_t size_type;
    typedef std::ptrdiff_t difference_type;

    static size_type static_size();

    // iteration
    iterator begin();
    iterator end();
    const_iterator begin() const;
    const_iterator end() const;

    size_type size() const;

    bool is_nil() const;
    
    enum variant_type {
        variant_ncs, // NCS backward compatibility
        variant_rfc_4122, // defined in RFC 4122 document
        variant_microsoft, // Microsoft Corporation backward compatibility
        variant_future // future definition
    };
    variant_type variant() const;

    enum version_type {
        version_unknown = -1,
        version_time_based = 1,
        version_dce_security = 2,
        version_name_based_md5 = 3,
        version_random_number_based = 4,
        version_name_based_sha1 = 5
    };
    version_type version() const;

    // Swap function
    void swap(uuid& rhs);

    uint8_t data[static_size()];
};

// standard operators
bool operator==(uuid const& lhs, uuid const& rhs);
bool operator!=(uuid const& lhs, uuid const& rhs);
bool operator<(uuid const& lhs, uuid const& rhs);
bool operator>(uuid const& lhs, uuid const& rhs);
bool operator<=(uuid const& lhs, uuid const& rhs);
bool operator>=(uuid const& lhs, uuid const& rhs);

void swap(uuid& lhs, uuid& rhs);

std::size_t hash_value(uuid const& u);

}} // namespace boost::uuids

Size

The size of a uuid (in bytes) can be obtained either by calling the function boost::uuids::uuid::size() or by calling the static function boost::uuids::uuid::static_size(), both always return 16.

    boost::uuids::uuid u;
    assert(16 == u.size());
    assert(16 == boost::uuids::uuid::static_size());

Iteration

Both iterators and constant iterators are provided.

    boost::uuids::uuid u;
    for (boost::uuids::uuid::const_iterator it=u.begin(); it!=u.end(); ++it) {
        boost::uuids::uuid::value_type v = *it;
    }
    for (boost::uuids::uuid::iterator it=u.begin(); it!=u.end(); ++it) {
        *it = 0;
    }

Nil uuid

The function, boost::uuids::uuid::is_null() returns true if and only if the uuid is equal to {00000000-0000-0000-0000-000000000000}.

Variant

Three bits of a uuid determine the variant.

    boost::uuids::uuid u;
    boost::uuids::uuid::variant_type v = u.variant();

Version

Four bits of a uuid determine the variant, that is the mechanism used to generate the uuid.

    boost::uuids::uuid u;
    boost::uuids::uuid::version_type v = u.version();

Swap

Both boost::uuids::uuid::swap() and boost::uuids::swap() are provided.

    boost::uuids::uuid u1, u2;
    u1.swap(u2);
    swap(u1, u2);

Operators

All of the standard numeric operators are defined for the uuid class. These include:

    operator==
    operator!=
    operator<
    operator>
    operator<=
    operator>=

Hash Function

This function allows uuids to be used with boost::hash

boost::hash<boost::uuids::uuid> uuid_hasher;
std::size_t uuid_hash_value = uuid_hasher(boost::uuids::uuid());

boost/uuid/uuid_generators.hpp

Synopsis

This file includes all the uuid generators for convenience.
#include <boost/uuid/nil_generator.hpp>
#include <boost/uuid/string_generator.hpp>
#include <boost/uuid/name_generator.hpp>
#include <boost/uuid/random_generator.hpp>

boost/uuid/nil_generator.hpp

Synopsis

namespace boost {
namespace uuids {

struct nil_generator {
    typedef uuid result_type;
    
    uuid operator()() const;
};
uuid nil_uuid();

}} //namespace boost::uuids

Nil Generator

The boost::uuids::nil_generator class always generates a nil uuid.

boost::uuids::nil_generator gen;
boost::uuids::uuid u = gen();
assert(u.is_nil() == true);

// or for convenience
boost::uuids::uuid u = boost::uuids::nil_uuid();
assert(u.is_nil() == true);

boost/uuid/string_generator.hpp

Synopsis

namespace boost {
namespace uuids {

struct string_generator {
    typedef uuid result_type;

    template <typename ch, typename char_traits, typename alloc>
        uuid operator()(std::basic_string<ch, char_traits, alloc> const& s) const;
};

}} //namespace boost::uuids

String Generator

The boost::uuids::string_generator class generates a uuid from a string.

boost::uuids::string_generator gen;
boost::uuids::uuid u1 = gen("{01234567-89ab-cdef-0123456789abcdef}");
boost::uuids::uuid u2 = gen(L"01234567-89ab-cdef-0123456789abcdef");
boost::uuids::uuid u3 = gen(std::string("0123456789abcdef0123456789abcdef"));
boost::uuids::uuid u4 = gen(std::wstring(L"01234567-89ab-cdef-0123456789abcdef"));

boost/uuid/name_generator.hpp

Synopsis

namespace boost {
namespace uuids {

class name_generator {
public:
    typedef uuid result_type;
    
    explicit name_generator(uuid const& namespace_uuid);

    uuid operator()(const char* name) const;
    uuid operator()(const wchar_t* name) const;
    tempate <typename ch, typename char_traits, typename alloc>
        uuid operator()(std::basic_string<ch, char_traits, alloc> const& name) const;
    uuid operator()(void const* buffer, std::size_t byte_count) const;
};

}} //namespace boost::uuids

Name Generator

The boost::uuids::name_generator class generates a name based uuid from a namespace uuid and a name.

boost::uuids::uuid dns_namespace_uuid; // initialize to {6ba7b810-9dad-11d1-80b4-00c04fd430c8}

boost::uuids::name_generator gen(dns_namespace_uuid);
boost::uuids::uuid u = gen("boost.org");

boost/uuid/random_generator.hpp

Synopsis

namespace boost {
namespace uuids {

template <typename UniformRandomNumberGenerator>
class basic_random_generator {
public:
    typedef uuid result_type;
    
    basic_random_generator();
    explicit basic_random_generator(UniformRandomNumberGenerator& gen);
    explicit basic_random_generator(UniformRandomNumberGenerator* pGen);
    
    uuid operator()();
};
typedef basic_random_generator<mt19937> random_generator;

}} // namespace boost::uuids

Random Generator

The boost::uuids::basic_random_generator class generates a random number based uuid from a random number generator (one that conforms to the UniformRandomNumberGenerator concept).

//default construct the random number generator and seed it
boost::uuids::basic_random_generator<boost::mt19937> gen;
boost::uuids::uuid u = gen();

//for convenience boost::uuids::random_generator
//is equivalent to boost::uuids::basic_random_generator<boost::mt19937>
boost::uuids::random_generator gen;
boost::uuids::uuid u = gen();

//use an existing random number generator
//pass either a reference or a pointer to the random number generator
boost::mt19937 ran;
boost::uuids::basic_random_generator<boost::mt19937> gen(&ran);
boost::uuids::uuid u = gen();

boost/uuid/uuid_io.hpp

Synopsis

namespace boost {
namespace uuids {

template <typename ch, typename char_traits>
    std::basic_ostream<ch, char_traits>& operator<<(std::basic_ostream<ch, char_traits> &os, uuid const& u);

template <typename ch, typename char_traits>
    std::basic_istream<ch, char_traits>& operator>>(std::basic_istream<ch, char_traits> &is, uuid &u);

}} // namespace boost::uuids

Input and Output

Input and output stream operators << and >> are provided by including boost/uuid/uuid_io.hpp. The external representation of a uuid is a string of hexidecimal digits of the following form: hhhhhhhh-hhhh-hhhh-hhhh-hhhhhhhhhhhh

boost::uuids::uuid u;

std::stringstream ss;
ss << u;
ss >> u;

boost/uuid/uuid_serialize.hpp

Synopsis

namespace boost {
namespace uuids {

BOOST_CLASS_IMPLEMENTATION(boost::uuids::uuid, boost::serialization::primitive_type)

}} // namespace boost::uuids

Serialization

Serialization is accomplished with the Boost Serialization library. A uuid is serialized as a primitive type, thus only the uuid value will be saved to/loaded from an archive.

Include boost/uuid/uuid_serialize.hpp to enable serialization for uuids.

Design notes

The document, http://www.itu.int/ITU-T/studygroups/com17/oid/X.667-E.pdf, was used to design and implement the boost::uuids::uuid struct.

The boost::uuids::basic_random_generator class' default constructor seeds the random number generator with a SHA-1 hash of a number of different values including std::time(0), std::clock(), uninitialized data, value return from new unsigned int, etc..

Using Valgrind produces a number of false positives with the default constructor of boost::uuids::basic_random_generator. One solution is to suppress the errors as described in Valgrind's documentation. Another solution is to use a different constructor of boost::uuids::basic_random_generator and explicitly pass in a random number generator.

boost::mt19937 ran;
ran.seed(time(NULL)); // one should likely seed in a better way
boost::uuids::basic_random_generator<boost::mt19937> gen(&ran);
boost::uuids::uuid u = gen();

The boost::uuids::name_generator class uses the SHA-1 hash function to compute the uuid.

All functions are re-entrant. Classes are as thread-safe as an int. That is an instance can not be shared between threads without proper synchronization.

History and Acknowledgements

A number of people on the boost.org mailing list provided useful comments and greatly helped to shape the library.

Revised  February 6, 2010


Copyright Andy Tompkins, 2006

Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at www.boost.org/LICENSE_1_0.txt)