UuidUuidA 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).
// 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);
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.
Note that boost::is_pod is specialized for boost::uuids::uuid and depends on Boost.TypeTraits. Define BOOST_UUID_NO_TYPE_TRAITS before including boost/uuid/uuid.hpp to remove the dependency on Boost.TypeTraits.
// 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);
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.
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
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());
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;
}
The function, boost::uuids::uuid::is_null() returns true if and only if the uuid is equal to {00000000-0000-0000-0000-000000000000}.
Three bits of a uuid determine the variant.
boost::uuids::uuid u;
boost::uuids::uuid::variant_type v = u.variant();
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();
Both boost::uuids::uuid::swap() and boost::uuids::swap() are provided.
boost::uuids::uuid u1, u2;
u1.swap(u2);
swap(u1, u2);
All of the standard numeric operators are defined for the uuid class. These include:
operator==
operator!=
operator<
operator>
operator<=
operator>=
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());
#include <boost/uuid/nil_generator.hpp> #include <boost/uuid/string_generator.hpp> #include <boost/uuid/name_generator.hpp> #include <boost/uuid/random_generator.hpp>
namespace boost {
namespace uuids {
struct nil_generator {
typedef uuid result_type;
uuid operator()() const;
};
uuid nil_uuid();
}} //namespace boost::uuids
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);
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
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-0123-456789abcdef}");
boost::uuids::uuid u2 = gen(L"01234567-89ab-cdef-0123-456789abcdef");
boost::uuids::uuid u3 = gen(std::string("0123456789abcdef0123456789abcdef"));
boost::uuids::uuid u4 = gen(std::wstring(L"01234567-89ab-cdef-0123-456789abcdef"));
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
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");
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
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();
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);
std::string to_string(uuid const& u);
std::wstring to_wstring(uuid const& u);
}} // namespace boost::uuids
The standard input and output stream operators << and >> are provided by including boost/uuid/uuid_io.hpp. The string representation of a uuid is hhhhhhhh-hhhh-hhhh-hhhh-hhhhhhhhhhhh where h is a hexidecimal digit.
boost::uuids::uuid u1; // initialize uuid std::stringstream ss; ss << u1; boost::uuids::uuid u2; ss >> u2; assert(u1, u2);
One can also use boost::lexical_cast.
boost::uuids::uuid u1; // initialize uuid std::string s = boost::lexical_cast<std::string>(u); boost::uuids::uuid u2 = boost::lexical_cast<boost::uuids::uuid>(s); assert(u1 == u2);
The functions to_string and to_wstring are provided as a convenience to convert a uuid to a string. They are also likely faster than the stream operators or using boost::lexical_cast.
boost::uuids::uuid u; // initialize uuid std::string s1 = to_string(u); std::wstring s2 = to_wstring(u);
namespace boost {
namespace uuids {
BOOST_CLASS_IMPLEMENTATION(boost::uuids::uuid, boost::serialization::primitive_type)
}} // namespace boost::uuids
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.
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.
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)