Chapter 65. Boost.Uuid

Boost.Uuid provides generators for UUIDs. UUIDs are universally unique identifiers that don’t depend on a central coordinating instance. There is, for example, no database storing all generated UUIDs that can be checked to see whether a new UUID has been used.

UUIDs are used by distributed systems that have to uniquely identify components. For example, Microsoft uses UUIDs to identify interfaces in the COM world. For new interfaces developed for COM, unique identifiers can be easily assigned.

UUIDs are 128-bit numbers. Various methods exist to generate UUIDs. For example, a computer’s network address can be used to generate a UUID. The generators provided by Boost.Uuid are based on a random number generator to avoid generating UUIDs that can be traced back to the computer generating them.

All classes and functions from Boost.Uuid are defined in the namespace boost::uuids. There is no master header file to get access to all of them.

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

using namespace boost::uuids;

int main()
{
  random_generator gen;
  uuid id = gen();
  std::cout << id << '\n';
}

Example 65.1 generates a random UUID. It uses the class boost::uuids::random_generator, which is defined in boost/uuid/uuid_generators.hpp. This header file provides access to all generators provided by Boost.Uuid.

boost::uuids::random_generator is used like the generators from the C++11 standard library or from Boost.Random. This class overloads operator() to generate random UUIDs.

The type of a UUID is boost::uuids::uuid. boost::uuids::uuid is a POD – plain old data. You can’t create objects of type boost::uuids::uuid without a generator. But then, it’s a lean type that allocates exactly 128 bits. The class is defined in boost/uuid/uuid.hpp.

An object of type boost::uuids::uuid can be written to the standard output stream. However, you must include boost/uuid/uuid_io.hpp. This header file provides the overloaded operator to write objects of type boost::uuids::uuid to an output stream.

Example 65.1 displays output that looks like the following: 0cb6f61f-be68-5afc-8686-c52e3fc7a50d. Using dashes is the preferred way of displaying UUIDs.

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

using namespace boost::uuids;

int main()
{
  random_generator gen;
  uuid id = gen();
  std::cout << id.size() << '\n';
  std::cout << std::boolalpha << id.is_nil() << '\n';
  std::cout << id.variant() << '\n';
  std::cout << id.version() << '\n';
}

boost::uuids::uuid provides only a few member functions, some of which are introduced in Example 65.2. size() returns the size of a UUID in bytes. Because a UUID is always 128 bits, size() always returns 16. is_nil() returns true if the UUID is a nil UUID. The nil UUID is 00000000-0000-0000-0000-000000000000. variant() and version() specify the kind of UUID and how it was generated. In Example 65.2, variant() returns 1, which means the UUID conforms to RFC 4122. version() returns 4, which means that the UUID was created by a random number generator.

boost::uuids::uuid also provides member functions like begin(), end(), and swap() .

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

using namespace boost::uuids;

int main()
{
  nil_generator nil_gen;
  uuid id = nil_gen();
  std::cout << std::boolalpha << id.is_nil() << '\n';

  string_generator string_gen;
  id = string_gen("CF77C981-F61B-7817-10FF-D916FCC3EAA4");
  std::cout << id.variant() << '\n';

  name_generator name_gen(id);
  std::cout << name_gen("theboostcpplibraries.com") << '\n';
}

Example 65.3 contains more generators from Boost.Uuid. nil_generator generates a nil UUID. is_nil() returns true only if the UUID is nil.

You use string_generator if you want to use an existing UUID. You can generate UUIDs at sites such as http://www.uuidgenerator.net/. For the UUID in Example 65.3, variant() returns 0, which means that the UUID conforms to the backwards compatible NCS standard. name_generator is used to generate UUIDs in namespaces.

Please note the spelling of UUIDs when using string_generator. You can pass a UUID without dashes, but if you use dashes, they must be in the right places. Case (upper or lower) is ignored.

#include <boost/uuid/uuid.hpp>
#include <boost/uuid/uuid_generators.hpp>
#include <boost/uuid/uuid_io.hpp>
#include <boost/lexical_cast.hpp>
#include <string>
#include <iostream>

using namespace boost::uuids;

int main()
{
  random_generator gen;
  uuid id = gen();

  std::string s = to_string(id);
  std::cout << s << '\n';

  std::cout << boost::lexical_cast<std::string>(id) << '\n';
}

Boost.Uuid provides the functions boost::uuids::to_string() and boost::uuids::to_wstring() to convert a UUID to a string (see Example 65.4). It is also possible to use boost::lexical_cast() for the conversion.