# A Proposal for a Universally Unique Identifier Library
## I. Motivation
Universally unique identifiers (*uuid*), also known as Globally Unique Identifiers (*guid*s), 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 UUIDs based on the operation system facilities, serialize and deserialize UUIDs to and from strings, validate UUIDs and other common ensuing operations.
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 input argument must have the form `xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx` where `x` is a hexadecimal digit. Should the argument be of a different format a nil UUID would be created.
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 result is a nil `uuid`. This constructor follows the conventions of other containers in the standard library.
Member function `size()` indicates the number of bytes in the UUID. Because this is a fixed size structure this function always returns 16.
Member function `nil()` indicates whether the `uuid` has all the bits set to 0. A nil uuid is created by the default constructor or by the string conversion constructors when failing to parse the input argument.
```
uuid id;
assert(id.nil());
assert(id.size() == 16);
```
### String conversion
Member functions `string` and `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.
Member functions `variant()` and `version()` allows to check the variant type of the uuid and respetively the version type. These are defined by two strongly typed enums called `uuid_variant` and `uuid_version`.
Both member and non-member swap functions including a `std::swap<>` specialization that enable the swapping of `uuid` values.
```
uuid empty;
uuid id("47183823-2574-4bfd-b411-99ed177d3e43");
assert(empty.nil());
assert(!id.nil());
std::swap(empty, id);
assert(!empty.nil());
assert(id.nil());
empty.swap(id);
assert(empty.nil());
assert(!id.nil());
```
### `operator==` and `operator!=`
Non-member operators == and != are provided in order to test the equality/inequality of two `uuid` values.
```
uuid empty;
uuid id("47183823-2574-4bfd-b411-99ed177d3e43");
assert(empty == empty);
assert(id == id);
assert(empty != id);
```
### `operator<`
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 some containers such as `std::set` that by default use the operator to compare keys.
A `std::hash<>` specialization for `uuid` is provided in order to enable the use of `uuid`s in associative unordered containers such as `std::unordered_set`.
Non-member `make_uuid` function creates a new uuid by relying on the operating system APIs for this purpose. In practice, all the major operating system APIs (`CoCreateGuid` on Windows, `uuid_generate` on Linux, `CFUUIDCreate` on Max OS) produce version 4 UUIDs of the RFC variant.
The library does not support creating different versions of UUIDs. Generating version 3 (name-based versiong using MD5 hashing) and version 5 (name-based version using SHA1 hashing) are made impossible by the lack of standard support for MD5 and SHA1 cryptographic hash function. Instead, the library relies on the underlying operating system functionalities for creating UUIDs.
This limitation should be irrelevant for most practical use cases. In the event UUIDs need to be generated with a specific version other than the one provided by the underlying system, users can resort to a 3rd party implementation such as `boost::uuid`.
