QUuid Class
The QUuid class stores a Universally Unique Identifier (UUID). More...
| Header: | #include <QUuid> |
| qmake: | QT += core |
Note: All functions in this class are reentrant.
Public Types
| enum | StringFormat { WithBraces, WithoutBraces, Id128 } |
| enum | Variant { VarUnknown, NCS, DCE, Microsoft, Reserved } |
| enum | Version { VerUnknown, Time, EmbeddedPOSIX, Name, Md5, …, Sha1 } |
Detailed Description
Using Universally Unique IDentifiers (UUID) is a standard way to uniquely identify entities in a distributed computing environment. A UUID is a 16-byte (128-bit) number generated by some algorithm that is meant to guarantee that the UUID will be unique in the distributed computing environment where it is used. The acronym GUID is often used instead, Globally Unique IDentifiers, but it refers to the same thing.
Actually, the GUID is one variant of UUID. Multiple variants are in use. Each UUID contains a bit field that specifies which type (variant) of UUID it is. Call variant() to discover which type of UUID an instance of QUuid contains. It extracts the three most significant bits of byte 8 of the 16 bytes. In QUuid, byte 8 is QUuid::data4[0]. If you create instances of QUuid using the constructor that accepts all the numeric values as parameters, use the following table to set the three most significant bits of parameter b1, which becomes QUuid::data4[0] and contains the variant field in its three most significant bits. In the table, 'x' means don't care.
| msb0 | msb1 | msb2 | Variant |
|---|---|---|---|
| 0 | x | x | NCS (Network Computing System) |
| 1 | 0 | x | DCE (Distributed Computing Environment) |
| 1 | 1 | 0 | Microsoft (GUID) |
| 1 | 1 | 1 | Reserved for future expansion |
If variant() returns QUuid::DCE, the UUID also contains a version field in the four most significant bits of QUuid::data3, and you can call version() to discover which version your QUuid contains. If you create instances of QUuid using the constructor that accepts all the numeric values as parameters, use the following table to set the four most significant bits of parameter w2, which becomes QUuid::data3 and contains the version field in its four most significant bits.
| msb0 | msb1 | msb2 | msb3 | Version |
|---|---|---|---|---|
| 0 | 0 | 0 | 1 | Time |
| 0 | 0 | 1 | 0 | Embedded POSIX |
| 0 | 0 | 1 | 1 | Md5(Name) |
| 0 | 1 | 0 | 0 | Random |
| 0 | 1 | 0 | 1 | Sha1 |
The field layouts for the DCE versions listed in the table above are specified in the Network Working Group UUID Specification.
Most platforms provide a tool for generating new UUIDs, e.g. uuidgen and guidgen. You can also use createUuid(). UUIDs generated by createUuid() are of the random type. Their QUuid::Version bits are set to QUuid::Random, and their QUuid::Variant bits are set to QUuid::DCE. The rest of the UUID is composed of random numbers. Theoretically, this means there is a small chance that a UUID generated by createUuid() will not be unique. But it is a very small chance.
UUIDs can be constructed from numeric values or from strings, or using the static createUuid() function. They can be converted to a string with toString(). UUIDs have a variant() and a version(), and null UUIDs return true from isNull().
Member Type Documentation
enum QUuid::StringFormat
This enum is used by toString(StringFormat) to control the formatting of the string representation. The possible values are:
| Constant | Value | Description |
|---|---|---|
QUuid::WithBraces | 0 | The default, toString() will return five hex fields, separated by dashes and surrounded by braces. Example: {00000000-0000-0000-0000-000000000000}. |
QUuid::WithoutBraces | 1 | Only the five dash-separated fields, without the braces. Example: 00000000-0000-0000-0000-000000000000. |
QUuid::Id128 | 3 | Only the hex digits, without braces or dashes. Note that QUuid cannot parse this back again as input. |
This enum was introduced or modified in Qt 5.11.
enum QUuid::Variant
This enum defines the values used in the variant field of the UUID. The value in the variant field determines the layout of the 128-bit value.
| Constant | Value | Description |
|---|---|---|
QUuid::VarUnknown | -1 | Variant is unknown |
QUuid::NCS | 0 | Reserved for NCS (Network Computing System) backward compatibility |
QUuid::DCE | 2 | Distributed Computing Environment, the scheme used by QUuid |
QUuid::Microsoft | 6 | Reserved for Microsoft backward compatibility (GUID) |
QUuid::Reserved | 7 | Reserved for future definition |
enum QUuid::Version
This enum defines the values used in the version field of the UUID. The version field is meaningful only if the value in the variant field is QUuid::DCE.
| Constant | Value | Description |
|---|---|---|
QUuid::VerUnknown | -1 | Version is unknown |
QUuid::Time | 1 | Time-based, by using timestamp, clock sequence, and MAC network card address (if available) for the node sections |
QUuid::EmbeddedPOSIX | 2 | DCE Security version, with embedded POSIX UUIDs |
QUuid::Name | Md5 | Name-based, by using values from a name for all sections |
QUuid::Md5 | 3 | Alias for Name |
QUuid::Random | 4 | Random-based, by using random numbers for all sections |
QUuid::Sha1 | 5 |