mongo::OID Class Reference

Object ID type. More...

#include <oid.h>

Classes
struct	Hasher
	Functor compatible with std::hash for std::unordered_{map,set} Warning: The hash function is subject to change. More...
struct	Increment
struct	InstanceUnique

Public Member Functions
	OID (const std::string &s)
	init from a 24 char hex string
	OID (const unsigned char(&arr)[kOIDSize])
	init from a reference to a 12-byte array
void	clear ()
	initialize to 'null'
std::string	toString () const
std::string	toIncString () const
void	init ()
	sets the contents to a new oid / randomized value
void	init (const std::string &s)
	init from a 24 char hex string
void	init (Date_t date, bool max=false)
	Set to the min/max OID that could be generated at given timestamp. More...
void	hash_combine (size_t &seed) const
	this is not consistent do not store on disk

Static Public Member Functions
static void MONGO_CLIENT_FUNC	justForked ()
	call this after a fork to update the process id

Detailed Description

Object ID type.

BSON objects typically have an _id field for the object id. This field should be the first member of the object when present. The OID class is a special type that is a 12 byte id which is likely to be unique to the system. You may also use other types for _id's. When _id field is missing from a BSON object, on an insert the database may insert one automatically in certain circumstances.

The BSON ObjectID is a 12-byte value consisting of a 4-byte timestamp (seconds since epoch), in the highest order 4 bytes followed by a 5 byte value unique to this machine AND process, followed by a 3 byte counter.

          4 byte timestamp    5 byte process unique   3 byte counter
        |<----------------->|<---------------------->|<------------->

OID layout: [-—|-—|-—|-—|-—|-—|-—|-—|-—|-—|-—|-—] 0 4 8 12

The timestamp is a big endian 4 byte signed-integer.

The process unique is an arbitrary sequence of 5 bytes. There are no endianness concerns since it is never interpreted as a multi-byte value.

The counter is a big endian 3 byte unsigned integer.

Note: The timestamp and counter are big endian (in contrast to the rest of BSON) because we use memcmp to order OIDs, and we want to ensure an increasing order.

Warning: You MUST call OID::justForked() after a fork(). This ensures that each process will generate unique OIDs.

Member Function Documentation

void mongo::OID::init	(	Date_t	date,
		bool	max = false
	)

Set to the min/max OID that could be generated at given timestamp.

std::string mongo::OID::toIncString

(

)

const

Returns

the random/sequential part of the object ID as 6 hex digits

std::string mongo::OID::toString

(

)

const

Returns

the object ID output as 24 hex digits

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The documentation for this class was generated from the following file:

• src/mongo/bson/oid.h