Using Caché Objects - InterSystems Documentation

Referring to an Object — OREF, OID, and ID
3.1.1 OID and ID Values
Caché makes a further distinction between a “universal” object identifier (OID) and a “local”
object identifier (ID). An OID value uniquely identifies any object stored in a database.
Specifically, an OID value contains a class name along with an ID value. An ID value uniquely
identifies an object belonging to a specific “extent” . An extent is a set of related objects,
such as all instances of a specific class or all instances of a class and its subclasses.
Caché provides two versions of methods that refer to persistent objects: one using OID and
the other using ID values. For example, using Caché ObjectScript you can open an object
instance using an OID value:
Set object = ##class(MyApp.Person).%Open(oid)
Or you can open it using an ID value:
Set object = ##class(MyApp.Person).%OpenId(id)
The ID versions have “Id” added to their name. In practice, applications most often use ID
values; there are few circumstances in which you need to locate an object in which you do
not know what extent (set of objects) an object belongs to.
Note that these two forms are also available in Caché Basic:
obj = Open MyApp.Person(oid)
obj = OpenId MyApp.Person(id)
3.1.2 OREFs and Reference Counting
References to in-memory objects (OREFs) automatically manage a reference count—the
number of items (variables or objects) currently referring to an object. Whenever you set a
variable or object property to refer to a object, the object's reference count is automatically
incremented. When a variable stops referring to an object (if it goes out of scope, is killed,
or is set to a new value) the reference count is decremented. When this count goes to 0, the
object is automatically destroyed (removed from memory) and its %OnClose method (if
present) is called.
For example, consider the following method:
Method Test()
{
Set person = ##class(Sample.Person).%OpenId(1)
}
Set person = ##class(Sample.Person).%OpenId(2)
Using Caché Objects 15