9.2.5. Type Instances

Every user defined type can have zero or more instances. Every instance knows it's type. The instances are SQL values. They are copyable. Instances for SQL types contain placeholders for the type's members. Instances for non-SQL types contain a reference to the real instance in type's hosted environment. (Java VM for JAVA and the CLR virtual machine for CLR). So copying an instance is different for SQL and non-SQL types. When a SQL type's instance is copied a new set of members placeholders is created and all members values are copied. This way the copy does not hold any link to it's original and they can be considered as two different instances. This means that changing members' value in the copy will not affect the original.

The non-SQL types instances hold only a reference to the real instance. So copying such an instance is equivalent of making another reference to the foreign object. Thus changing the member's value in the copy WILL affect the original. Usually the foreign virtual machines have a means of explicitly copying an instance, but they are not used by the Virtuoso server when copying the SQL values.

As with the other SQL values, an instance gets destroyed when it is no longer referenced.