Oracle C++ Call Interface Programmer's Guide

Type Inheritance
Instance substitutability refers to the ability to use an object value of a subtype in a
context declared in terms of a supertype. REF substitutability refers to the ability to
use a REF to a subtype in a context declared in terms of a REF to a supertype.
REF type attributes are substitutable, that is, an attribute defined as REF T can hold
a REF to an instance of T or any of its subtypes.
Object type attributes are substitutable, that is, an attribute defined to be of (an
object) type T can hold an instance of T or any of its subtypes.
Collection element types are substitutable, that is, if we define a collection of
elements of type T, then it can hold instances of type T and any of its subtypes. Here
is an example of object attribute substitutability:
CREATE TYPE Book_t AS OBJECT
( title VARCHAR2(30),
author Person_t /* substitutable */);
Thus, a Book_t instance can be created by specifying a title string and a Person_t
(or any subtype of Person_t) object:
Book_t(‘My Oracle Experience’,
Employee_t(12345, ‘Joe’, ‘SF’, 1111, NULL))
NOT INSTANTIABLE Types and Methods
A type can be declared NOT INSTANTIABLE, which means that there is no
constructor (default or user defined) for the type. Thus, it will not be possible to
construct instances of this type. The typical usage would be to define instantiable
subtypes for such a type. Here is how this property is used:
CREATE TYPE Address_t AS OBJECT(...) NOT INSTANTIABLE NOT FINAL;
CREATE TYPE USAddress_t UNDER Address_t(...);
CREATE TYPE IntlAddress_t UNDER Address_t(...);
A method of a type can be declared to be NOT INSTANTIABLE. Declaring a method
as NOT INSTANTIABLE means that the type is not providing an implementation
for that method. Further, a type that contains any NOT INSTANTIABLE methods
must necessarily be declared as NOT INSTANTIABLE. For example:
CREATE TYPE T AS OBJECT
(
x NUMBER,
NOT INSTANTIABLE MEMBER FUNCTION func1() RETURN NUMBER
) NOT INSTANTIABLE;
Object Programming 3-27