Class Inheritance Semantics

Classes use the same pass-by-value semantics as structures. If you pass a class instance to an operation, the class and all its members are passed. The usual type compatibility rules apply: you can pass a derived instance where a base instance is expected. If the receiver has static type knowledge of the actual derived run-time type, it receives the derived instance; otherwise, if the receiver does not have static type knowledge of the derived type, depending on the format used to encode the class, it will either fail to read the instance or slice the instance to the base type.

For an example, suppose we have the following definitions:

// In file

class TimeOfDay {
    short hour;         // 0 - 23
    short minute;       // 0 - 59
    short second;       // 0 - 59

interface Clock {
    TimeOfDay getTime();
    void setTime(TimeOfDay time);

// In file

#include <>

class DateTime extends TimeOfDay {
    short day;          // 1 - 31
    short month;        // 1 - 12
    short year;         // 1753 onwards

Because DateTime is a sub-class of TimeOfDay, the server can return a DateTime instance from getTime, and the client can pass a DateTime instance to setTime. In this case, if both client and server are linked to include the code generated for both and, they each receive the actual derived DateTime instance, that is, the actual run-time type of the instance is preserved.

Contrast this with the case where the server is linked to include the code generated for both and, but the client is linked only with the code generated for In other words, the server understands the type DateTime and can return a DateTime instance from getTime, but the client only understands TimeOfDay. In this case, there are two possible outcomes depending on the format used by the server to encode the instance:

  • with the sliced format, the derived DateTime instance returned by the server is sliced to its TimeOfDay base type in the client
  • with the compact format, getTime fails with the Ice::NoObjectFactoryException exception

See Design Considerations for Objects for additional information on the sliced and compact formats.

Class hierarchies are useful if you need polymorphic values (instead of polymorphic interfaces). For example:

class Shape {
    // Definitions for shapes, such as size, center, etc.

class Circle extends Shape {
    // Definitions for circles, such as radius...

class Rectangle extends Shape {
    // Definitions for rectangles, such as width and length...

sequence<Shape> ShapeSeq;

interface ShapeProcessor {
    void processShapes(ShapeSeq ss);

Note the definition of ShapeSeq and its use as a parameter to the processShapes operation: the class hierarchy allows us to pass a polymorphic sequence of shapes (instead of having to define a separate operation for each type of shape).

The receiver of a ShapeSeq can iterate over the elements of the sequence and down-cast each element to its actual run-time type. (The receiver can also ask each element for its type ID to determine its type.)

See Also