Facets

Facets provide a general-purpose mechanism for non-intrusively extending the type system of an application, by loosely coupling new type instances to existing ones.

Ice Objects as Collections of Facets

Up to this point, we have presented an Ice object as a single conceptual entity, that is, as an object with a single most-derived interface and a single identity, with the object being implemented by a single servant. However, an Ice object is more correctly viewed as a collection of one or more sub-objects known as facets, as shown below:

The diagram above shows a single Ice object with five facets. Each facet has a name, known as the facet name. Within a single Ice object, all facets must have unique names. Facet names are arbitrary strings that are assigned by the server that implements an Ice object. A facet with an empty facet name is legal and known as the default facet. Unless you arrange otherwise, an Ice object has a single default facet; by default, operations that involve Ice objects and servants operate on the default facet.

Note that all the facets of an Ice object share the same single identity, but have different facet names.

Even though Ice objects usually consist of just the default facet, it is entirely legal for an Ice object to consist of facets that all have non-empty names (that is, it is legal for an Ice object not to have a default facet).

Each facet has a single most-derived interface. There is no need for the interface types of the facets of an Ice object to be unique. It is legal for two facets of an Ice object to implement the same most-derived interface.

Each facet is implemented by a servant. Typically, each facet of an Ice object has a separate servant, although, if two facets of an Ice object have the same type, they can also be implemented by a single servant (for example, using a default servant).