Defining New Data Types

There is a limited form of user-defined type, the Message type.

The MLDesigner contains a heterogeneous message interface, which provides a mechanism for primitives to transmit arbitrary objects to other primitives. It’s design fulfills the following requirements:

• Existing primitives (primitives that were written before the message interface was added) that handle ANYTYPE work with message particles without change.
• Message portholes can send different types of messages during the same simulation. This is especially useful for modeling communication networks.
• It avoids copying large messages by using a reference count mechanism, as in many C++ classes (for example, string classes).
• It is possible to safely modify large messages without excessive memory allocation and deallocation.
• It is (relatively) easy for users to define their own message types. No change to the kernel is required to support new message types.

The message type is interpreted by MLDesigner as a particle containing a message. There are three classes that implement the support for message types:

• The Message class is the base class from which all other message data types are derived. A user who wishes to define an application-specific message type derives a new class from Message.
• The Envelope class contains a pointer to a derived class from Message. When an Envelope object is copied or duplicated, the new envelope simply sets its own pointer to the pointer contained in the original. Thus, several envelopes can reference the same Message object. Each Message object contains a reference count, which tracks how many Envelope objects reference it. When the last reference is removed, the Message is deleted.
• The MessageParticle class is a type of Particle (like IntParticle, FloatParticle, etc.). It contains an Envelope. Ports of type message transmit and receive objects of this type.

Class Particle contains two member functions for message support: getMessage, to receive a message, and the << operator with an Envelope as the right argument, to load a message into a particle. These functions return errors in the base class. They are overridden in the MessageParticle class with functions that perform the expected operation.