Node instances are physical manifestations of node classes. A node instance has the following functions:

• It can generate messages. An instance can create a compound message. For example, an instance of the hardware class can create a message from a stream of bytes sent by a sound card. The hardware instance collects bytes until an entire compound message arrives. At this point, it has a complete compound message ready for further processing.

• It can be linked to another instance. Compound messages travel from one instance to another. The link tells the instance where to send the message, if it decides to send it at all.

• It can send a message to another instance. The link tells the instance where to send the message. Note that the instance can decide at run-time whether to send the message. For example, an instance that performs filtering can decide to forward messages that fit a certain criteria. Messages that are not forwarded are lost.

• It can be configured. An application can send configuration data to a particular instance at any time. The class determines whether configuration data is accepted, but the instance processes the data. Typically, the configuration data changes some of the parameters of the instance. For example, the filter instance (mentioned in the previous list item) can accept data that changes the filter criteria. Another example is an instance that can accept actual machine language instructions and execute them whenever a message arrives. This feature would make that class very powerful, and very dangerous.

• It can be identified with a unique number. This 32-bit number is called the instance number and it is used internally as a 16:16 pointer to the instance data. When another driver or an application wishes to reference a particular instance, it uses the instance number.

RTMIDI comes with pre-defined classes. Each of these is discussed in detail in Pre-defined Classes.