Interface allows entrance to communicate forced (forced good - standardization of interfaces)
Proprietary internal interface enables active protection obtained (active)
Between the object and the object, the message still does not involve passing inside the object.
Message is transmitted by way of assignment.
Objects are input and output through the object's interface,
Example Interface (corresponding to the program): passive input function object (received parameter) / active output (return)
Active input (friend function input port) / passive output (friend class declaration)
Passive definition: not by their own change and change
Active definition: change by its own change
Active and passive symmetrical
