22.8. Indicating Collaboration Among Objects

In this section, we concentrate on the collaborations (interactions) among objects in our ATM system. When two objects communicate with each other to accomplish a task, they are said to collaborate—they do this by invoking one another’s operations. A collaboration consists of an object of one class sending a message to an object of another class. Messages are sent in C++ via member-function calls.

In Section 22.7, we determined many of the operations of the system’s classes. Next, we concentrate on the messages that invoke these operations. To identify the collaborations, we return to the requirements document in Section 22.3. Recall that this document specifies the range of activities that occur during an ATM session (e.g., authenticating a user, performing transactions). The steps used to describe how the system must perform each of these tasks are our first indication of the collaborations in our system. As we proceed through this and the remaining sections, we may discover additional collaborations.