Prefactoring

10.5. Interface Splitting

The actions in the RentalOperations class introduced in Chapter 8 are definitely separate from the actions for searching a catalog. The search operations go into their own class, CatalogOperations. The only overlap is that methods in CatalogOperations need to find out if a particular CDDisc is rented, by invoking is_cd_disc_rented( ). That method is currently in RentalOperations.

Status operations do not affect a system's state. They are just reporting mechanisms for the state of an object. We split the current RentalOperations interface into two interfaces: RentalOperations and StatusOperations. The rental status operation (is_cd_disc_rented( )) is placed in StatusOperations.

Now, CatalogOperations needs StatusOperations, but it does not need RentalOperations, as shown in Figure 10-2. Splitting the interfaces allows us to better control the access to each function ("If You Forget Security, You're Not Secure"). The program used by a customer to perform catalog searches involves CatalogOperations, which has access to StatusOperations but not to RentalOperations.

Figure 10-2. Split interfaces

SPLIT INTERFACES Split a single interface into multiple interfaces if multiple clients use different portions of the interface.[*]

^[*] A reviewer suggested that you could find more information on splitting interfaces in Large-Scale C++ Software Design by John ...