11.2 Class-constrained virtual class

In chapter , we introduced class Set, which has a class ElmType as a parameter. Class ElmType is an example of a class-constrained virtual class.

We first show how to define class ElmType as a local attribute and not a parameter to illustrate the similarity with respect to virtual class TravelInfo defined in the previous section. To be able to distinguish from the Set defined in chapter , the class below is called ObjectSet:

class ObjectSet:
   class ElmType:< Object
   insert(E: ref ElmType): ...
   has(E: ref ElmType): ...
   remove(E: ref ElmType): ...
   ...

The declaration of class ElmType here specifies a local class attribute like class TravelInfo in the previous section. The difference is that class ElmType is constrained by a class, in this case the pre-defined class Object. The constraint of class ElmType specifies that it must be a subclass of Object.

We may extend the specification of ElmType in subclasses of ObjectSet. Here we define a class AccountSet:

class AccountSet: ObjectSet
   class ElmType ::< Account  

With this binding/extension of ElmType to Account, instances of AccountSet may only contain objects of type Account.

as: obj AccountSet
r: obj Customer
a: obj Account
as.insert(r)  -- illegal
as.insert(a)  -- legal

We may further extend ElmType in subclasses of AccountSet:

class SavingsAccountSet: AccountSet
   class ElmType::< SavingsAccount

Instances of SavingsAccountSet may only contain objects of type SavingsAccount.

Virtual classes as parameters

As mentioned, class Set as introduced in chapter has class ElmType as a parameter.

class Set(class ElmType:< Object):  
   insert(e: ref ElmType): ...
   has(e: ref ElmType): ...
   remove(e: ref ElmType): ...
   ...

A parameter is in general similar to a local attribute – the difference being that a value (an actual parameter) for the parameter must be supplied when the class (or method) is instantiated. You may also supply the actual parameter when a class or method is used as a superclass or supermethod respectively.

Declaring class ElmType as a parameter thus works in the same way as for the ObjectSet with a local virtual class ElmType.

In chapter , we showed how to create an instance of class Set and supplying an actual parameter for element:

theAccountsFile: obj Set(Account)

In this case, the specification of the virtual class is extended by supplying an actual parameter for ElmType. This is actually a binding of ElmType to Account. We may use class ObjectSet to make a similar object:

theAccountsFileX: obj ObjectSet
   class ElmType::< Account 
   ...

Here the binding of ElmType to Account also extends the description of ElmType in a sub-descriptor of class ObjectSet.