2 Why We Need Model-Based Testing

Advanced Topics 249
}
}
public override void Initialize() { this.salary = 0; }
// ...
Model program Payroll1 and Payroll2 behave similarly. In the case of objectoriented
Payroll2, the NModel framework will create the field map for salary
automatically. The field map will record the associated value of each reachable
instance of class Employee. An object is reachable if it occurs within a global state
variable, or if it occurs within an instance field of a reachable object. Reachability
will be discussed in more detail in Section 15.2.
When inherited by type T the LabeledInstance class provides the following
features:
• An implicit field map for instance fields of T, as explained above.
• A serialized term label for each instance of T. For example, the first instance of
class Employee may appear as the term Employee(1) inside of an action argument.
The role of the term label in composition is discussed in Section 15.3. The role
of the term label in test harnessing is discussed in Section 15.4.
• A parameter generator for type T named new. You can use the attribute [Domain
("new")] on an action parameter whenever a new object should appear. This is
covered in Section 15.2.
• A static factory method called Create. This factory method is used instead of
C# operator new in contexts where a instance of T needs to be created within a
method. This is an advanced feature; most model programs will use the parameter
generator [Domain("new")] as a way to introduce new objects instead of the
Create factory method.
• A virtual method Initialize. The model program should override this method
for type T and reset all instance fields to their initial state.
In the following sections we present some topics that relate to the use of LabeledInstance
for object-oriented modeling.
15.2 Creating instances
Whenever you model objects, you have to deal with the issue of instantiation. How
do instances come into existence? How are they deleted?
For example, we might extend our employee example as follows with actions
that add and remove employees from the system. Here we program in an objectoriented
style, but without using LabeledInstance, managing the IDs and the field
map explicitly in our code.
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