11 DEFERRED CLASSES
The inheritance mechanism includes one more major component: deferred
routines and classes.
Declaring a routine r as deferred in a class C expresses
that there is no default implementation of r in C; such implementations
will appear in eventual descendants of C. A class which has one
or more deferred routines is itself said to be deferred. A non-deferred
routine or class is said to be effective.
For example, a system used by a Department of Motor Vehicles to register
vehicles could include a class of the form
- deferred class VEHICLE feature
- dues_paid (year: INTEGER): BOOLEAN is
- do... end
- valid_plate (year: INTEGER): BOOLEAN is
- do... end
- register (year: INTEGER) is
- -- Register vehicle for year.
- require
- dues_paid (year)
- deferred
- ensure
- valid_plate (year)
- end
- ... Other features, deferred or effective...
end -- class VEHICLE
This example assumes that no single registration algorithm applies to
all kinds of vehicle; passenger cars, motorcycles, trucks etc. are all
registered differently. But the same precondition and postcondition apply
in all cases. The solution is to treat register as a deferred routine,
making VEHICLE a deferred class. Descendants of class VEHICLE,
such as CAR or TRUCK, effect this routine, that is
to say, give effective versions. An effecting is similar to a redefinition;
only here there is no effective definition in the original class, just
a specification in the form of a deferred routine. The term redeclaration
covers both redefinition and effecting.
Vehicle inheritance hierarchy
Deferred classes describe a group of implementations of an abstract
data type rather than just a single implementation. A deferred class may
not be instantiated: create v is invalid if v is an entity declared
of type VEHICLE. But such an entity may be assigned a reference
to an instance of a non-deferred descendant of VEHICLE. For example,
assuming CAR and TRUCK provide effective definitions for
all deferred routines of VEHICLE, the following will be valid:
- v: VEHICLE; c: CAR; t: TRUCK;
- ...
- create c ...; create t ...;...
- if "Some test" then
v := c
else
v := t
end
- v.register (1996)
This example fully exploits polymorphism: depending on the outcome of
"Some test", v will be treated as a car or a truck, and
the appropriate registration algorithm will be applied. Also, "Some
test" may depend on some event whose outcome is impossible to predict
until run-time, for example the user clicking with the mouse to select
one among several vehicle icons displayed on the screen.
Deferred classes are particularly useful at the design stage.
The first version of a module may be a deferred class, which will later
be refined into one or more effective (non-deferred) classes. Particularly
important for this application is the possibility of associating a precondition
and a postcondition with a routine even though it is a deferred routine
(as with register above), and an invariant with a class even though
it is a deferred class. This enables the designer to attach precise semantics
to a module at the design stage, long before making any implementation
choices.
These possibilities make Eiffel an attractive alternative to PDLs (Program
Design Languages). The combination of deferred classes to capture partially
understood concepts, assertions to express what is known about their semantics,
and the language's other structuring facilities (information hiding, inheritance,
genericity) to obtain clear, convincing architectures, yields a higher-level
design method. A further benefit, of course, is that the notation is also
a programming language, making the development cycle smoother by reducing
the gap between design and implementation.
The role of Eiffel in this cycle is not limited to design and implementation
but extends to the earliest stage of development, analysis. Deferred
classes written at that stage describe not software objects, but objects
from the external reality's model -- documents, airplanes, investments.
Here again the presence of assertions to express constraints, and the language's
other structuring facilities, provide an attractive alternative to older
methods and notations.
  
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