C++ for Scientists - Technische Universität Dresden

206 CHAPTER 7. EFFECTIVE PROGRAMMING: THE POLYMORPHIC WAY
7.3 Programming with Objects
Programming with objects may be viewed as an evolution from the structured imperative
paradigm. It, on the one hand, tries to address the issue of code reuseability by providing a
specific type of polymorphism, sub-typing. On the other hand it addresses the issue of unchecked
modification of state by enforcing data encapsulation, thus enforcing changes through defined
interfaces. Both of these notions are attached to an entity called an object. Therefore an object
serves as a self contained unit which interacts with the environment via messages. It thus
accomplishes a decoupling of the internal implementation within the object and the interaction
with the sourrounding environment. Thus enforcing (clean) interfaces, which is essential for
effective programming. The algorithms are expressed much more by the notion of what is to be
done as an interaction and modification of objects, where the details of how, are encapsulated
to a great extent within the objects themselves.
Another benefit for programming with objects is, that these entities can be placed in libraries.
This saves the effort of continually rewriting the same code for every new program. Furthermore,
because objects can be made polymorphic, object libraries offer the programmer more flexibility
and functionality than subroutine libraries (their counterparts in the procedural paradigm).
Technically, object libraries are quite feasible, and the advantages of extensibility can be significant.
However, the real challenge to making code reusable is not technical. Rather, it is
identifying functionality that other people both understand and want. People who use procedural
languages have been writing and using subroutine libraries for decades. These libraries are
most successful when they perform simple, clearly defined functions, such as calculating square
roots or computing trigonometric functions. An object library can provide complex functions
more easily than a subroutine library. However, unless those functions are clearly defined, well
understood and generally useful, the library is unlikely to be used widely.
To give an intuitive specification of the programming approach with objects, the following list
specifies different points in the object world:
• Identity is the quantization of data in discrete, distinguishable entities called objects
• Classification is the grouping of objects with the same structure and behavior into classes
• Polymorphism is the differentiation of behavior of the same operation on different classes
• Inheritance is the sharing of structure and behavior among classes in a hierarchical relationship
But one of the biggest problems of this programming approach is the interaction of objects with
algorithms. The problem can easily be seen using the example of a simple sorting algorithm.
Should the algorithm be placed into the object? Should an algorithm work on a class hierarchy
with a common interface?
The problem cannot be solved easily within this paradigm. A possible solution is some kind of
polymorphism, which is explained in Section 7.5.2.
7.3.1 Object-Based Programming
In languages which support identity and classification the object-based paradigm can be used
efficiently.