Object-Oriented Analysis and Design in C++

The Development Cycle

Many volumes have been written about the development cycle. Some propose a "waterfall" method, in which designers determine what the program should do; architects determine how the program will be built, what classes will be used, and so forth; and then programmers implement the design and architecture. By the time the design and architecture is given to the programmer, it is complete; all the programmer needs to do is implement the required functionality.

Even if the waterfall method worked, it would probably be a poor method for writing good programs. As the programmer proceeds, there is a necessary and natural feedback between what has been written so far and what remains to be done. While it is true that good C++ programs are designed in great detail before a line of code is written, it is not true that that design remains unchanged throughout the cycle.

The amount of design that must be finished "up front," before programming begins, is a function of the size of the program. A highly complex effort, involving dozens of programmers working for many months, will require a more fully articulated architecture than a quick-and-dirty utility written in one day by a single programmer.

This article will focus on the design of large, complex programs which will be expanded and enhanced over many years. Many programmers enjoy working at the bleeding edge of technology; they tend to write programs whose complexity pushes at the limits of their tools and understanding. In many ways, C++ was designed to extend the complexity that a programmer or team of programmers could manage.
This chapter will examine a number of design problems from an object-oriented perspective. The goal will be to review the analysis process, and then to understand how you apply the syntax of C++ to implement these design objectives.

Simulating an Alarm System

A simulation is a computer model of a part of a real-world system. There are many reasons to build a simulation, but a good design must start with an understanding of what questions you hope the simulation will answer.

As a starting point, examine this problem: You have been asked to simulate the alarm system for a house. The house is a center hall colonial with four bedrooms, a finished basement, and an under-the-house garage.

The downstairs has the following windows: three in the kitchen, four in the dining room, one in the half-bathroom, two each in the living room and the family room, and two small windows next to the door. All four bedrooms are upstairs, each of which has two windows except for the master bedroom, which has four. There are two baths, each with one window. Finally, there are four half-windows in the basement, and one window in the garage.

Normal access to the house is through the front door. Additionally, the kitchen has a sliding glass door, and the garage has two doors for the cars and one door for easy access to the basement. There is also a cellar door in the backyard.

All the windows and doors are alarmed, and there are panic buttons on each phone and next to the bed. The grounds are alarmed as well, though these are carefully calibrated so that they are not set off by small animals or birds.

There is a central alarm system in the basement, which sounds a warning chirp when the alarm has been tripped. If the alarm is not disabled within a setable amount of time, the police are called. If a panic button is pushed, the police are called immediately.

The alarm is also wired into the fire and smoke detectors and the sprinkler system, and the alarm system itself is fault tolerant, has its own internal backup power supply, and is encased in a fireproof box.

Preliminary Design

You begin by asking, "What questions might this simulation answer?" For example, you might be able to use the simulation to answer the questions, "How long might a sensor be broken before anyone notices?" or "Is there a way to defeat the window alarms without the police being notified?"

Once you understand the purpose of the simulation you will know what parts of the real system the program must model. Once that is well understood, it becomes much easier to design the program itself. Read more about Objects and Classes
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