C++ Primer Plus

In C++, a class is a specification describing such a new data form, and an object is a particular data structure constructed according to that plan. For example, a class could describe the general properties of a corporation executive (name, title, salary, unusual abilities, for example), while an object would represent a specific executive (Guilford Sheepblat, vice president, $925,000, knows how to restore the Windows registry). In general, a class defines what data is used to represent an object and the operations that can be performed on that data. For example, suppose you were developing a computer drawing program capable of drawing a rectangle. You could define a class to describe a rectangle. The data part of the specification could include such things as the location of the corners, the height and width, the color and style of the boundary line, and the color and pattern used to fill the rectangle. The operations part of the specification could include methods for moving the rectangle, resizing it, rotating it, changing colors and patterns, and copying the rectangle to another location. If you then used your program to draw a rectangle, it would create an object according to the class specification. That object would hold all the data values describing the rectangle, and you could use the class methods to modify that rectangle. If you drew two rectangles, the program would create two objects, one for each rectangle.

The OOP approach to program design is to first design classes that accurately represent those things with which the program deals. For example, a drawing program might define classes to represent rectangles, lines, circles, brushes, pens, and the like. The class definitions, recall, include a description of permissible operations for each class, such as moving a circle or rotating a line. Then you would proceed to design a program, using objects of those classes. The process of going from a lower level of organization, such as classes, to a higher level, such as program design, is called bottom-up programming.

There’s more to OOP than the binding of data and methods into a class definition. For example, OOP facilitates creating reusable code, and that can eventually save a lot of work. Information hiding safeguards data from improper access. Polymorphism lets you create multiple definitions for operators and functions, with the programming context determining which definition is used. Inheritance lets you derive new classes from old ones. As you can see, OOP introduces many new ideas and involves a different approach to programming than does procedural programming. Instead of concentrating on tasks, you concentrate on representing concepts. Instead of taking a top-down programming approach, you sometimes take a bottom-up approach. This book will guide you through all these points, with plenty of easily grasped examples.

Designing a useful, reliable class can be a difficult task. Fortunately, OOP languages make it simple to incorporate existing classes into your own programming. Vendors provide a variety of useful class libraries, including libraries of classes designed to simplify creating programs for environments such as Windows or the Macintosh. One of the real benefits of C++ is that it lets you easily reuse and adapt existing, well-tested code.

C++ and Generic Programming

Generic programming is yet another programming paradigm supported by C++. It shares with OOP the aim of making it simpler to reuse code and the technique of abstracting general concepts. But whereas OOP emphasizes the data aspect of programming, generic programming emphasizes independence from a particular data type. And its focus is different. OOP is a tool for managing large projects, whereas generic programming provides tools for performing common tasks, such as sorting data or merging lists. The term generic refers to code that is type independent. C++ data representations come in many types—integers, numbers with fractional parts, characters, strings of characters, and user-defined compound structures of several types. If, for example, you wanted to sort data of these various types, you would normally have to create a separate sorting function for each type. Generic programming involves extending the language so that you can write a function for a generic (that is, an unspecified) type once and use it for a variety of actual types. C++ templates provide a mechanism for doing that.

The Genesis of C++