C++ Primer Plus

Let’s see, what am I keeping track of? A ball player, of course. So I want an object that represents the whole player, not just her batting average or times at bat. Yeah, that’ll be my fundamental data unit, an object representing the name and statistics for a player. I’ll need some methods to handle this object. Hmmm, I guess I need a method to get basic information into this unit. The computer should calculate some of the stuff, like the batting averages—I can add methods to do calculations. And the program should do those calculations automatically, without the user having to remember to ask to have them done. Also I’ll need methods for updating and displaying the information. So the user gets three ways to interact with the data: initialization, updating, and reporting. That’s the user interface.

In short, with an OOP approach, you concentrate on the object as the user perceives it, thinking about the data you need to describe the object and the operations that will describe the user’s interaction with the data. After you develop a description of that interface, you move on to decide how to implement the interface and data storage. Finally, you put together a program to use your new design.

Abstraction and Classes

Life is full of complexities, and one way we cope with complexity is to frame simplifying abstractions. You are a collection of more than an octillion atoms. Some students of the mind would say that your mind is a collection of several semiautonomous agents. But it’s much simpler to think of yourself as a single entity. In computing, abstraction is the crucial step of representing information in terms of its interface with the user. That is, you abstract the essential operational features of a problem and express a solution in those terms. In the softball statistics example, the interface describes how the user initializes, updates, and displays the data. From abstraction, it is a short step to the user-defined type, which in C++ is a class design that implements the abstract interface.

What Is a Type?

Let’s think a little more about what constitutes a type. For example, what is a nerd? If you subscribe to the popular stereotype, you might think of a nerd in visual terms—thick, black-rimmed glasses, pocket protector full of pens, and so on. After a little reflection, you might conclude that a nerd is better defined operationally—for example, in terms of how he or she responds to an awkward social situation. You have a similar situation, if you don’t mind stretched analogies, with a procedural language such as C. At first, you tend to think of a data type in terms of its appearance—how it is stored in memory. A char, for example, is 1 byte of memory, and a double is often 8 bytes of memory. But a little reflection leads you to conclude that a data type is also defined in terms of the operations that can be performed on it. For example, the int type can use all the arithmetic operations. You can add, subtract, multiply, and divide integers. You can also use the modulus operator (%) with them.

On the other hand, consider pointers. A pointer might very well require the same amount of memory as an int. It might even be represented internally as an integer. But a pointer doesn’t allow the same operations that an integer does. You can’t, for example, multiply two pointers by each other. The concept makes no sense, so C++ doesn’t implement it. Thus, when you declare a variable as an int or as a pointer-to-float, you’re not just allocating memory—you are also establishing which operations can be performed with the variable. In short, specifying a basic type does three things:

• It determines how much memory is needed for a data object.

• It determines how the bits in memory are interpreted. (A long and a float might use the same number of bits in memory, but they are translated into numeric values differently.)

• It determines what operations, or methods, can be performed using the data object.

For built-in types, the information about operations is built in to the compiler. But when you define a user-defined type in C++, you have to provide the same kind of information yourself. In exchange for this extra work, you gain the power and flexibility to custom fit new data types to match real-world requirements.

Classes in C++

A class is a C++ vehicle for translating an abstraction to a user-defined type. It combines data representation and methods for manipulating that data into one neat package. Let’s look at a class that represents stocks.