Chapter 8: User-Defined Data Types

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Overview

Throughout this book you have been using a variety of different variables of different types (int, float, bool, etc.). However, all these data types have something in common; they each store a single value. That statement may seem so obvious as to not need stating. However, having a variable store only a single value is not the only option you have when programming with C++. Another option is to create user-defined data types. User-defined data types are data types that you create. They allow you to store several, hopefully related, values under a single variable name. You literally create your own new data type. The difference between your data types and the ones that are part of the C++ language is that yours are complex and hold more than one value.

A structure is one of the most common sorts of user-defined data type. That means that you, the programmer, essentially create your own new data type to suit your needs. Structures are just one category of user-defined data types, but they are perhaps the most commonly used. Other user-defined types include typedefs, unions, and enumerations. Each of these user-defined data types will be explored and demonstrated in this chapter. The different categories of user-defined types are summarized in Table 8.1. Each will be explained in detail later in the chapter.

Table 8.1: User-Defined Types

User-Defined Type	Purpose	Example
Structure	A compound data type that simply combines several related pieces of data.	Struct employee { char lastname[30]; float salary; };
Typedef	Provides a different name, or an alias for an existing data type.	Typedef int RETURN; Typedef double ANSWER;
Union	A combination of different data types.	Union NumericType { int iValue; long lValue; double dValue; }
Enumeration	An enumeration is a user- defined type that defines named constants.	enum test{value1,value2,value3};
Bit Field	A type of structure that allows you access to individual bits.	Struct status { unsigned :cleartosend; unsigned :ringing; }

Table 8.1 shows you the basics of the various user-defined data types. The following sections of this chapter will explore each of these in detail, with examples. The structure is perhaps the most commonly encountered user-defined data type, and it is essential to master it. However, the other user-defined types are also important and should not be ignored.