The bool Type

The ANSI/ISO C++ Standard has added a new type (new to C++, that is), called bool. It’s named in honor of the English mathematician George Boole, who developed a mathematical representation of the laws of logic. In computing, a Boolean variable is one whose value can be either true or false. In the past, C++, like C, has not had a Boolean type. Instead, as you’ll see in greater detail in Chapters 5 and 6, C++ interprets nonzero values as true and zero values as false. Now, however, you can use the bool type to represent true and false, and the predefined literals true and false represent those values. That is, you can make statements like the following:

bool is_ready = true;

The literals true and false can be converted to type int by promotion, with true converting to 1 and false to 0:

int ans = true;           // ans assigned 1
int promise = false;      // promise assigned 0

Also any numeric or pointer value can be converted implicitly (that is, without an explicit type cast) to a bool value. Any nonzero value converts to true, whereas a zero value converts to false:

bool start = -100;       // start assigned true
bool stop = 0;           // stop assigned false

After the book introduces if statements (in Chapter 6, “Branching Statements and Logical Operators”), the bool type will become a common feature in the examples.

The const Qualifier

Now let’s return to the topic of symbolic names for constants. A symbolic name can suggest what the constant represents. Also if the program uses the constant in several places and you need to change the value, you can just change the single symbol definition. The note about #define statements earlier in this chapter (see the sidebar “Symbolic Constants the Preprocessor Way”) promises that C++ has a better way to handle symbolic constants. That way is to use the const keyword to modify a variable declaration and initialization. Suppose, for example, that you want a symbolic constant for the number of months in a year. Just enter this line in a program:

const int Months = 12;  // Months is symbolic constant for 12

Now you can use Months in a program instead of 12. (A bare 12 in a program might represent the number of inches in a foot or the number of donuts in a dozen, but the name Months tells you what the value 12 represents.) After you initialize a constant such as Months, its value is set. The compiler does not let you subsequently change the value Months. If you try to, for example, g++ gives an error message that the program used an assignment of a read-only variable. The keyword const is termed a qualifier because it qualifies the meaning of a declaration.

A common practice is to capitalize the first character in a name to help remind yourself that Months is a constant. This is by no means a universal convention, but it helps separate the constants from the variables when you read a program. Another convention is to make all the characters uppercase; this is the usual convention for constants created using #define. Yet another convention is to begin constant names with the letter k, as in kmonths. And there are yet other conventions. Many organizations have particular coding conventions they expect their programmers to follow.

The general form for creating a constant is this:

const type name = value;

Note that you initialize a const in the declaration. The following sequence is no good:

const int toes;    // value of toes undefined at this point
toes = 10;         // too late!

If you don’t provide a value when you declare the constant, it ends up with an unspecified value that you cannot modify.

If your background is in C, you might feel that the #define statement, which is discussed earlier, already does the job adequately. But const is better. For one thing, it lets you specify the type explicitly. Second, you can use C++’s scoping rules to limit the definition to particular functions or files. (Scoping rules describe how widely known a name is to different modules; you’ll learn about this in more detail in Chapter 9, “Memory Models and Namespaces.”) Third, you can use const with more elaborate types, such as arrays and structures, as discussed in Chapter 4.

ANSI C also uses the const qualifier, which it borrows from C++. If you’re familiar with the ANSI C version, you should be aware that the C++ version is slightly different. One difference relates to the scope rules, and Chapter 9 covers that point. The other main difference is that in C++ (but not in C), you can use a const value to declare the size of an array. You’ll see examples in Chapter 4.

Floating-Point Numbers

Now that you have seen the complete line of C++ integer types, let’s look at the floating-point types, which compose the second major group of fundamental C++ types. These numbers let you represent numbers with fractional parts, such as the gas mileage of an M1 tank (0.56 MPG). They also provide a much greater range in values. If a number is too large to be represented as type long—for example, the number of bacterial cells in a human body (estimated to be greater than 100,000,000,000,000)—you can use one of the floating-point types.

With floating-point types, you can represent numbers such as 2.5 and 3.14159 and 122442.32—that is, numbers with fractional parts. A computer stores such values in two parts. One part represents a value, and the other part scales that value up or down. Here’s an analogy. Consider the two numbers 34.1245 and 34124.5. They’re identical except for scale. You can represent the first one as 0.341245 (the base value) and 100 (the scaling factor). You can represent the second as 0.341245 (the same base value) and 100,000 (a bigger scaling factor). The scaling factor serves to move the decimal point, hence the term floating-point. C++ uses a similar method to represent floating-point numbers internally, except it’s based on binary numbers, so the scaling is by factors of 2 instead of by factors of 10. Fortunately, you don’t have to know much about the internal representation. The main points are that floating-point numbers let you represent fractional, very large, and very small values, and they have internal representations much different from those of integers.