C++ Primer Plus

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.

Tip

If you are coming to C++ from C and you are about to use #define to define a symbolic constant, use const instead.

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)—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.

Writing Floating-Point Numbers

C++ has two ways of writing floating-point numbers. The first is to use the standard decimal-point notation you’ve been using much of your life:

12.34             // floating-point

939001.32         // floating-point

0.00023           // floating-point

8.0               // still floating-point

Even if the fractional part is 0, as in 8.0, the decimal point ensures that the number is represented in floating-point format and not as an integer. (The C++ Standard does allow for implementations to represent different locales—for example, providing a mechanism for using the European method of using a comma instead of a period for the decimal point. However, these choices govern how the numbers can appear in input and output, not in code.)

The second method for representing floating-point values is called E notation, and it looks like this: 3.45E6. This means that the value 3.45 is multiplied by 1,000,000; the E6 means 10 to the 6th power, which is 1 followed by 6 zeros. Thus 3.45E6 means 3,450,000. The 6 is called an exponent, and the 3.45 is termed the mantissa. Here are more examples:

2.52e+8             // can use E or e, + is optional

8.33E-4             // exponent can be negative