C++ Primer Plus

Now look at reasons why you might use another type. If a variable represents something that is never negative, such as the number of words in a document, you can use an unsigned type; that way the variable can represent higher values.

If you know that the variable might have to represent integer values too great for a 16-bit integer, you should use long. This is true even if int is 32 bits on your system. That way, if you transfer your program to a system with a 16-bit int, your program won’t embarrass you by suddenly failing to work properly (see Figure 3.2). And if a mere two billion is inadequate for your needs, you can move up to long long.

Figure 3.2. For portability, use long for big integers.

Using short can conserve memory if short is smaller than int. Most typically, this is important only if you have a large array of integers. (An array is a data structure that stores several values of the same type sequentially in memory.) If it is important to conserve space, you should use short instead of int, even if the two are the same size. Suppose, for example, that you move your program from a 16-bit int system to a 32-bit int system. That doubles the amount of memory needed to hold an int array, but it doesn’t affect the requirements for a short array. Remember, a bit saved is a bit earned.

If you need only a single byte, you can use char. We’ll examine that possibility soon.

Integer Literals

An integer literal, or constant, is one you write out explicitly, such as 212 or 1776. C++, like C, lets you write integers in three different number bases: base 10 (the public favorite), base 8 (the old Unix favorite), and base 16 (the hardware hacker’s favorite). Appendix A, “Number Bases,” describes these bases; here we’ll look at the C++ representations. C++ uses the first digit or two to identify the base of a number constant. If the first digit is in the range 1–9, the number is base 10 (decimal); thus 93 is base 10. If the first digit is 0 and the second digit is in the range 1–7, the number is base 8 (octal); thus 042 is octal and equal to 34 decimal. If the first two characters are 0x or 0X, the number is base 16 (hexadecimal); thus 0x42 is hex and equal to 66 decimal. For hexadecimal values, the characters a–f and A–F represent the hexadecimal digits corresponding to the values 10–15. 0xF is 15 and 0xA5 is 165 (10 sixteens plus 5 ones). Listing 3.3 is tailor-made to show the three bases.

Listing 3.3. hexoct1.cpp

// hexoct1.cpp -- shows hex and octal literals

#include

int main()

{

    using namespace std;

    int chest = 42;     // decimal integer literal

    int waist = 0x42;   // hexadecimal integer literal

    int inseam = 042;   // octal integer literal

    cout << "Monsieur cuts a striking figure!\n";

    cout << "chest = " << chest << " (42 in decimal)\n";

    cout << "waist = " << waist << " (0x42 in hex)\n";

    cout << "inseam = " << inseam << " (042 in octal)\n";

    return 0;

}

By default, cout displays integers in decimal form, regardless of how they are written in a program, as the following output shows:

Monsieur cuts a striking figure!

chest = 42 (42 in decimal)

waist = 66 (0x42 in hex)

inseam = 34 (042 in octal)

Keep in mind that these notations are merely notational conveniences. For example, if you belong to a vintage PC club and read that the CGA video memory segment is B000 in hexadecimal, you don’t have to convert the value to base 10 45,056 before using it in your program. Instead, you can simply use 0xB000. But whether you write the value ten as 10, 012, or 0xA, it’s stored the same way in the computer—as a binary (base 2) value.

By the way, if you want to display a value in hexadecimal or octal form, you can use some special features of cout. Recall that the iostream header file provides the endl manipulator to give cout the message to start a new line. Similarly, it provides the dec, hex, and oct manipulators to give cout the messages to display integers in decimal, hexadecimal, and octal formats, respectively. Listing 3.4 uses hex and oct to display the decimal value 42 in three formats. (Decimal is the default format, and each format stays in effect until you change it.)

Listing 3.4. hexoct2.cpp

// hexoct2.cpp -- display values in hex and octal

#include

using namespace std;

int main()

{

    using namespace std;

    int chest = 42;

    int waist = 42;

    int inseam = 42;

    cout << "Monsieur cuts a striking figure!"  << endl;

    cout << "chest = " << chest << " (decimal for 42)" << endl;

    cout << hex;      // manipulator for changing number base

    cout << "waist = " << waist << " (hexadecimal for 42)" << endl;

    cout << oct;      // manipulator for changing number base

    cout << "inseam = " << inseam << " (octal for 42)" << endl;

    return 0;

}

Here’s the program output for Listing 3.4:

Monsieur cuts a striking figure!

chest = 42 (decimal for 42)

waist = 2a (hexadecimal for 42)

inseam = 52 (octal for 42)

Note that code like the following doesn’t display anything onscreen: