C++ Primer Plus

Listing 17.2 illustrates the output defaults. It displays a colon (:) after each value so you can see the width of the field used in each case. The program uses the expression 1.0 / 9.0 to generate a nonterminating fraction so you can see how many places get printed.

Note

Not all compilers generate output formatted in accordance with the current C++ Standard. Also the current standard does allow for regional variations. For example, a European implementation can follow the continental fashion of using a comma instead of a period for displaying decimal fractions. That is, it may write 2,54 instead of 2.54. The locale library (header file locale) provides a mechanism for imbuing an input or output stream with a particular style, so a single compiler can offer more than one locale choice. This chapter uses the U.S. locale.

Listing 17.2. defaults.cpp

// defaults.cpp -- cout default formats

#include

int main()

{

    using std::cout;

    cout << "12345678901234567890\n";

    char ch = 'K';

    int t = 273;

    cout << ch << ":\n";

    cout << t << ":\n";

    cout << -t <<":\n";

    double f1 = 1.200;

    cout << f1 << ":\n";

    cout << (f1 + 1.0 / 9.0) << ":\n";

    double f2 = 1.67E2;

    cout << f2 << ":\n";

    f2 += 1.0 / 9.0;

    cout << f2 << ":\n";

    cout << (f2 * 1.0e4) << ":\n";

    double f3 = 2.3e-4;

    cout << f3 << ":\n";

    cout << f3 / 10 << ":\n";

    return 0;

}

Here is the output of the program in Listing 17.2:

12345678901234567890

K:

273:

-273:

1.2:

1.31111:

167:

167.111:

1.67111e+006:

0.00023:

2.3e-005:

Each value fills its field. Note that the trailing zeros of 1.200 are not displayed but that floating-point values without terminating zeros have six places displayed. Also this particular implementation displays three digits in the exponent; others might use two.

Changing the Number Base Used for Display

The ostream class inherits from the ios class, which inherits from the ios_base class. The ios_base class stores information that describes the format state. For example, certain bits in one class member determine the number base used, whereas another member determines the field width. By using manipulators, you can control the number base used to display integers. By using ios_base member functions, you can control the field width and the number of places displayed to the right of the decimal. Because the ios_base class is an indirect base class for ostream, you can use its methods with ostream objects (or descendants), such as cout.

Note

The members and methods found in the ios_base class were formerly found in the ios class. Now ios_base is a base class to ios. In the new system, ios is a template class with char and wchar_t specializations, and ios_base contains the nontemplate features.

Let’s look at how to set the number base to be used in displaying integers. To control whether integers are displayed in base 10, base 16, or base 8, you can use the dec, hex, and oct manipulators. For example, the following function call sets the number base format state for the cout object to hexadecimal:

hex(cout);

After you do this, a program will print integer values in hexadecimal form until you set the format state to another choice. Note that the manipulators are not member functions, hence they don’t have to be invoked by an object.

Although the manipulators really are functions, you normally see them used this way:

cout << hex;

The ostream class overloads the << operator to make this usage equivalent to the function call hex(cout). The manipulators are in the std namespace. Listing 17.3 illustrates using these manipulators. It shows the value of an integer and its square in three different number bases. Note that you can use a manipulator separately or as part of a series of insertions.

Listing 17.3. manip.cpp

// manip.cpp -- using format manipulators

#include

int main()

{

    using namespace std;

    cout << "Enter an integer: ";

    int n;

    cin >> n;

    cout << "n     n*n\n";

    cout << n << "     " << n * n << " (decimal)\n";

// set to hex mode

    cout << hex;

    cout << n << "     ";

    cout << n * n << " (hexadecimal)\n";

// set to octal mode

    cout << oct << n << "     " << n * n << " (octal)\n";

// alternative way to call a manipulator

    dec(cout);

    cout << n << "     " << n * n << " (decimal)\n";

    return 0;

}

Here is some sample output from the program in Listing 17.3:

Enter an integer: 13

n     n*n

13     169 (decimal)

d     a9 (hexadecimal)

15     251 (octal)

13     169 (decimal)

Adjusting Field Widths

You probably noticed that the columns in output from Listing 17.3 don’t line up; that’s because the numbers have different field widths. You can use the width member function to place differently sized numbers in fields that have equal widths. The method has these prototypes:

int width();

int width(int i);