C++ Primer Plus

Setting some format values, such as the field width, can be awkward using the iostream tools. To make life easier, C++ supplies additional manipulators in the iomanip header file. They provide the same services already discussed, but in a notationally more convenient manner. The three most commonly used are setprecision() for setting the precision, setfill() for setting the fill character, and setw() for setting the field width. Unlike the manipulators discussed previously, these take arguments. The setprecision() manipulator takes an integer argument that specifies the precision, the setfill() manipulator takes a char argument that indicates the fill character, and the setw() manipulator takes an integer argument that specifies the field width. Because they are manipulators, they can be concatenated in a cout statement. This makes the setw() manipulator particularly convenient when you’re displaying several columns of values. Listing 17.10 illustrates this by changing the field width and fill character several times for one output line. It also uses some of the newer standard manipulators.

Note

Some C++ systems don’t automatically search the math library. As mentioned before, some Unix systems require that you use the following to access the math library:

$ CC iomanip.C -lm

Listing 17.10. iomanip.cpp

// iomanip.cpp -- using manipulators from iomanip

// some systems require explicitly linking the math library

#include

#include

#include

int main()

{

    using namespace std;

    // use new standard manipulators

    cout << fixed << right;

    // use iomanip manipulators

    cout << setw(6) << "N" << setw(14) << "square root"

         << setw(15) << "fourth root\n";

    double root;

    for (int n = 10; n <=100; n += 10)

    {

        root = sqrt(double(n));

        cout << setw(6) << setfill('.') << n << setfill(' ')

               << setw(12) << setprecision(3) << root

               << setw(14) << setprecision(4) << sqrt(root)

               << endl;

    }

    return 0;

}

Here is the output of the program in Listing 17.10:

     N   square root   fourth root

....10       3.162        1.7783

....20       4.472        2.1147

....30       5.477        2.3403

....40       6.325        2.5149

....50       7.071        2.6591

....60       7.746        2.7832

....70       8.367        2.8925

....80       8.944        2.9907

....90       9.487        3.0801

...100      10.000        3.1623

Now you can produce neatly aligned columns. Using the fixed manipulator causes trailing zeros to be displayed.

Input with cin

Now it’s time to turn to input and getting data into a program. The cin object represents the standard input as a stream of bytes. Normally, you generate that stream of characters at the keyboard. If you type the character sequence 2011, the cin object extracts those characters from the input stream. You may intend that input to be part of a string, to be an int value, to be a float value, or to be some other type. Thus, extraction also involves type conversion. The cin object, guided by the type of variable designated to receive the value, must use its methods to convert that character sequence into the intended type of value.

Typically, you use cin as follows:

cin >> value_holder;

Here value_holder identifies the memory location in which to store the input. It can be the name of a variable, a reference, a dereferenced pointer, or a member of a structure or of a class. How cin interprets the input depends on the data type for value_holder. The istream class, defined in the iostream header file, overloads the >> extraction operator to recognize the following basic types:

• signed char &

• unsigned char &

• char &

• short &

• unsigned short &

• int &

• unsigned int &

• long &

• unsigned long &

• long long & (C++11)

• unsigned long long & (C++11)

• float &

• double &

• long double &

These are referred to as formatted input functions because they convert the input data to the format indicated by the target.

A typical operator function has a prototype like the following:

istream & operator>>(int &);

Both the argument and the return value are references. With a reference argument (see Chapter 8, “Adventures in Functions”), a statement such as the following causes the operator>>() function to work with the variable staff_size itself rather than with a copy, as would be the case with a regular argument:

cin >> staff_size;

Because the argument type is a reference, cin is able to directly modify the value of a variable used as an argument. The preceding statement, for example, directly modifies the value of the staff_size variable. We’ll get to the significance of a reference return value in a moment. First, let’s examine the type conversion aspect of the extraction operator. For arguments of each type in the preceding list of types, the extraction operator converts the character input to the indicated type of value. For example, suppose staff_size is type int. In this case, the compiler matches

cin >> staff_size;

to the following prototype:

istream & operator>>(int &);