28.3 — Output with ostream and ios

In this section, we will look at various aspects of the iostream output class (ostream).

The insertion operator

The insertion operator (<<) is used to put information into an output stream. C++ has predefined insertion operations for all of the built-in data types, and you’ve already seen how you can overload the insertion operator for your own classes.

In the lesson on streams, you saw that both istream and ostream were derived from a class called ios. One of the jobs of ios (and ios_base) is to control the formatting options for output.

Formatting

There are two ways to change the formatting options: flags, and manipulators. You can think of flags as boolean variables that can be turned on and off. Manipulators are objects placed in a stream that affect the way things are input and output.

To switch a flag on, use the setf() function, with the appropriate flag as a parameter. For example, by default, C++ does not print a + sign in front of positive numbers. However, by using the std::ios::showpos flag, we can change this behavior:

std::cout.setf(std::ios::showpos); // turn on the std::ios::showpos flag
std::cout << 27 << '\n';

This results in the following output:

+27

It is possible to turn on multiple ios flags at once using the Bitwise OR (|) operator:

std::cout.setf(std::ios::showpos | std::ios::uppercase); // turn on the std::ios::showpos and std::ios::uppercase flag
std::cout << 1234567.89f << '\n';

This outputs:

+1.23457E+06

To turn a flag off, use the unsetf() function:

std::cout.setf(std::ios::showpos); // turn on the std::ios::showpos flag
std::cout << 27 << '\n';
std::cout.unsetf(std::ios::showpos); // turn off the std::ios::showpos flag
std::cout << 28 << '\n';

This results in the following output:

+27
28

There’s one other bit of trickiness when using setf() that needs to be mentioned. Many flags belong to groups, called format groups. A format group is a group of flags that perform similar (sometimes mutually exclusive) formatting options. For example, a format group named “basefield” contains the flags “oct”, “dec”, and “hex”, which controls the base of integral values. By default, the “dec” flag is set. Consequently, if we do this:

std::cout.setf(std::ios::hex); // try to turn on hex output
std::cout << 27 << '\n';

We get the following output:

27

It didn’t work! The reason why is because setf() only turns flags on -- it isn’t smart enough to turn mutually exclusive flags off. Consequently, when we turned std::hex on, std::ios::dec was still on, and std::ios::dec apparently takes precedence. There are two ways to get around this problem.

First, we can turn off std::ios::dec so that only std::hex is set:

std::cout.unsetf(std::ios::dec); // turn off decimal output
std::cout.setf(std::ios::hex); // turn on hexadecimal output
std::cout << 27 << '\n';

Now we get output as expected:

1b

The second way is to use a different form of setf() that takes two parameters: the first parameter is the flag to set, and the second is the formatting group it belongs to. When using this form of setf(), all of the flags belonging to the group are turned off, and only the flag passed in is turned on. For example:

// Turn on std::ios::hex as the only std::ios::basefield flag
std::cout.setf(std::ios::hex, std::ios::basefield);
std::cout << 27 << '\n';

This also produces the expected output:

1b

Using setf() and unsetf() tends to be awkward, so C++ provides a second way to change the formatting options: manipulators. The nice thing about manipulators is that they are smart enough to turn on and off the appropriate flags. Here is an example of using some manipulators to change the base:

std::cout << std::hex << 27 << '\n'; // print 27 in hex
std::cout << 28 << '\n'; // we're still in hex
std::cout << std::dec << 29 << '\n'; // back to decimal

This program produces the output:

1b
1c
29

In general, using manipulators is much easier than setting and unsetting flags. Many options are available via both flags and manipulators (such as changing the base), however, other options are only available via flags or via manipulators, so it’s important to know how to use both.

Useful formatters

Here is a list of some of the more useful flags, manipulators, and member functions. Flags live in the std::ios class, manipulators live in the std namespace, and the member functions live in the std::ostream class.

Example:

std::cout << true << ' ' << false << '\n';

std::cout.setf(std::ios::boolalpha);
std::cout << true << ' ' << false << '\n';

std::cout << std::noboolalpha << true << ' ' << false << '\n';

std::cout << std::boolalpha << true << ' ' << false << '\n';

Result:

1 0
true false
1 0
true false

Example:

std::cout << 5 << '\n';

std::cout.setf(std::ios::showpos);
std::cout << 5 << '\n';

std::cout << std::noshowpos << 5 << '\n';

std::cout << std::showpos << 5 << '\n';

Result:

5
+5
5
+5

Example:

std::cout << 12345678.9 << '\n';

std::cout.setf(std::ios::uppercase);
std::cout << 12345678.9 << '\n';

std::cout << std::nouppercase << 12345678.9 << '\n';

std::cout << std::uppercase << 12345678.9 << '\n';

Result:

1.23457e+007
1.23457E+007
1.23457e+007
1.23457E+007

Example:

std::cout << 27 << '\n';

std::cout.setf(std::ios::dec, std::ios::basefield);
std::cout << 27 << '\n';

std::cout.setf(std::ios::oct, std::ios::basefield);
std::cout << 27 << '\n';

std::cout.setf(std::ios::hex, std::ios::basefield);
std::cout << 27 << '\n';

std::cout << std::dec << 27 << '\n';
std::cout << std::oct << 27 << '\n';
std::cout << std::hex << 27 << '\n';

Result:

27
27
33
1b
27
33
1b

By now, you should be able to see the relationship between setting formatting via flag and via manipulators. In future examples, we will use manipulators unless they are not available.

Precision, notation, and decimal points

Using manipulators (or flags), it is possible to change the precision and format with which floating point numbers are displayed. There are several formatting options that combine in somewhat complex ways, so we will take a closer look at this.

If fixed or scientific notation is used, precision determines how many decimal places in the fraction is displayed. Note that if the precision is less than the number of significant digits, the number will be rounded.

std::cout << std::fixed << '\n';
std::cout << std::setprecision(3) << 123.456 << '\n';
std::cout << std::setprecision(4) << 123.456 << '\n';
std::cout << std::setprecision(5) << 123.456 << '\n';
std::cout << std::setprecision(6) << 123.456 << '\n';
std::cout << std::setprecision(7) << 123.456 << '\n';

std::cout << std::scientific << '\n';
std::cout << std::setprecision(3) << 123.456 << '\n';
std::cout << std::setprecision(4) << 123.456 << '\n';
std::cout << std::setprecision(5) << 123.456 << '\n';
std::cout << std::setprecision(6) << 123.456 << '\n';
std::cout << std::setprecision(7) << 123.456 << '\n';

Produces the result:

123.456
123.4560
123.45600
123.456000
123.4560000

1.235e+002
1.2346e+002
1.23456e+002
1.234560e+002
1.2345600e+002

If neither fixed nor scientific are being used, precision determines how many significant digits should be displayed. Again, if the precision is less than the number of significant digits, the number will be rounded.

std::cout << std::setprecision(3) << 123.456 << '\n';
std::cout << std::setprecision(4) << 123.456 << '\n';
std::cout << std::setprecision(5) << 123.456 << '\n';
std::cout << std::setprecision(6) << 123.456 << '\n';
std::cout << std::setprecision(7) << 123.456 << '\n';

Produces the following result:

123
123.5
123.46
123.456
123.456

Using the showpoint manipulator or flag, you can make the stream write a decimal point and trailing zeros.

std::cout << std::showpoint << '\n';
std::cout << std::setprecision(3) << 123.456 << '\n';
std::cout << std::setprecision(4) << 123.456 << '\n';
std::cout << std::setprecision(5) << 123.456 << '\n';
std::cout << std::setprecision(6) << 123.456 << '\n';
std::cout << std::setprecision(7) << 123.456 << '\n';

Produces the following result:

123.
123.5
123.46
123.456
123.4560

Here’s a summary table with some more examples:

Width, fill characters, and justification

Typically when you print numbers, the numbers are printed without any regard to the space around them. However, it is possible to left or right justify the printing of numbers. In order to do this, we have to first define a field width, which defines the number of output spaces a value will have. If the actual number printed is smaller than the field width, it will be left or right justified (as specified). If the actual number is larger than the field width, it will not be truncated -- it will overflow the field.

In order to use any of these formatters, we first have to set a field width. This can be done via the width(int) member function, or the setw() manipulator. Note that right justification is the default.

std::cout << -12345 << '\n'; // print default value with no field width
std::cout << std::setw(10) << -12345 << '\n'; // print default with field width
std::cout << std::setw(10) << std::left << -12345 << '\n'; // print left justified
std::cout << std::setw(10) << std::right << -12345 << '\n'; // print right justified
std::cout << std::setw(10) << std::internal << -12345 << '\n'; // print internally justified

This produces the result:

-12345
    -12345
-12345
    -12345
-    12345

One thing to note is that setw() and width() only affect the next output statement. They are not persistent like some other flags/manipulators.

Now, let’s set a fill character and do the same example:

std::cout.fill('*');
std::cout << -12345 << '\n'; // print default value with no field width
std::cout << std::setw(10) << -12345 << '\n'; // print default with field width
std::cout << std::setw(10) << std::left << -12345 << '\n'; // print left justified
std::cout << std::setw(10) << std::right << -12345 << '\n'; // print right justified
std::cout << std::setw(10) << std::internal << -12345 << '\n'; // print internally justified

This produces the output:

-12345
****-12345
-12345****
****-12345
-****12345

Note that all the blank spaces in the field have been filled up with the fill character.

The ostream class and iostream library contain other output functions, flags, and manipulators that may be useful, depending on what you need to do. As with the istream class, those topics are really more suited for a tutorial or book focusing on the standard library (such as the excellent book “The C++ Standard Template Library” by Nicolai M. Josuttis).