C++ defines some format flags for standard input and output, which can be manipulated with the flags, setf, and unsetf functions. For example,

   cout.setf(ios_base::left, ios_base::adjustfield);

turns on left justification for all output directed to cout.

These flags and fields are defined in the base class ios_base.

You can also manipulate flags indirectly, using the following manipulators. Most programmers are familiar with the endl manipulator, which might give you an idea of how manipulators are used. For example, to set the boolalpha flag, you might use the following command:

    cout << boolalpha;

This is equivalent to:

    cout.setf(ios_base::boolalpha);

Manipulators from <ios> may be used on both input and output streams.

Formatting controlled by a boolean has two manipulators with names matching the flag name.

• boolalpha / noboolalpha
• showbase / noshowbase
• showpoint / noshowpoint
• showpos / noshowpos
• uppercase / nouppercase
• unitbuf / nounitbuf

Formatting controlled by a field has one manipulator for each flag, which turns off the others:

• ios_base::adjustfield
  • internal
  • left
  • right
• ios_base::basefield
  • dec
  • hex
  • oct
• ios_base::floatfield
  • fixed
  • scientific

For example, the following two statements are equivalent:

   cout << left;
   cout.setf(ios_base::left, ios_base::adjustfield);

There is one input manipulator, ws, which extracts whitespace from the stream, effectively skipping it before the next operation. This is rarely used directly, as streams with the skipws flag set should skip whitespace before any extraction. (All the standard extractors do this, user-defined extractors should use istream::sentry, which will also do this.)

These manipulators may only be used with output streams.

These manipulators may be used with both input and output streams.

Unlike the other format flags and fields, the width field is automatically set whenever the « operator is used to output text or numbers. This can lead to surprising results. The loop

    for(int i=1;i<1000;i*=10){
       cout << setw(6) << i << " |" << i << endl;
    }

does not pad the second column as might be expected, but produces:

     1 |1
    10 |10
   100 |100

To pad both columns, the width field should be set before each number is printed,

    for(int i=1;i<1000;i*=10){
       cout << setw(6) << i << " |" << setw(6) << i << endl;
    }

producing the more pleasing alignment:

    1 |     1
   10 |    10
  100 |   100

Why does the standard require this extra setw(6)? If the standard did not specify that the width field automatically be reset, the width of the seperator ” |” would need to be set to 0 so that extra spaces would not be added around it. If the language were defined this way, we would have to write:

    for(int i=1;i<1000;i*=10){
       cout << setw(6) << i << setw(0) << " |" << setw(6) << i << endl;
    }

The width is reset regardless of whether the function cout.width(n) or the stream manipulator cout « setw(n) is used. It is the only field or flag which is reset by the operator «.

The I/O stream state flags tell you the current state of an I/O stream. The flags are:

The I/O stream mode flags allow you to access files in different ways. The flags are: