C++ I/O Flags
Format flags
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::left);
turns on left justification for all output directed to cout.
| Flag |
Meaning |
| boolalpha |
Boolean values can be input/output using the words "true" and "false". |
| dec |
Numeric values are displayed in decimal. |
| fixed |
Display floating point values using normal notation (as opposed to scientific). |
| hex |
Numeric values are displayed in hexidecimal. |
| internal |
If a numeric value is padded to fill a field, spaces are inserted between the sign and base character. |
| left |
Output is left justified. |
| oct |
Numeric values are displayed in octal. |
| right |
Output is right justified. |
| scientific |
Display floating point values using scientific notation. |
| showbase |
Display the base of all numeric values. |
| showpoint |
Display a decimal and extra zeros, even when not needed. |
| showpos |
Display a leading plus sign before positive numeric values. |
| skipws |
Discard whitespace characters (spaces, tabs, newlines) when reading from a stream. |
| unitbuf |
Flush the buffer after each insertion. |
| uppercase |
Display the "e" of scientific notation and the "x" of hexidecimal notation as capital letters. |
Manipulators
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 dec flag, you might use the following command:
cout << dec;
| Manipulators defined in <iostream> |
| Manipulator |
Description |
Input |
Output |
| boolalpha |
Turns on the boolalpha flag |
X |
X |
| dec |
Turns on the dec flag |
X |
X |
| endl |
Output a newline character, flush the stream |
|
X |
| ends |
Output a null character |
|
X |
| fixed |
Turns on the fixed flag |
|
X |
| flush |
Flushes the stream |
|
X |
| hex |
Turns on the hex flag |
X |
X |
| internal |
Turns on the internal flag |
|
X |
| left |
Turns on the left flag |
|
X |
| noboolalpha |
Turns off the boolalpha flag |
X |
X |
| noshowbase |
Turns off the showbase flag |
|
X |
| noshowpoint |
Turns off the showpoint flag |
|
X |
| noshowpos |
Turns off the showpos flag |
|
X |
| noskipws |
Turns off the skipws flag |
X |
|
| nounitbuf |
Turns off the unitbuf flag |
|
X |
| nouppercase |
Turns off the uppercase flag |
|
X |
| oct |
Turns on the oct flag |
X |
X |
| right |
Turns on the right flag |
|
X |
| scientific |
Turns on the scientific flag |
|
X |
| showbase |
Turns on the showbase flag |
|
X |
| showpoint |
Turns on the showpoint flag |
|
X |
| showpos |
Turns on the showpos flag |
|
X |
| skipws |
Turns on the skipws flag |
X |
|
| unitbuf |
Turns on the unitbuf flag |
|
X |
| uppercase |
Turns on the uppercase flag |
|
X |
| ws |
Skip any leading whitespace |
X |
|
| Manipulators defined in <iomanip> |
| Manipulator |
Description |
Input |
Output |
| resetiosflags( long f ) |
Turn off the flags specified by f |
X |
X |
| setbase( int base ) |
Sets the number base to base |
|
X |
| setfill( int ch ) |
Sets the fill character to ch |
|
X |
| setiosflags( long f ) |
Turn on the flags specified by f |
X |
X |
| setprecision( int p ) |
Sets the number of digits of precision |
|
X |
| setw( int w ) |
Sets the field width to w |
|
X |
State flags
The I/O stream state flags tell you the current state of an I/O stream. The flags are:
| Flag |
Meaning |
| badbit |
a fatal error has occurred |
| eofbit |
EOF has been found |
| failbit |
a nonfatal error has occurred |
| goodbit |
no errors have occurred |
Mode flags
The I/O stream mode flags allow you to access files in different ways. The flags are:
| Mode |
Meaning |
| ios::app |
append output |
| ios::ate |
seek to EOF when opened |
| ios::binary |
open the file in binary mode |
| ios::in |
open the file for reading |
| ios::out |
open the file for writing |
| ios::trunc |
overwrite the existing file |