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