Having created a stream and connected it to a file, you
can now write to the file just as you would to cout -- using
the stream insertion operator <<.
outStream.open( "output.dat" );
int age = 42;
string name = "Bob";
outStream << name << " is " << age << " years old!" << endl;
Although the method is simple, the look of your output is at the mercy
of the C++ default formatting parameters (field width, justification,
number of decimal places, etc.)
These can be controlled in a number of different ways.
Formatting commands control what your output looks like and can be
used on any output stream (including cout).
Every output stream has a function named setf( ) that
can be used to change the format of your output. The most common
formatting commands are shown below. The text has a table of
formatting commands on page 520.
Note that once issued, these commands
stay in affect until reset.
Other output stream functions
In addition to setf( ) output streams also have the following
- outStream.setf( ios::fixed );
causes floating point numbers to be written with a decimal point. This is analagous
to printf()'s %f
- outStream.setf( ios::showpoint );
forces the decimal point and trailing zeros to always be shown even if
not strictly mathematically necessary
- outStream.setf( ios::right);
causes the output to be right-justified in the output field. This
is the default setting for the output stream.
- outStream.setf( ios::left);
causes the output to be left-justified in the output field.
- outStream.precision( int places);
when used in conjunction with setf( ios::fixed ), precision( )
specifies the number of places to the right of the decimal point to be displayed.
A call to precision() affects all floating point I/O operations for a stream
until the next call to precision()
- outStream.width( int size);
specifies the (minimum) number of spaces to use to output the next item.
The format of your output can often be controlled by inserting "manipulators"
directly into the output stream, rather than calling an output stream function.
Note that not all manipulators are supported by all compilers. To use the
manipulators, you must #include <iomanip>
The example code shows the effect of these formatting options.
- setprecision( int places ) has the same effect as the
output stream precision( ) function.
outStream << setprecision( 2 ) << 4.3 << endl;
has the same effect as
outStream.precision( 2 );
outStream << 4.3 << endl;
- setw( int size ) has the same effect as width()
outStream << setw( 5 ) << 4.3 << endl;
is the same as
outStream.width( 5 );
outStream << 4.3 << endl;
- fixed has the same effect as setf( ios::fixed )
- showpoint has the same effect as setf( ios::showpoint )
Well written functions should have no "side effects". This means that
when writing functions that perform output, it is good programming practice
to save the current settings of the stream's formatting flags and restore them
after the function's output is complete. Use code such as
int savePrecision = outStream.precision( );
int saveFlags = outStream.flags( );
to read the current file settings and then reset them with code such as
outStream.precision( savePrecision );
outStream.flags( saveFlags ):
Page 523 of the text has a more complete example of this technique.
Last Modified: Monday, 28-Aug-2006 10:15:54 EDT