Thinking In C++. Volume 2: Practical Programming

The standard error output stream cerr has unit buffering turned on by default. There is a cost for unit buffering, of course, so if an output stream is heavily used, don’t enable unit buffering unless efficiency is not a consideration^.

Format fields

The second type of formatting flags work in a group. Only one of these flags can be, like the buttons on old car radios—you push one in, the rest pop out. Unfortunately this doesn’t happen automatically, and you have to pay attention to what flags you’re setting so that you don’t accidentally call the wrong setf( ) function. For example, there’s a flag for each of the number bases: hexadecimal, decimal, and octal. Collectively, these flags are referred to as the ios::basefield. If the ios::dec flag is set and you call setf(ios::hex), you’ll set the ios::hex flag, but you won’t clear the ios::dec bit, resulting in undefined behavior. The proper thing to do is call the second form of setf( ) like this: setf(ios::hex, ios::basefield). This function first clears all the bits in the ios::basefield and then sets ios::hex. Thus, this form of setf( ) ensures that the other flags in the group "pop out" whenever you set one. Of course, the ios::hex manipulator does all this for you, automatically, so you don’t have to concern yourself with the internal details of the implementation of this class or to even care that it’s a set of binary flags. Later you’ll see that there are manipulators to provide equivalent functionality in all the places you would use setf( )^.

Here are the flag groups and their effects:

ios::basefield	effect
ios::dec	Format integral values in base 10 (decimal) (the default radix—no prefix is visible).
ios::hex	Format integral values in base 16 (hexadecimal).
ios::oct	Format integral values in base 8 (octal).

ios::floatfield	effect
ios::scientific	Display floating-point numbers in scientific format. Precision field indicates number of digits after the decimal point.
ios::fixed	Display floating-point numbers in fixed format. Precision field indicates number of digits after the decimal point.
"automatic" (Neither bit is set.)	Precision field indicates the total number of significant digits.

ios::adjustfield	Effect
ios::left	Left-align values; pad on the right with the fill character.
ios::right	Right-align values. Pad on the left with the fill character. This is the default alignment.
ios::internal	Add fill characters after any leading sign or base indicator, but before the value. (In other words, the sign, if printed, is left-justified while the number is right-justified).

Width, fill, and precision

The internal variables that control the width of the output field, the fill character used to pad an output field, and the precision for printing floating-point numbers are read and written by member functions of the same name^.