31 Input/output library [input.output]

31.7 Formatting and manipulators [iostream.format]

31.7.6 Output streams [output.streams] Formatted output functions [ostream.formatted] Common requirements [ostream.formatted.reqmts]

Each formatted output function begins execution by constructing an object of class sentry.
If that object returns true when converted to a value of type bool, the function endeavors to generate the requested output.
If the generation fails, then the formatted output function does setstate(ios_base​::​failbit), which can throw an exception.
If an exception is thrown during output, then ios_base​::​badbit is set291 in *this's error state.
If (exceptions()&badbit) != 0 then the exception is rethrown.
Whether or not an exception is thrown, the sentry object is destroyed before leaving the formatted output function.
If no exception is thrown, the result of the formatted output function is *this.
The descriptions of the individual formatted output functions describe how they perform output and do not mention the sentry object.
If a formatted output function of a stream os determines padding, it does so as follows.
Given a charT character sequence seq where charT is the character container type of the stream, if the length of seq is less than os.width(), then enough copies of os.fill() are added to this sequence as necessary to pad to a width of os.width() characters.
If (os.flags() & ios_base​::​adjustfield) == ios_base​::​left is true, the fill characters are placed after the character sequence; otherwise, they are placed before the character sequence.
This is done without causing an ios_base​::​failure to be thrown.