C++ named requirements: FormattedOutputFunction

< cpp‎ | named req
C++ named requirements
Type properties
Container Elements

Stream I/O

Random Numbers


[edit] Requirements

A FormattedOutputFunction is a stream output function that performs the following:

  • Constructs an object of type basic_ostream::sentry with automatic storage duration, which performs the following
  • if eofbit or badbit are set on the output stream, sets the failbit as well, and if exceptions on failbit are enabled in this output stream's exception mask ((exceptions() & failbit) != 0), throws ios_base::failure.
  • flushes the tie()'d output stream, if applicable.
  • Checks the status of the sentry by calling sentry::operator bool(), which is equivalent to basic_ios::good.
  • If the sentry returned false or sentry's constructor threw an exception, no output takes place
  • If the sentry returned true, attempts to perform the desired output by inserting the characters into the output stream as if by calling rdbuf()->sputc(). Other public members of std::basic_ostream may also be used, but virtual members of rdbuf() except overflow(), xsputn() and sync() will never be called.
  • If the output could not be generated, sets failbit. If exceptions on failbit are enabled in this stream's exception mask ((exceptions() & failbit) != 0), throws ios_base::failure.
  • if an exception is thrown during output, sets badbit in the output stream. If exceptions on badbit are enabled in this stream's exception mask ((exceptions() & badbit) != 0), the exception is also rethrown.
  • If no exception was thrown, returns *this.
  • In any event, whether terminating by exception or returning, the sentry's destructor is called before leaving this function.

[edit] Standard library

The following standard library functions are FormattedOutputFunctions.

except that (given os is an output stream object):
  • any exception thrown by the call to std::vformat is propagated without regard to the value of os.exceptions() and without turning on ios_base::badbit in the error state of os;
  • if inserting into os fails, calls os.setstate(ios_base::badbit) (which may throw ios_base::failure).
(since C++23)

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 160 C++98 the process of determining whether the exception caught
is rethrown mentioned a non-existing function exception()
corrected to exceptions()
LWG 165 C++98 the only virtual member allowed to be
called on rdbuf() was overflow()
also allowed
xsputn() and sync()