# C++ named requirements: FormattedOutputFunction

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### 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.

### Padding

Formatted output functions determine padding according to `std::num_put::do_put()` stage 3. | (until C++14)

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 os, 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() & std::ios_base::adjustfield ) == std::ios_base::left is true, the fill characters are placed after the character sequence; otherwise, they are placed before the character sequence.

(since C++14)

### Standard library

The following standard library functions are FormattedOutputFunctions.

- `basic_ostream::operator<<(std::basic_ostream&, int / long / double / void* / bool)`
- `operator<<(std::basic_ostream&, char)`
- `operator<<(std::basic_ostream&, char*)`
- `operator<<(std::basic_ostream&, const std::bitset&)`
- `operator<<(std::basic_ostream&, const std::basic_string&)`
- `operator<<` , when called on the return value of std::put_money

- `operator<<(std::basic_ostream&, std::basic_string_view)`

(since C++17)

- `print(std::ostream&, std::format_string<Args...>, Args&&...)`,
- `println(std::ostream&, std::format_string<Args...>, Args&&...)`,
- `vprint_unicode(std::ostream&, std::string_view, std::format_args)`,
- `vprint_nonunicode(std::ostream&, std::string_view, std::format_args)`,

  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)

### 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()`

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