C++ named requirements: FormattedOutputFunction
Requirements
A FormattedOutputFunction is a stream output function that performs the following:
- Constructs an object of type basic_istream::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.
- 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
- 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 ofrdbuf()
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() & failbit) != 0)
, the exception is also rethrown. - If no exception was thrown, returns
*this
.
- If the output could not be generated, sets failbit. If exceptions on failbit are enabled in this stream's
exception mask
- 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.
(do 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 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.
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::get_money
- operator<<(std::basic_ostream&, std::basic_string_view) (od 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 ofos
; - if inserting into
os
fails, callsos.setstate(ios_base::badbit)
(which may throw ios_base::failure).
- any exception thrown by the call to std::vformat is propagated without regard to the value
of
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() |