31 Input/output library [input.output]

31.7 Formatting and manipulators [iostream.format]

31.7.6 Output streams [output.streams]

31.7.6.3 Formatted output functions [ostream.formatted]

31.7.6.3.1 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.

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This is done without causing an ios_base::failure to be thrown.

31.7.6.3.2 Arithmetic inserters [ostream.inserters.arithmetic]

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basic_ostream& operator<<(bool val);
basic_ostream& operator<<(short val);
basic_ostream& operator<<(unsigned short val);
basic_ostream& operator<<(int val);
basic_ostream& operator<<(unsigned int val);
basic_ostream& operator<<(long val);
basic_ostream& operator<<(unsigned long val);
basic_ostream& operator<<(long long val);
basic_ostream& operator<<(unsigned long long val);
basic_ostream& operator<<(float val);
basic_ostream& operator<<(double val);
basic_ostream& operator<<(long double val);
basic_ostream& operator<<(const void* val);

Effects: The classes num_get<> and num_put<> handle locale-dependent numeric formatting and parsing.

These inserter functions use the imbued locale value to perform numeric formatting.

When val is of type bool, long, unsigned long, long long, unsigned long long, double, long double, or const void*, the formatting conversion occurs as if it performed the following code fragment: bool failed = use_facet<num_put<charT, ostreambuf_iterator<charT, traits>>>( getloc()).put(*this, *this, fill(), val).failed();

When val is of type short the formatting conversion occurs as if it performed the following code fragment: ios_base::fmtflags baseflags = ios_base::flags() & ios_base::basefield; bool failed = use_facet<num_put<charT, ostreambuf_iterator<charT, traits>>>( getloc()).put(*this, *this, fill(), baseflags == ios_base::oct || baseflags == ios_base::hex ? static_cast<long>(static_cast<unsigned short>(val)) : static_cast<long>(val)).failed();

When val is of type int the formatting conversion occurs as if it performed the following code fragment: ios_base::fmtflags baseflags = ios_base::flags() & ios_base::basefield; bool failed = use_facet<num_put<charT, ostreambuf_iterator<charT, traits>>>( getloc()).put(*this, *this, fill(), baseflags == ios_base::oct || baseflags == ios_base::hex ? static_cast<long>(static_cast<unsigned int>(val)) : static_cast<long>(val)).failed();

When val is of type unsigned short or unsigned int the formatting conversion occurs as if it performed the following code fragment: bool failed = use_facet<num_put<charT, ostreambuf_iterator<charT, traits>>>( getloc()).put(*this, *this, fill(), static_cast<unsigned long>(val)).failed();

When val is of type float the formatting conversion occurs as if it performed the following code fragment: bool failed = use_facet<num_put<charT, ostreambuf_iterator<charT, traits>>>( getloc()).put(*this, *this, fill(), static_cast<double>(val)).failed();

The first argument provides an object of the ostreambuf_iterator<> class which is an iterator for class basic_ostream<>.

It bypasses ostreams and uses streambufs directly.

Class locale relies on these types as its interface to iostreams, since for flexibility it has been abstracted away from direct dependence on ostream.

The second parameter is a reference to the base class subobject of type ios_base.

It provides formatting specifications such as field width, and a locale from which to obtain other facets.

If failed is true then does setstate(badbit), which may throw an exception, and returns.

Returns: *this.

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basic_ostream& operator<<(const volatile void* p);

Effects: Equivalent to: return operator<<(const_cast<const void*>(p));

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basic_ostream& operator<<(extended-floating-point-type val);

Effects: If the floating-point conversion rank of extended-floating-point-type is less than or equal to that of double, the formatting conversion occurs as if it performed the following code fragment: bool failed = use_facet<num_put<charT, ostreambuf_iterator<charT, traits>>>( getloc()).put(*this, *this, fill(), static_cast<double>(val)).failed();

Otherwise, if the floating-point conversion rank of extended-floating-point-type is less than or equal to that of long double, the formatting conversion occurs as if it performed the following code fragment: bool failed = use_facet<num_put<charT, ostreambuf_iterator<charT, traits>>>( getloc()).put(*this, *this, fill(), static_cast<long double>(val)).failed();

Otherwise, an invocation of the operator function is conditionally supported with implementation-defined semantics.

If failed is true then does setstate(badbit), which may throw an exception, and returns.

Returns: *this.

31.7.6.3.3 basic_ostream::operator<< [ostream.inserters]

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basic_ostream& operator<<(basic_ostream& (*pf)(basic_ostream&));

Effects: None.

Does not behave as a formatted output function (as described in [ostream.formatted.reqmts]).

Returns: pf(*this).292

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basic_ostream& operator<<(basic_ios<charT, traits>& (*pf)(basic_ios<charT, traits>&));

Effects: Calls pf(*this).

This inserter does not behave as a formatted output function (as described in [ostream.formatted.reqmts]).

Returns: *this.293

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basic_ostream& operator<<(ios_base& (*pf)(ios_base&));

Effects: Calls pf(*this).

This inserter does not behave as a formatted output function (as described in [ostream.formatted.reqmts]).

Returns: *this.

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basic_ostream& operator<<(basic_streambuf<charT, traits>* sb);

Effects: Behaves as an unformatted output function ([ostream.unformatted]).

After the sentry object is constructed, if sb is null calls setstate(badbit) (which may throw ios_base::failure).

Gets characters from sb and inserts them in *this.

Characters are read from sb and inserted until any of the following occurs:

(8.1)
end-of-file occurs on the input sequence;
(8.2)
inserting in the output sequence fails (in which case the character to be inserted is not extracted);
(8.3)
an exception occurs while getting a character from sb.

If the function inserts no characters, it calls setstate(failbit) (which may throw ios_base::failure ([iostate.flags])).

If an exception was thrown while extracting a character, the function sets failbit in the error state, and if failbit is set in exceptions() the caught exception is rethrown.

Returns: *this.

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basic_ostream& operator<<(nullptr_t);

Effects: Equivalent to: return *this << s; where s is an implementation-defined NTCTS .

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See, for example, the function signature endl(basic_ostream&) ([ostream.manip]).

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See, for example, the function signature dec(ios_base&) ([basefield.manip]).

31.7.6.3.4 Character inserter function templates [ostream.inserters.character]

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template<class charT, class traits>
  basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& out, charT c);
template<class charT, class traits>
  basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& out, char c);
// specialization
template<class traits>
  basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>& out, char c);
// signed and unsigned
template<class traits>
  basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>& out, signed char c);
template<class traits>
  basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>& out, unsigned char c);

Effects: Behaves as a formatted output function of out.

Constructs a character sequence seq.

If c has type char and the character container type of the stream is not char, then seq consists of out.widen(c); otherwise seq consists of c.

Determines padding for seq as described in [ostream.formatted.reqmts].

Inserts seq into out.

Calls os.width(0).

Returns: out.

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template<class charT, class traits>
  basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& out, const charT* s);
template<class charT, class traits>
  basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& out, const char* s);
template<class traits>
  basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>& out, const char* s);
template<class traits>
  basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>& out, const signed char* s);
template<class traits>
  basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>& out,
                                          const unsigned char* s);

Preconditions: s is not a null pointer.

Effects: Behaves like a formatted inserter (as described in [ostream.formatted.reqmts]) of out.

Creates a character sequence seq of n characters starting at s, each widened using out.widen() ([basic.ios.members]), where n is the number that would be computed as if by:

(4.1)
traits::length(s) for the overload where the first argument is of type basic_ostream<charT, traits>& and the second is of type const charT*, and also for the overload where the first argument is of type basic_ostream<char, traits>& and the second is of type const char*,
(4.2)
char_traits<char>::length(s) for the overload where the first argument is of type basic_ostream<charT, traits>& and the second is of type const char*,
(4.3)
traits::length(reinterpret_cast<const char*>(s)) for the other two overloads.

Determines padding for seq as described in [ostream.formatted.reqmts].

Inserts seq into out.

Calls width(0).

Returns: out.

31.7.6.3.5 Print [ostream.formatted.print]

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template<class... Args>
  void print(ostream& os, format_string<Args...> fmt, Args&&... args);

Effects: If the ordinary literal encoding ([lex.charset]) is UTF-8, equivalent to: vprint_unicode(os, fmt.str, make_format_args(args...));

Otherwise, equivalent to: vprint_nonunicode(os, fmt.str, make_format_args(args...));

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template<class... Args>
  void println(ostream& os, format_string<Args...> fmt, Args&&... args);

Effects: Equivalent to: print(os, "{}\n", format(fmt, std::forward<Args>(args)...));

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void println(ostream& os);

Effects: Equivalent to: print(os, "\n");

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void vprint_unicode(ostream& os, string_view fmt, format_args args);
void vprint_nonunicode(ostream& os, string_view fmt, format_args args);

Effects: Behaves as a formatted output function ([ostream.formatted.reqmts]) of os, except that:

(4.1)
failure to generate output is reported as specified below, and
(4.2)
any exception thrown by the call to vformat is propagated without regard to the value of os.exceptions() and without turning on ios_base::badbit in the error state of os.

After constructing a sentry object, the function initializes a variable with automatic storage duration via string out = vformat(os.getloc(), fmt, args);

If the function is vprint_unicode and os is a stream that refers to a terminal capable of displaying Unicode which is determined in an implementation-defined manner, writes out to the terminal using the native Unicode API; if out contains invalid code units, the behavior is undefined and implementations are encouraged to diagnose it.

If the native Unicode API is used, the function flushes os before writing out.

Otherwise (if os is not such a stream or the function is vprint_nonunicode), inserts the character sequence [out.begin(), out.end()) into os.

If writing to the terminal or inserting into os fails, calls os.setstate(ios_base::badbit) (which may throw ios_base::failure).

Recommended practice: For vprint_unicode, if invoking the native Unicode API requires transcoding, implementations should substitute invalid code units with U+fffd replacement character per the Unicode Standard, Chapter 3.9 U+fffd Substitution in Conversion.

31 Input/output library [input.output]

31.7 Formatting and manipulators [iostream.format]

31.7.6 Output streams [output.streams]

31.7.6.3 Formatted output functions [ostream.formatted]

31.7.6.3.1 Common requirements [ostream.formatted.reqmts]

31.7.6.3.2 Arithmetic inserters [ostream.inserters.arithmetic]

31.7.6.3.3 basic_ostream​::​operator<< [ostream.inserters]

31.7.6.3.4 Character inserter function templates [ostream.inserters.character]

31.7.6.3.5 Print [ostream.formatted.print]

31.7.6.3.3 basic_ostream::operator<< [ostream.inserters]