31 Input/output library [input.output]

31.1 General [input.output.general]

This Clause describes components that C++ programs may use to perform input/output operations.
The following subclauses describe requirements for stream parameters, and components for forward declarations of iostreams, predefined iostreams objects, base iostreams classes, stream buffering, stream formatting and manipulators, string streams, and file streams, as summarized in Table 117.
Table 117: Input/output library summary [tab:iostreams.summary]
Subclause
Header
Requirements
Forward declarations
<iosfwd>
Standard iostream objects
<iostream>
Iostreams base classes
<ios>
Stream buffers
<streambuf>
Formatting and manipulators
<istream>, <ostream>, <iomanip>, <print>
String streams
<sstream>
Span-based streams
<spanstream>
File streams
<fstream>
Synchronized output streams
<syncstream>
File systems
<filesystem>
C library files
<cstdio>, <cinttypes>

31.2 Iostreams requirements [iostreams.requirements]

31.2.1 Imbue limitations [iostream.limits.imbue]

No function described in [input.output] except for ios_base​::​imbue and basic_filebuf​::​pubimbue causes any instance of basic_ios​::​imbue or basic_streambuf​::​imbue to be called.
If any user function called from a function declared in [input.output] or as an overriding virtual function of any class declared in [input.output] calls imbue, the behavior is undefined.

31.2.2 Types [stream.types]

using streamoff = implementation-defined;
The type streamoff is a synonym for one of the signed basic integral types of sufficient size to represent the maximum possible file size for the operating system.264
using streamsize = implementation-defined;
The type streamsize is a synonym for one of the signed basic integral types.
It is used to represent the number of characters transferred in an I/O operation, or the size of I/O buffers.265
264)264)
Typically long long.
265)265)
Most places where streamsize is used would use size_t in ISO C, or ssize_t in POSIX.

31.2.3 Positioning type limitations [iostreams.limits.pos]

The classes of [input.output] with template arguments charT and traits behave as described if traits​::​pos_type and traits​::​off_type are streampos and streamoff respectively.
Except as noted explicitly below, their behavior when traits​::​pos_type and traits​::​off_type are other types is implementation-defined.
[Note 1: 
For each of the specializations of char_traits defined in [char.traits.specializations], state_type denotes mbstate_t, pos_type denotes fpos<mbstate_t>, and off_type denotes streamoff.
β€” end note]
In the classes of [input.output], a template parameter with name charT represents a member of the set of types containing char, wchar_t, and any other implementation-defined character types that meet the requirements for a character on which any of the iostream components can be instantiated.

31.2.4 Thread safety [iostreams.threadsafety]

Concurrent access to a stream object ([string.streams], [file.streams]), stream buffer object ([stream.buffers]), or C Library stream ([c.files]) by multiple threads may result in a data race ([intro.multithread]) unless otherwise specified ([iostream.objects]).
[Note 1: 
Data races result in undefined behavior ([intro.multithread]).
β€” end note]
If one thread makes a library call a that writes a value to a stream and, as a result, another thread reads this value from the stream through a library call b such that this does not result in a data race, then a's write synchronizes with b's read.

31.3 Forward declarations [iostream.forward]

31.3.1 Header <iosfwd> synopsis [iosfwd.syn]

namespace std { template<class charT> struct char_traits; template<> struct char_traits<char>; template<> struct char_traits<char8_t>; template<> struct char_traits<char16_t>; template<> struct char_traits<char32_t>; template<> struct char_traits<wchar_t>; template<class T> class allocator; template<class charT, class traits = char_traits<charT>> class basic_ios; template<class charT, class traits = char_traits<charT>> class basic_streambuf; template<class charT, class traits = char_traits<charT>> class basic_istream; template<class charT, class traits = char_traits<charT>> class basic_ostream; template<class charT, class traits = char_traits<charT>> class basic_iostream; template<class charT, class traits = char_traits<charT>, class Allocator = allocator<charT>> class basic_stringbuf; template<class charT, class traits = char_traits<charT>, class Allocator = allocator<charT>> class basic_istringstream; template<class charT, class traits = char_traits<charT>, class Allocator = allocator<charT>> class basic_ostringstream; template<class charT, class traits = char_traits<charT>, class Allocator = allocator<charT>> class basic_stringstream; template<class charT, class traits = char_traits<charT>> class basic_spanbuf; template<class charT, class traits = char_traits<charT>> class basic_ispanstream; template<class charT, class traits = char_traits<charT>> class basic_ospanstream; template<class charT, class traits = char_traits<charT>> class basic_spanstream; template<class charT, class traits = char_traits<charT>> class basic_filebuf; template<class charT, class traits = char_traits<charT>> class basic_ifstream; template<class charT, class traits = char_traits<charT>> class basic_ofstream; template<class charT, class traits = char_traits<charT>> class basic_fstream; template<class charT, class traits = char_traits<charT>, class Allocator = allocator<charT>> class basic_syncbuf; template<class charT, class traits = char_traits<charT>, class Allocator = allocator<charT>> class basic_osyncstream; template<class charT, class traits = char_traits<charT>> class istreambuf_iterator; template<class charT, class traits = char_traits<charT>> class ostreambuf_iterator; using ios = basic_ios<char>; using wios = basic_ios<wchar_t>; using streambuf = basic_streambuf<char>; using istream = basic_istream<char>; using ostream = basic_ostream<char>; using iostream = basic_iostream<char>; using stringbuf = basic_stringbuf<char>; using istringstream = basic_istringstream<char>; using ostringstream = basic_ostringstream<char>; using stringstream = basic_stringstream<char>; using spanbuf = basic_spanbuf<char>; using ispanstream = basic_ispanstream<char>; using ospanstream = basic_ospanstream<char>; using spanstream = basic_spanstream<char>; using filebuf = basic_filebuf<char>; using ifstream = basic_ifstream<char>; using ofstream = basic_ofstream<char>; using fstream = basic_fstream<char>; using syncbuf = basic_syncbuf<char>; using osyncstream = basic_osyncstream<char>; using wstreambuf = basic_streambuf<wchar_t>; using wistream = basic_istream<wchar_t>; using wostream = basic_ostream<wchar_t>; using wiostream = basic_iostream<wchar_t>; using wstringbuf = basic_stringbuf<wchar_t>; using wistringstream = basic_istringstream<wchar_t>; using wostringstream = basic_ostringstream<wchar_t>; using wstringstream = basic_stringstream<wchar_t>; using wspanbuf = basic_spanbuf<wchar_t>; using wispanstream = basic_ispanstream<wchar_t>; using wospanstream = basic_ospanstream<wchar_t>; using wspanstream = basic_spanstream<wchar_t>; using wfilebuf = basic_filebuf<wchar_t>; using wifstream = basic_ifstream<wchar_t>; using wofstream = basic_ofstream<wchar_t>; using wfstream = basic_fstream<wchar_t>; using wsyncbuf = basic_syncbuf<wchar_t>; using wosyncstream = basic_osyncstream<wchar_t>; template<class state> class fpos; using streampos = fpos<char_traits<char>::state_type>; using wstreampos = fpos<char_traits<wchar_t>::state_type>; using u8streampos = fpos<char_traits<char8_t>::state_type>; using u16streampos = fpos<char_traits<char16_t>::state_type>; using u32streampos = fpos<char_traits<char32_t>::state_type>; }
Default template arguments are described as appearing both in <iosfwd> and in the synopsis of other headers but it is well-formed to include both <iosfwd> and one or more of the other headers.266
266)266)
It is the implementation's responsibility to implement headers so that including <iosfwd> and other headers does not violate the rules about multiple occurrences of default arguments.

31.3.2 Overview [iostream.forward.overview]

The class template specialization basic_ios<charT, traits> serves as a virtual base class for the class templates basic_istream, basic_ostream, and class templates derived from them.
basic_iostream is a class template derived from both basic_istream<charT, traits> and basic_ostream<charT, traits>.
The class template specialization basic_streambuf<charT, traits> serves as a base class for class templates basic_stringbuf, basic_filebuf, and basic_syncbuf.
The class template specialization basic_istream<charT, traits> serves as a base class for class templates basic_istringstream and basic_ifstream.
The class template specialization basic_ostream<charT, traits> serves as a base class for class templates basic_ostringstream, basic_ofstream, and basic_osyncstream.
The class template specialization basic_iostream<charT, traits> serves as a base class for class templates basic_stringstream and basic_fstream.
[Note 1: 
For each of the class templates above, the program is ill-formed if traits​::​char_type is not the same type as charT ([char.traits]).
β€” end note]
Other typedef-names define instances of class templates specialized for char or wchar_t types.
Specializations of the class template fpos are used for specifying file position information.
[Example 1: 
The types streampos and wstreampos are used for positioning streams specialized on char and wchar_t respectively.
β€” end example]
[Note 2: 
This synopsis suggests a circularity between streampos and char_traits<char>.
An implementation can avoid this circularity by substituting equivalent types.
β€” end note]

31.4 Standard iostream objects [iostream.objects]

31.4.1 Header <iostream> synopsis [iostream.syn]

#include <ios> // see [ios.syn] #include <streambuf> // see [streambuf.syn] #include <istream> // see [istream.syn] #include <ostream> // see [ostream.syn] namespace std { extern istream cin; extern ostream cout; extern ostream cerr; extern ostream clog; extern wistream wcin; extern wostream wcout; extern wostream wcerr; extern wostream wclog; }

31.4.2 Overview [iostream.objects.overview]

In this Clause, the type name FILE refers to the type FILE declared in <cstdio>.
The header <iostream> declares objects that associate objects with the standard C streams provided for by the functions declared in <cstdio>, and includes all the headers necessary to use these objects.
The objects are constructed and the associations are established at some time prior to or during the first time an object of class ios_base​::​Init is constructed, and in any case before the body of main ([basic.start.main]) begins execution.
The objects are not destroyed during program execution.267
Recommended practice: If it is possible for them to do so, implementations should initialize the objects earlier than required.
The results of including <iostream> in a translation unit shall be as if <iostream> defined an instance of ios_base​::​Init with static storage duration.
Each C++ library module ([std.modules]) in a hosted implementation shall behave as if it contains an interface unit that defines an unexported ios_base​::​Init variable with ordered initialization ([basic.start.dynamic]).
[Note 1: 
As a result, the definition of that variable is appearance-ordered before any declaration following the point of importation of a C++ library module.
Whether such a definition exists is unobservable by a program that does not reference any of the standard iostream objects.
β€” end note]
Mixing operations on corresponding wide- and narrow-character streams follows the same semantics as mixing such operations on FILEs, as specified in the C standard library.
Concurrent access to a synchronized ([ios.members.static]) standard iostream object's formatted and unformatted input ([istream]) and output ([ostream]) functions or a standard C stream by multiple threads does not result in a data race ([intro.multithread]).
[Note 2: 
Unsynchronized concurrent use of these objects and streams by multiple threads can result in interleaved characters.
β€” end note]
See also: ISO/IEC 9899:2018, 7.21.2
267)267)
Constructors and destructors for objects with static storage duration can access these objects to read input from stdin or write output to stdout or stderr.

31.4.3 Narrow stream objects [narrow.stream.objects]

istream cin;
The object cin controls input from a stream buffer associated with the object stdin, declared in <cstdio>.
After the object cin is initialized, cin.tie() returns &cout.
Its state is otherwise the same as required for basic_ios<char>​::​init.
ostream cout;
The object cout controls output to a stream buffer associated with the object stdout, declared in <cstdio>.
ostream cerr;
The object cerr controls output to a stream buffer associated with the object stderr, declared in <cstdio>.
After the object cerr is initialized, cerr.flags() & unitbuf is nonzero and cerr.tie() returns &cout.
Its state is otherwise the same as required for basic_ios<char>​::​init.
ostream clog;
The object clog controls output to a stream buffer associated with the object stderr, declared in <cstdio>.

31.4.4 Wide stream objects [wide.stream.objects]

wistream wcin;
The object wcin controls input from a stream buffer associated with the object stdin, declared in <cstdio>.
After the object wcin is initialized, wcin.tie() returns &wcout.
Its state is otherwise the same as required for basic_ios<wchar_t>​::​init.
wostream wcout;
The object wcout controls output to a stream buffer associated with the object stdout, declared in <cstdio>.
wostream wcerr;
The object wcerr controls output to a stream buffer associated with the object stderr, declared in <cstdio>.
After the object wcerr is initialized, wcerr.flags() & unitbuf is nonzero and wcerr.tie() returns &wcout.
Its state is otherwise the same as required for basic_ios<wchar_t>​::​init.
wostream wclog;
The object wclog controls output to a stream buffer associated with the object stderr, declared in <cstdio>.

31.5 Iostreams base classes [iostreams.base]

31.5.1 Header <ios> synopsis [ios.syn]

#include <iosfwd> // see [iosfwd.syn] namespace std { using streamoff = implementation-defined; using streamsize = implementation-defined; template<class stateT> class fpos; class ios_base; template<class charT, class traits = char_traits<charT>> class basic_ios; // [std.ios.manip], manipulators ios_base& boolalpha (ios_base& str); ios_base& noboolalpha(ios_base& str); ios_base& showbase (ios_base& str); ios_base& noshowbase (ios_base& str); ios_base& showpoint (ios_base& str); ios_base& noshowpoint(ios_base& str); ios_base& showpos (ios_base& str); ios_base& noshowpos (ios_base& str); ios_base& skipws (ios_base& str); ios_base& noskipws (ios_base& str); ios_base& uppercase (ios_base& str); ios_base& nouppercase(ios_base& str); ios_base& unitbuf (ios_base& str); ios_base& nounitbuf (ios_base& str); // [adjustfield.manip], adjustfield ios_base& internal (ios_base& str); ios_base& left (ios_base& str); ios_base& right (ios_base& str); // [basefield.manip], basefield ios_base& dec (ios_base& str); ios_base& hex (ios_base& str); ios_base& oct (ios_base& str); // [floatfield.manip], floatfield ios_base& fixed (ios_base& str); ios_base& scientific (ios_base& str); ios_base& hexfloat (ios_base& str); ios_base& defaultfloat(ios_base& str); // [error.reporting], error reporting enum class io_errc { stream = 1 }; template<> struct is_error_code_enum<io_errc> : public true_type { }; error_code make_error_code(io_errc e) noexcept; error_condition make_error_condition(io_errc e) noexcept; const error_category& iostream_category() noexcept; }

31.5.2 Class ios_base [ios.base]

31.5.2.1 General [ios.base.general]

namespace std { class ios_base { public: class failure; // see below // [ios.fmtflags], fmtflags using fmtflags = T1; static constexpr fmtflags boolalpha = unspecified; static constexpr fmtflags dec = unspecified; static constexpr fmtflags fixed = unspecified; static constexpr fmtflags hex = unspecified; static constexpr fmtflags internal = unspecified; static constexpr fmtflags left = unspecified; static constexpr fmtflags oct = unspecified; static constexpr fmtflags right = unspecified; static constexpr fmtflags scientific = unspecified; static constexpr fmtflags showbase = unspecified; static constexpr fmtflags showpoint = unspecified; static constexpr fmtflags showpos = unspecified; static constexpr fmtflags skipws = unspecified; static constexpr fmtflags unitbuf = unspecified; static constexpr fmtflags uppercase = unspecified; static constexpr fmtflags adjustfield = see below; static constexpr fmtflags basefield = see below; static constexpr fmtflags floatfield = see below; // [ios.iostate], iostate using iostate = T2; static constexpr iostate badbit = unspecified; static constexpr iostate eofbit = unspecified; static constexpr iostate failbit = unspecified; static constexpr iostate goodbit = see below; // [ios.openmode], openmode using openmode = T3; static constexpr openmode app = unspecified; static constexpr openmode ate = unspecified; static constexpr openmode binary = unspecified; static constexpr openmode in = unspecified; static constexpr openmode noreplace = unspecified; static constexpr openmode out = unspecified; static constexpr openmode trunc = unspecified; // [ios.seekdir], seekdir using seekdir = T4; static constexpr seekdir beg = unspecified; static constexpr seekdir cur = unspecified; static constexpr seekdir end = unspecified; class Init; // [fmtflags.state], fmtflags state fmtflags flags() const; fmtflags flags(fmtflags fmtfl); fmtflags setf(fmtflags fmtfl); fmtflags setf(fmtflags fmtfl, fmtflags mask); void unsetf(fmtflags mask); streamsize precision() const; streamsize precision(streamsize prec); streamsize width() const; streamsize width(streamsize wide); // [ios.base.locales], locales locale imbue(const locale& loc); locale getloc() const; // [ios.base.storage], storage static int xalloc(); long& iword(int idx); void*& pword(int idx); // destructor virtual ~ios_base(); // [ios.base.callback], callbacks enum event { erase_event, imbue_event, copyfmt_event }; using event_callback = void (*)(event, ios_base&, int idx); void register_callback(event_callback fn, int idx); ios_base(const ios_base&) = delete; ios_base& operator=(const ios_base&) = delete; static bool sync_with_stdio(bool sync = true); protected: ios_base(); private: static int index; // exposition only long* iarray; // exposition only void** parray; // exposition only }; }
ios_base defines several member types:
  • a type failure, defined as either a class derived from system_error or a synonym for a class derived from system_error;
  • a class Init;
  • three bitmask types, fmtflags, iostate, and openmode;
  • an enumerated type, seekdir.
It maintains several kinds of data:
  • state information that reflects the integrity of the stream buffer;
  • control information that influences how to interpret (format) input sequences and how to generate (format) output sequences;
  • additional information that is stored by the program for its private use.
[Note 1: 
For the sake of exposition, the maintained data is presented here as:
  • static int index, specifies the next available unique index for the integer or pointer arrays maintained for the private use of the program, initialized to an unspecified value;
  • long* iarray, points to the first element of an arbitrary-length long array maintained for the private use of the program;
  • void** parray, points to the first element of an arbitrary-length pointer array maintained for the private use of the program.
β€” end note]

31.5.2.2 Types [ios.types]

31.5.2.2.1 Class ios_base​::​failure [ios.failure]

namespace std { class ios_base::failure : public system_error { public: explicit failure(const string& msg, const error_code& ec = io_errc::stream); explicit failure(const char* msg, const error_code& ec = io_errc::stream); }; }
An implementation is permitted to define ios_base​::​failure as a synonym for a class with equivalent functionality to class ios_base​::​failure shown in this subclause.
[Note 1: 
When ios_base​::​failure is a synonym for another type, that type is required to provide a nested type failure to emulate the injected-class-name.
β€” end note]
The class failure defines the base class for the types of all objects thrown as exceptions, by functions in the iostreams library, to report errors detected during stream buffer operations.
When throwing ios_base​::​failure exceptions, implementations should provide values of ec that identify the specific reason for the failure.
[Note 2: 
Errors arising from the operating system would typically be reported as system_category() errors with an error value of the error number reported by the operating system.
Errors arising from within the stream library would typically be reported as error_code(io_errc​::​stream, iostream_category()).
β€” end note]
explicit failure(const string& msg, const error_code& ec = io_errc::stream);
Effects: Constructs the base class with msg and ec.
explicit failure(const char* msg, const error_code& ec = io_errc::stream);
Effects: Constructs the base class with msg and ec.

31.5.2.2.2 Type ios_base​::​fmtflags [ios.fmtflags]

using fmtflags = T1;
The type fmtflags is a bitmask type ([bitmask.types]).
Setting its elements has the effects indicated in Table 118.
Table 118: fmtflags effects [tab:ios.fmtflags]
Element
Effect(s) if set
boolalpha
insert and extract bool type in alphabetic format
dec
converts integer input or generates integer output in decimal base
fixed
generate floating-point output in fixed-point notation
hex
converts integer input or generates integer output in hexadecimal base
internal
adds fill characters at a designated internal point in certain generated output, or identical to right if no such point is designated
left
adds fill characters on the right (final positions) of certain generated output
oct
converts integer input or generates integer output in octal base
right
adds fill characters on the left (initial positions) of certain generated output
scientific
generates floating-point output in scientific notation
showbase
generates a prefix indicating the numeric base of generated integer output
showpoint
generates a decimal-point character unconditionally in generated floating-point output
showpos
generates a + sign in non-negative generated numeric output
skipws
skips leading whitespace before certain input operations
unitbuf
flushes output after each output operation
uppercase
replaces certain lowercase letters with their uppercase equivalents in generated output
Type fmtflags also defines the constants indicated in Table 119.
Table 119: fmtflags constants [tab:ios.fmtflags.const]
Constant
Allowable values
adjustfield
left | right | internal
basefield
dec | oct | hex
floatfield
scientific | fixed

31.5.2.2.3 Type ios_base​::​iostate [ios.iostate]

using iostate = T2;
The type iostate is a bitmask type ([bitmask.types]) that contains the elements indicated in Table 120.
Table 120: iostate effects [tab:ios.iostate]
Element
Effect(s) if set
badbit
indicates a loss of integrity in an input or output sequence (such as an irrecoverable read error from a file);
eofbit
indicates that an input operation reached the end of an input sequence;
failbit
indicates that an input operation failed to read the expected characters, or that an output operation failed to generate the desired characters.
Type iostate also defines the constant:

31.5.2.2.4 Type ios_base​::​openmode [ios.openmode]

using openmode = T3;
The type openmode is a bitmask type ([bitmask.types]).
It contains the elements indicated in Table 121.
Table 121: openmode effects [tab:ios.openmode]
Element
Effect(s) if set
app
seek to end before each write
ate
open and seek to end immediately after opening
binary
perform input and output in binary mode (as opposed to text mode)
in
open for input
noreplace
open in exclusive mode
out
open for output
trunc
truncate an existing stream when opening

31.5.2.2.5 Type ios_base​::​seekdir [ios.seekdir]

using seekdir = T4;
The type seekdir is an enumerated type ([enumerated.types]) that contains the elements indicated in Table 122.
Table 122: seekdir effects [tab:ios.seekdir]
Element
Meaning
beg
request a seek (for subsequent input or output) relative to the beginning of the stream
cur
request a seek relative to the current position within the sequence
end
request a seek relative to the current end of the sequence

31.5.2.2.6 Class ios_base​::​Init [ios.init]

namespace std { class ios_base::Init { public: Init(); Init(const Init&) = default; ~Init(); Init& operator=(const Init&) = default; private: static int init_cnt; // exposition only }; }
The class Init describes an object whose construction ensures the construction of the eight objects declared in <iostream> ([iostream.objects]) that associate file stream buffers with the standard C streams provided for by the functions declared in <cstdio>.
For the sake of exposition, the maintained data is presented here as:
  • static int init_cnt, counts the number of constructor and destructor calls for class Init, initialized to zero.
Init();
Effects: Constructs and initializes the objects cin, cout, cerr, clog, wcin, wcout, wcerr, and wclog if they have not already been constructed and initialized.
~Init();
Effects: If there are no other instances of the class still in existence, calls cout.flush(), cerr.flush(), clog.flush(), wcout.flush(), wcerr.flush(), wclog.flush().

31.5.2.3 State functions [fmtflags.state]

fmtflags flags() const;
Returns: The format control information for both input and output.
fmtflags flags(fmtflags fmtfl);
Postconditions: fmtfl == flags().
Returns: The previous value of flags().
fmtflags setf(fmtflags fmtfl);
Effects: Sets fmtfl in flags().
Returns: The previous value of flags().
fmtflags setf(fmtflags fmtfl, fmtflags mask);
Effects: Clears mask in flags(), sets fmtfl & mask in flags().
Returns: The previous value of flags().
void unsetf(fmtflags mask);
Effects: Clears mask in flags().
streamsize precision() const;
Returns: The precision to generate on certain output conversions.
streamsize precision(streamsize prec);
Postconditions: prec == precision().
Returns: The previous value of precision().
streamsize width() const;
Returns: The minimum field width (number of characters) to generate on certain output conversions.
streamsize width(streamsize wide);
Postconditions: wide == width().
Returns: The previous value of width().

31.5.2.4 Functions [ios.base.locales]

locale imbue(const locale& loc);
Effects: Calls each registered callback pair (fn, idx) ([ios.base.callback]) as (*fn)(imbue_event, *this, idx) at such a time that a call to ios_base​::​getloc() from within fn returns the new locale value loc.
Postconditions: loc == getloc().
Returns: The previous value of getloc().
locale getloc() const;
Returns: If no locale has been imbued, a copy of the global C++ locale, locale(), in effect at the time of construction.
Otherwise, returns the imbued locale, to be used to perform locale-dependent input and output operations.

31.5.2.5 Static members [ios.members.static]

static bool sync_with_stdio(bool sync = true);
Effects: If any input or output operation has occurred using the standard streams prior to the call, the effect is implementation-defined.
Otherwise, called with a false argument, it allows the standard streams to operate independently of the standard C streams.
Returns: true if the previous state of the standard iostream objects was synchronized and otherwise returns false.
The first time it is called, the function returns true.
Remarks: When a standard iostream object str is synchronized with a standard stdio stream f, the effect of inserting a character c by fputc(f, c); is the same as the effect of str.rdbuf()->sputc(c); for any sequences of characters; the effect of extracting a character c by c = fgetc(f); is the same as the effect of c = str.rdbuf()->sbumpc(); for any sequences of characters; and the effect of pushing back a character c by ungetc(c, f); is the same as the effect of str.rdbuf()->sputbackc(c); for any sequence of characters.268
268)268)
This implies that operations on a standard iostream object can be mixed arbitrarily with operations on the corresponding stdio stream.
In practical terms, synchronization usually means that a standard iostream object and a standard stdio object share a buffer.

31.5.2.6 Storage functions [ios.base.storage]

static int xalloc();
Returns: index ++.
Remarks: Concurrent access to this function by multiple threads does not result in a data race.
long& iword(int idx);
Preconditions: idx is a value obtained by a call to xalloc.
Effects: If iarray is a null pointer, allocates an array of long of unspecified size and stores a pointer to its first element in iarray.
The function then extends the array pointed at by iarray as necessary to include the element iarray[idx].
Each newly allocated element of the array is initialized to zero.
The reference returned is invalid after any other operation on the object.269
However, the value of the storage referred to is retained, so that until the next call to copyfmt, calling iword with the same index yields another reference to the same value.
If the function fails270 and *this is a base class subobject of a basic_ios<> object or subobject, the effect is equivalent to calling basic_ios<>​::​setstate(badbit) on the derived object (which may throw failure).
Returns: On success iarray[idx].
On failure, a valid long& initialized to 0.
void*& pword(int idx);
Preconditions: idx is a value obtained by a call to xalloc.
Effects: If parray is a null pointer, allocates an array of pointers to void of unspecified size and stores a pointer to its first element in parray.
The function then extends the array pointed at by parray as necessary to include the element parray[idx].
Each newly allocated element of the array is initialized to a null pointer.
The reference returned is invalid after any other operation on the object.
However, the value of the storage referred to is retained, so that until the next call to copyfmt, calling pword with the same index yields another reference to the same value.
If the function fails271 and *this is a base class subobject of a basic_ios<> object or subobject, the effect is equivalent to calling basic_ios<>​::​setstate(badbit) on the derived object (which may throw failure).
Returns: On success parray[idx].
On failure a valid void*& initialized to 0.
Remarks: After a subsequent call to pword(int) for the same object, the earlier return value may no longer be valid.
269)269)
An implementation is free to implement both the integer array pointed at by iarray and the pointer array pointed at by parray as sparse data structures, possibly with a one-element cache for each.
270)270)
For example, because it cannot allocate space.
271)271)
For example, because it cannot allocate space.

31.5.2.7 Callbacks [ios.base.callback]

void register_callback(event_callback fn, int idx);
Preconditions: The function fn does not throw exceptions.
Effects: Registers the pair (fn, idx) such that during calls to imbue() ([ios.base.locales]), copyfmt(), or ~ios_base() ([ios.base.cons]), the function fn is called with argument idx.
Functions registered are called when an event occurs, in opposite order of registration.
Functions registered while a callback function is active are not called until the next event.
Remarks: Identical pairs are not merged.
A function registered twice will be called twice.

31.5.2.8 Constructors and destructor [ios.base.cons]

ios_base();
Effects: Each ios_base member has an indeterminate value after construction.
The object's members shall be initialized by calling basic_ios​::​init before the object's first use or before it is destroyed, whichever comes first; otherwise the behavior is undefined.
~ios_base();
Effects: Calls each registered callback pair (fn, idx) ([ios.base.callback]) as (*fn)(​erase_event, *this, idx) at such time that any ios_base member function called from within fn has well-defined results.
Then, any memory obtained is deallocated.

31.5.3 Class template fpos [fpos]

namespace std { template<class stateT> class fpos { public: // [fpos.members], members stateT state() const; void state(stateT); private: stateT st; // exposition only }; }

31.5.3.1 Members [fpos.members]

void state(stateT s);
Effects: Assigns s to st.
stateT state() const;
Returns: Current value of st.

31.5.3.2 Requirements [fpos.operations]

An fpos type specifies file position information.
It holds a state object whose type is equal to the template parameter stateT.
Type stateT shall meet the Cpp17DefaultConstructible (Table 30), Cpp17CopyConstructible (Table 32), Cpp17CopyAssignable (Table 34), and Cpp17Destructible (Table 35) requirements.
If is_trivially_copy_constructible_v<stateT> is true, then fpos<stateT> has a trivial copy constructor.
If is_trivially_copy_assignable_v<stateT> is true, then fpos<stateT> has a trivial copy assignment operator.
If is_trivially_destructible_v<stateT> is true, then fpos<stateT> has a trivial destructor.
All specializations of fpos meet the Cpp17DefaultConstructible, Cpp17CopyConstructible, Cpp17CopyAssignable, Cpp17Destructible, and Cpp17EqualityComparable (Table 28) requirements.
In addition, the expressions shown in Table 123 are valid and have the indicated semantics.
In that table,
  • P refers to a specialization of fpos,
  • p and q refer to values of type P or const P,
  • pl and ql refer to modifiable lvalues of type P,
  • O refers to type streamoff, and
  • o and o2 refer to values of type streamoff or const streamoff.
Table 123: Position type requirements [tab:fpos.operations]
Expression
Return type
Operational
Assertion/note
semantics
pre-/post-condition
P(o)
P
converts from offset
Effects: Value-initializes the state object.
P p(o);
P p = o;
Effects: Value-initializes the state object.

Postconditions: p == P(o) is true.
P()
P
P(0)
P p;
P p(0);
O(p)
streamoff
converts to offset
P(O(p)) == p
p == q
bool
Remarks: For any two values o and o2, if p is obtained from o converted to P or from a copy of such P value and if q is obtained from o2 converted to P or from a copy of such P value, then p == q is true only if o == o2 is true.
p != q
bool
!(p == q)
p + o
P
+ offset
Remarks: With ql = p + o;, then: ql - o == p
pl += o
P&
+= offset
Remarks: With ql = pl; before the +=, then: pl - o == ql
p - o
P
- offset
Remarks: With ql = p - o;, then: ql + o == p
pl -= o
P&
-= offset
Remarks: With ql = pl; before the -=, then: pl + o == ql
o + p
convertible to P
p + o
P(o + p) == p + o
p - q
streamoff
distance
p == q + (p - q)
Stream operations that return a value of type traits​::​pos_type return P(O(-1)) as an invalid value to signal an error.
If this value is used as an argument to any istream, ostream, or streambuf member that accepts a value of type traits​::​pos_type then the behavior of that function is undefined.

31.5.4 Class template basic_ios [ios]

31.5.4.1 Overview [ios.overview]

namespace std { template<class charT, class traits = char_traits<charT>> class basic_ios : public ios_base { public: using char_type = charT; using int_type = typename traits::int_type; using pos_type = typename traits::pos_type; using off_type = typename traits::off_type; using traits_type = traits; // [iostate.flags], flags functions explicit operator bool() const; bool operator!() const; iostate rdstate() const; void clear(iostate state = goodbit); void setstate(iostate state); bool good() const; bool eof() const; bool fail() const; bool bad() const; iostate exceptions() const; void exceptions(iostate except); // [basic.ios.cons], constructor/destructor explicit basic_ios(basic_streambuf<charT, traits>* sb); virtual ~basic_ios(); // [basic.ios.members], members basic_ostream<charT, traits>* tie() const; basic_ostream<charT, traits>* tie(basic_ostream<charT, traits>* tiestr); basic_streambuf<charT, traits>* rdbuf() const; basic_streambuf<charT, traits>* rdbuf(basic_streambuf<charT, traits>* sb); basic_ios& copyfmt(const basic_ios& rhs); char_type fill() const; char_type fill(char_type ch); locale imbue(const locale& loc); char narrow(char_type c, char dfault) const; char_type widen(char c) const; basic_ios(const basic_ios&) = delete; basic_ios& operator=(const basic_ios&) = delete; protected: basic_ios(); void init(basic_streambuf<charT, traits>* sb); void move(basic_ios& rhs); void move(basic_ios&& rhs); void swap(basic_ios& rhs) noexcept; void set_rdbuf(basic_streambuf<charT, traits>* sb); }; }

31.5.4.2 Constructors [basic.ios.cons]

explicit basic_ios(basic_streambuf<charT, traits>* sb);
Effects: Assigns initial values to its member objects by calling init(sb).
basic_ios();
Effects: Leaves its member objects uninitialized.
The object shall be initialized by calling basic_ios​::​init before its first use or before it is destroyed, whichever comes first; otherwise the behavior is undefined.
~basic_ios();
Remarks: The destructor does not destroy rdbuf().
void init(basic_streambuf<charT, traits>* sb);
Postconditions: The postconditions of this function are indicated in Table 124.
Table 124: basic_ios​::​init() effects [tab:basic.ios.cons]
Element
Value
rdbuf()
sb
tie()
0
rdstate()
goodbit if sb is not a null pointer, otherwise badbit.
exceptions()
goodbit
flags()
skipws | dec
width()
0
precision()
6
fill()
widen(' ')
getloc()
a copy of the value returned by locale()
iarray
a null pointer
parray
a null pointer

31.5.4.3 Member functions [basic.ios.members]

basic_ostream<charT, traits>* tie() const;
Returns: An output sequence that is tied to (synchronized with) the sequence controlled by the stream buffer.
basic_ostream<charT, traits>* tie(basic_ostream<charT, traits>* tiestr);
Preconditions: If tiestr is not null, tiestr is not reachable by traversing the linked list of tied stream objects starting from tiestr->tie().
Postconditions: tiestr == tie().
Returns: The previous value of tie().
basic_streambuf<charT, traits>* rdbuf() const;
Returns: A pointer to the streambuf associated with the stream.
basic_streambuf<charT, traits>* rdbuf(basic_streambuf<charT, traits>* sb);
Effects: Calls clear().
Postconditions: sb == rdbuf().
Returns: The previous value of rdbuf().
locale imbue(const locale& loc);
Effects: Calls ios_base​::​imbue(loc) and if rdbuf() != 0 then rdbuf()->pubimbue(loc).
Returns: The prior value of ios_base​::​imbue().
char narrow(char_type c, char dfault) const;
Returns: use_facet<ctype<char_type>>(getloc()).narrow(c, dfault)
char_type widen(char c) const;
Returns: use_facet<ctype<char_type>>(getloc()).widen(c)
char_type fill() const;
Returns: The character used to pad (fill) an output conversion to the specified field width.
char_type fill(char_type fillch);
Postconditions: traits​::​eq(fillch, fill()).
Returns: The previous value of fill().
basic_ios& copyfmt(const basic_ios& rhs);
Effects: If (this == addressof(rhs)) is true does nothing.
Otherwise assigns to the member objects of *this the corresponding member objects of rhs as follows:
  • calls each registered callback pair (fn, idx) as (*fn)(erase_event, *this, idx);
  • then, assigns to the member objects of *this the corresponding member objects of rhs, except that
    • rdstate(), rdbuf(), and exceptions() are left unchanged;
    • the contents of arrays pointed at by pword and iword are copied, not the pointers themselves;272 and
    • if any newly stored pointer values in *this point at objects stored outside the object rhs and those objects are destroyed when rhs is destroyed, the newly stored pointer values are altered to point at newly constructed copies of the objects;
  • then, calls each callback pair that was copied from rhs as (*fn)(copyfmt_event, *this, idx);
  • then, calls exceptions(rhs.exceptions()).
[Note 1: 
The second pass through the callback pairs permits a copied pword value to be zeroed, or to have its referent deep copied or reference counted, or to have other special action taken.
β€” end note]
Postconditions: The postconditions of this function are indicated in Table 125.
Table 125: basic_ios​::​copyfmt() effects [tab:basic.ios.copyfmt]
Element
Value
rdbuf()
unchanged
tie()
rhs.tie()
rdstate()
unchanged
exceptions()
rhs.exceptions()
flags()
rhs.flags()
width()
rhs.width()
precision()
rhs.precision()
fill()
rhs.fill()
getloc()
rhs.getloc()
Returns: *this.
void move(basic_ios& rhs); void move(basic_ios&& rhs);
Postconditions: *this has the state that rhs had before the function call, except that rdbuf() returns nullptr.
rhs is in a valid but unspecified state, except that rhs.rdbuf() returns the same value as it returned before the function call, and rhs.tie() returns nullptr.
void swap(basic_ios& rhs) noexcept;
Effects: The states of *this and rhs are exchanged, except that rdbuf() returns the same value as it returned before the function call, and rhs.rdbuf() returns the same value as it returned before the function call.
void set_rdbuf(basic_streambuf<charT, traits>* sb);
Preconditions: sb != nullptr is true.
Effects: Associates the basic_streambuf object pointed to by sb with this stream without calling clear().
Postconditions: rdbuf() == sb is true.
Throws: Nothing.
272)272)
This suggests an infinite amount of copying, but the implementation can keep track of the maximum element of the arrays that is nonzero.

31.5.4.4 Flags functions [iostate.flags]

explicit operator bool() const;
Returns: !fail().
bool operator!() const;
Returns: fail().
iostate rdstate() const;
Returns: The error state of the stream buffer.
void clear(iostate state = goodbit);
Effects: If ((state | (rdbuf() ? goodbit : badbit)) & exceptions()) == 0, returns.
Otherwise, the function throws an object of class ios_base​::​failure ([ios.failure]), constructed with implementation-defined argument values.
Postconditions: If rdbuf() != 0 then state == rdstate(); otherwise rdstate() == (state | ios_base​::​badbit).
void setstate(iostate state);
Effects: Calls clear(rdstate() | state) (which may throw ios_base​::​failure ([ios.failure])).
bool good() const;
Returns: rdstate() == 0
bool eof() const;
Returns: true if eofbit is set in rdstate().
bool fail() const;
Returns: true if failbit or badbit is set in rdstate().273
bool bad() const;
Returns: true if badbit is set in rdstate().
iostate exceptions() const;
Returns: A mask that determines what elements set in rdstate() cause exceptions to be thrown.
void exceptions(iostate except);
Effects: Calls clear(rdstate()).
Postconditions: except == exceptions().
273)273)
Checking badbit also for fail() is historical practice.

31.5.5 ios_base manipulators [std.ios.manip]

31.5.5.1 fmtflags manipulators [fmtflags.manip]

Each function specified in this subclause is a designated addressable function ([namespace.std]).
ios_base& boolalpha(ios_base& str);
Effects: Calls str.setf(ios_base​::​boolalpha).
Returns: str.
ios_base& noboolalpha(ios_base& str);
Effects: Calls str.unsetf(ios_base​::​boolalpha).
Returns: str.
ios_base& showbase(ios_base& str);
Effects: Calls str.setf(ios_base​::​showbase).
Returns: str.
ios_base& noshowbase(ios_base& str);
Effects: Calls str.unsetf(ios_base​::​showbase).
Returns: str.
ios_base& showpoint(ios_base& str);
Effects: Calls str.setf(ios_base​::​showpoint).
Returns: str.
ios_base& noshowpoint(ios_base& str);
Effects: Calls str.unsetf(ios_base​::​showpoint).
Returns: str.
ios_base& showpos(ios_base& str);
Effects: Calls str.setf(ios_base​::​showpos).
Returns: str.
ios_base& noshowpos(ios_base& str);
Effects: Calls str.unsetf(ios_base​::​showpos).
Returns: str.
ios_base& skipws(ios_base& str);
Effects: Calls str.setf(ios_base​::​skipws).
Returns: str.
ios_base& noskipws(ios_base& str);
Effects: Calls str.unsetf(ios_base​::​skipws).
Returns: str.
ios_base& uppercase(ios_base& str);
Effects: Calls str.setf(ios_base​::​uppercase).
Returns: str.
ios_base& nouppercase(ios_base& str);
Effects: Calls str.unsetf(ios_base​::​uppercase).
Returns: str.
ios_base& unitbuf(ios_base& str);
Effects: Calls str.setf(ios_base​::​unitbuf).
Returns: str.
ios_base& nounitbuf(ios_base& str);
Effects: Calls str.unsetf(ios_base​::​unitbuf).
Returns: str.

31.5.5.2 adjustfield manipulators [adjustfield.manip]

Each function specified in this subclause is a designated addressable function ([namespace.std]).
ios_base& internal(ios_base& str);
Effects: Calls str.setf(ios_base​::​internal, ios_base​::​adjustfield).
Returns: str.
ios_base& left(ios_base& str);
Effects: Calls str.setf(ios_base​::​left, ios_base​::​adjustfield).
Returns: str.
ios_base& right(ios_base& str);
Effects: Calls str.setf(ios_base​::​right, ios_base​::​adjustfield).
Returns: str.

31.5.5.3 basefield manipulators [basefield.manip]

Each function specified in this subclause is a designated addressable function ([namespace.std]).
ios_base& dec(ios_base& str);
Effects: Calls str.setf(ios_base​::​dec, ios_base​::​basefield).
Returns: str.274
ios_base& hex(ios_base& str);
Effects: Calls str.setf(ios_base​::​hex, ios_base​::​basefield).
Returns: str.
ios_base& oct(ios_base& str);
Effects: Calls str.setf(ios_base​::​oct, ios_base​::​basefield).
Returns: str.
274)274)
The function signature dec(ios_base&) can be called by the function signature basic_ostream& stream​::​operator<<(ios_base& (*)(ios_base&)) to permit expressions of the form cout << dec to change the format flags stored in cout.

31.5.5.4 floatfield manipulators [floatfield.manip]

Each function specified in this subclause is a designated addressable function ([namespace.std]).
ios_base& fixed(ios_base& str);
Effects: Calls str.setf(ios_base​::​fixed, ios_base​::​floatfield).
Returns: str.
ios_base& scientific(ios_base& str);
Effects: Calls str.setf(ios_base​::​scientific, ios_base​::​floatfield).
Returns: str.
ios_base& hexfloat(ios_base& str);
Effects: Calls str.setf(ios_base​::​fixed | ios_base​::​scientific, ios_base​::​floatfield).
Returns: str.
[Note 1: 
ios_base​::​hex cannot be used to specify a hexadecimal floating-point format, because it is not part of ios_base​::​floatfield (Table 119).
β€” end note]
ios_base& defaultfloat(ios_base& str);
Effects: Calls str.unsetf(ios_base​::​floatfield).
Returns: str.

31.5.6 Error reporting [error.reporting]

error_code make_error_code(io_errc e) noexcept;
Returns: error_code(static_cast<int>(e), iostream_category()).
error_condition make_error_condition(io_errc e) noexcept;
Returns: error_condition(static_cast<int>(e), iostream_category()).
const error_category& iostream_category() noexcept;
Returns: A reference to an object of a type derived from class error_category.
The object's default_error_condition and equivalent virtual functions shall behave as specified for the class error_category.
The object's name virtual function shall return a pointer to the string "iostream".

31.6 Stream buffers [stream.buffers]

31.6.1 Header <streambuf> synopsis [streambuf.syn]

namespace std { template<class charT, class traits = char_traits<charT>> class basic_streambuf; using streambuf = basic_streambuf<char>; using wstreambuf = basic_streambuf<wchar_t>; }
The header <streambuf> defines types that control input from and output to character sequences.

31.6.2 Stream buffer requirements [streambuf.reqts]

Stream buffers can impose various constraints on the sequences they control.
Some constraints are:
  • The controlled input sequence can be not readable.
  • The controlled output sequence can be not writable.
  • The controlled sequences can be associated with the contents of other representations for character sequences, such as external files.
  • The controlled sequences can support operations directly to or from associated sequences.
  • The controlled sequences can impose limitations on how the program can read characters from a sequence, write characters to a sequence, put characters back into an input sequence, or alter the stream position.
Each sequence is characterized by three pointers which, if non-null, all point into the same charT array object.
The array object represents, at any moment, a (sub)sequence of characters from the sequence.
Operations performed on a sequence alter the values stored in these pointers, perform reads and writes directly to or from associated sequences, and alter β€œthe stream position” and conversion state as needed to maintain this subsequence relationship.
The three pointers are:
  • the beginning pointer, or lowest element address in the array (called xbeg here);
  • the next pointer, or next element address that is a current candidate for reading or writing (called xnext here);
  • the end pointer, or first element address beyond the end of the array (called xend here).
The following semantic constraints shall always apply for any set of three pointers for a sequence, using the pointer names given immediately above:
  • If xnext is not a null pointer, then xbeg and xend shall also be non-null pointers into the same charT array, as described above; otherwise, xbeg and xend shall also be null.
  • If xnext is not a null pointer and xnext < xend for an output sequence, then a write position is available.
    In this case, *xnext shall be assignable as the next element to write (to put, or to store a character value, into the sequence).
  • If xnext is not a null pointer and xbeg < xnext for an input sequence, then a putback position is available.
    In this case, xnext[-1] shall have a defined value and is the next (preceding) element to store a character that is put back into the input sequence.
  • If xnext is not a null pointer and xnext < xend for an input sequence, then a read position is available.
    In this case, *xnext shall have a defined value and is the next element to read (to get, or to obtain a character value, from the sequence).

31.6.3 Class template basic_streambuf [streambuf]

31.6.3.1 General [streambuf.general]

namespace std { template<class charT, class traits = char_traits<charT>> class basic_streambuf { public: using char_type = charT; using int_type = typename traits::int_type; using pos_type = typename traits::pos_type; using off_type = typename traits::off_type; using traits_type = traits; virtual ~basic_streambuf(); // [streambuf.locales], locales locale pubimbue(const locale& loc); locale getloc() const; // [streambuf.buffer], buffer and positioning basic_streambuf* pubsetbuf(char_type* s, streamsize n); pos_type pubseekoff(off_type off, ios_base::seekdir way, ios_base::openmode which = ios_base::in | ios_base::out); pos_type pubseekpos(pos_type sp, ios_base::openmode which = ios_base::in | ios_base::out); int pubsync(); // get and put areas // [streambuf.pub.get], get area streamsize in_avail(); int_type snextc(); int_type sbumpc(); int_type sgetc(); streamsize sgetn(char_type* s, streamsize n); // [streambuf.pub.pback], putback int_type sputbackc(char_type c); int_type sungetc(); // [streambuf.pub.put], put area int_type sputc(char_type c); streamsize sputn(const char_type* s, streamsize n); protected: basic_streambuf(); basic_streambuf(const basic_streambuf& rhs); basic_streambuf& operator=(const basic_streambuf& rhs); void swap(basic_streambuf& rhs); // [streambuf.get.area], get area access char_type* eback() const; char_type* gptr() const; char_type* egptr() const; void gbump(int n); void setg(char_type* gbeg, char_type* gnext, char_type* gend); // [streambuf.put.area], put area access char_type* pbase() const; char_type* pptr() const; char_type* epptr() const; void pbump(int n); void setp(char_type* pbeg, char_type* pend); // [streambuf.virtuals], virtual functions // [streambuf.virt.locales], locales virtual void imbue(const locale& loc); // [streambuf.virt.buffer], buffer management and positioning virtual basic_streambuf* setbuf(char_type* s, streamsize n); virtual pos_type seekoff(off_type off, ios_base::seekdir way, ios_base::openmode which = ios_base::in | ios_base::out); virtual pos_type seekpos(pos_type sp, ios_base::openmode which = ios_base::in | ios_base::out); virtual int sync(); // [streambuf.virt.get], get area virtual streamsize showmanyc(); virtual streamsize xsgetn(char_type* s, streamsize n); virtual int_type underflow(); virtual int_type uflow(); // [streambuf.virt.pback], putback virtual int_type pbackfail(int_type c = traits::eof()); // [streambuf.virt.put], put area virtual streamsize xsputn(const char_type* s, streamsize n); virtual int_type overflow(int_type c = traits::eof()); }; }
The class template basic_streambuf serves as a base class for deriving various stream buffers whose objects each control two character sequences:

31.6.3.2 Constructors [streambuf.cons]

basic_streambuf();
Effects: Initializes:275
  • all pointer member objects to null pointers,
  • the getloc() member to a copy of the global locale, locale(), at the time of construction.
Remarks: Once the getloc() member is initialized, results of calling locale member functions, and of members of facets so obtained, can safely be cached until the next time the member imbue is called.
basic_streambuf(const basic_streambuf& rhs);
Postconditions:
  • eback() == rhs.eback()
  • gptr() == rhs.gptr()
  • egptr() == rhs.egptr()
  • pbase() == rhs.pbase()
  • pptr() == rhs.pptr()
  • epptr() == rhs.epptr()
  • getloc() == rhs.getloc()
~basic_streambuf();
Effects: None.
275)275)
The default constructor is protected for class basic_streambuf to assure that only objects for classes derived from this class can be constructed.

31.6.3.3 Public member functions [streambuf.members]

31.6.3.3.1 Locales [streambuf.locales]

locale pubimbue(const locale& loc);
Effects: Calls imbue(loc).
Postconditions: loc == getloc().
Returns: Previous value of getloc().
locale getloc() const;
Returns: If pubimbue() has ever been called, then the last value of loc supplied, otherwise the current global locale, locale(), in effect at the time of construction.
If called after pubimbue() has been called but before pubimbue has returned (i.e., from within the call of imbue()) then it returns the previous value.

31.6.3.3.2 Buffer management and positioning [streambuf.buffer]

basic_streambuf* pubsetbuf(char_type* s, streamsize n);
Returns: setbuf(s, n).
pos_type pubseekoff(off_type off, ios_base::seekdir way, ios_base::openmode which = ios_base::in | ios_base::out);
Returns: seekoff(off, way, which).
pos_type pubseekpos(pos_type sp, ios_base::openmode which = ios_base::in | ios_base::out);
Returns: seekpos(sp, which).
int pubsync();
Returns: sync().

31.6.3.3.3 Get area [streambuf.pub.get]

streamsize in_avail();
Returns: If a read position is available, returns egptr() - gptr().
Otherwise returns showmanyc().
int_type snextc();
Effects: Calls sbumpc().
Returns: If that function returns traits​::​eof(), returns traits​::​eof().
Otherwise, returns sgetc().
int_type sbumpc();
Effects: If the input sequence read position is not available, returns uflow().
Otherwise, returns traits​::​to_int_type(*gptr()) and increments the next pointer for the input sequence.
int_type sgetc();
Returns: If the input sequence read position is not available, returns underflow().
Otherwise, returns traits​::​to_int_type(*gptr()).
streamsize sgetn(char_type* s, streamsize n);
Returns: xsgetn(s, n).

31.6.3.3.4 Putback [streambuf.pub.pback]

int_type sputbackc(char_type c);
Effects: If the input sequence putback position is not available, or if traits​::​eq(c, gptr()[-1]) is false, returns pbackfail(traits​::​to_int_type(c)).
Otherwise, decrements the next pointer for the input sequence and returns traits​::​to_int_type(*gptr()).
int_type sungetc();
Effects: If the input sequence putback position is not available, returns pbackfail().
Otherwise, decrements the next pointer for the input sequence and returns traits​::​to_int_type(*gptr()).

31.6.3.3.5 Put area [streambuf.pub.put]

int_type sputc(char_type c);
Effects: If the output sequence write position is not available, returns overflow(traits​::​to_int_type(c)).
Otherwise, stores c at the next pointer for the output sequence, increments the pointer, and returns traits​::​to_int_type(c).
streamsize sputn(const char_type* s, streamsize n);
Returns: xsputn(s, n).

31.6.3.4 Protected member functions [streambuf.protected]

31.6.3.4.1 Assignment [streambuf.assign]

basic_streambuf& operator=(const basic_streambuf& rhs);
Postconditions:
  • eback() == rhs.eback()
  • gptr() == rhs.gptr()
  • egptr() == rhs.egptr()
  • pbase() == rhs.pbase()
  • pptr() == rhs.pptr()
  • epptr() == rhs.epptr()
  • getloc() == rhs.getloc()
Returns: *this.
void swap(basic_streambuf& rhs);
Effects: Swaps the data members of rhs and *this.

31.6.3.4.2 Get area access [streambuf.get.area]

char_type* eback() const;
Returns: The beginning pointer for the input sequence.
char_type* gptr() const;
Returns: The next pointer for the input sequence.
char_type* egptr() const;
Returns: The end pointer for the input sequence.
void gbump(int n);
Effects: Adds n to the next pointer for the input sequence.
void setg(char_type* gbeg, char_type* gnext, char_type* gend);
Postconditions: gbeg == eback(), gnext == gptr(), and gend == egptr() are all true.

31.6.3.4.3 Put area access [streambuf.put.area]

char_type* pbase() const;
Returns: The beginning pointer for the output sequence.
char_type* pptr() const;
Returns: The next pointer for the output sequence.
char_type* epptr() const;
Returns: The end pointer for the output sequence.
void pbump(int n);
Effects: Adds n to the next pointer for the output sequence.
void setp(char_type* pbeg, char_type* pend);
Postconditions: pbeg == pbase(), pbeg == pptr(), and pend == epptr() are all true.

31.6.3.5 Virtual functions [streambuf.virtuals]

31.6.3.5.1 Locales [streambuf.virt.locales]

void imbue(const locale&);
Effects: Change any translations based on locale.
Remarks: Allows the derived class to be informed of changes in locale at the time they occur.
Between invocations of this function a class derived from streambuf can safely cache results of calls to locale functions and to members of facets so obtained.
Default behavior: Does nothing.

31.6.3.5.2 Buffer management and positioning [streambuf.virt.buffer]

basic_streambuf* setbuf(char_type* s, streamsize n);
Effects: Influences stream buffering in a way that is defined separately for each class derived from basic_streambuf in this Clause ([stringbuf.virtuals], [filebuf.virtuals]).
Default behavior: Does nothing.
Returns this.
pos_type seekoff(off_type off, ios_base::seekdir way, ios_base::openmode which = ios_base::in | ios_base::out);
Effects: Alters the stream positions within one or more of the controlled sequences in a way that is defined separately for each class derived from basic_streambuf in this Clause ([stringbuf.virtuals], [filebuf.virtuals]).
Default behavior: Returns pos_type(off_type(-1)).
pos_type seekpos(pos_type sp, ios_base::openmode which = ios_base::in | ios_base::out);
Effects: Alters the stream positions within one or more of the controlled sequences in a way that is defined separately for each class derived from basic_streambuf in this Clause ([stringbuf], [filebuf]).
Default behavior: Returns pos_type(off_type(-1)).
int sync();
Effects: Synchronizes the controlled sequences with the arrays.
That is, if pbase() is non-null the characters between pbase() and pptr() are written to the controlled sequence.
The pointers may then be reset as appropriate.
Returns: -1 on failure.
What constitutes failure is determined by each derived class ([filebuf.virtuals]).
Default behavior: Returns zero.

31.6.3.5.3 Get area [streambuf.virt.get]

streamsize showmanyc();276
Returns: An estimate of the number of characters available in the sequence, or .
If it returns a positive value, then successive calls to underflow() will not return traits​::​eof() until at least that number of characters have been extracted from the stream.
If showmanyc() returns , then calls to underflow() or uflow() will fail.277
Default behavior: Returns zero.
Remarks: Uses traits​::​eof().
streamsize xsgetn(char_type* s, streamsize n);
Effects: Assigns up to n characters to successive elements of the array whose first element is designated by s.
The characters assigned are read from the input sequence as if by repeated calls to sbumpc().
Assigning stops when either n characters have been assigned or a call to sbumpc() would return traits​::​eof().
Returns: The number of characters assigned.278
Remarks: Uses traits​::​eof().
int_type underflow();
The pending sequence of characters is defined as the concatenation of
  • the empty sequence if gptr() is null, otherwise the characters in [gptr(), egptr()), followed by
  • some (possibly empty) sequence of characters read from the input sequence.
The result character is the first character of the pending sequence if it is non-empty, otherwise the next character that would be read from the input sequence.
The backup sequence is the empty sequence if eback() is null, otherwise the characters in [eback(), gptr()).
Effects: The function sets up the gptr() and egptr() such that if the pending sequence is non-empty, then egptr() is non-null and the characters in [gptr(), egptr()) are the characters in the pending sequence, otherwise either gptr() is null or gptr() == egptr().
If eback() and gptr() are non-null then the function is not constrained as to their contents, but the β€œusual backup condition” is that either
  • the backup sequence contains at least gptr() - eback() characters, in which case the characters in [eback(), gptr()) agree with the last gptr() - eback() characters of the backup sequence, or
  • the characters in [gptr() - n, gptr()) agree with the backup sequence (where n is the length of the backup sequence).
Returns: traits​::​to_int_type(c), where c is the first character of the pending sequence, without moving the input sequence position past it.
If the pending sequence is null then the function returns traits​::​eof() to indicate failure.
Default behavior: Returns traits​::​eof().
Remarks: The public members of basic_streambuf call this virtual function only if gptr() is null or gptr() >= egptr().
int_type uflow();
Preconditions: The constraints are the same as for underflow(), except that the result character is transferred from the pending sequence to the backup sequence, and the pending sequence is not empty before the transfer.
Default behavior: Calls underflow().
If underflow() returns traits​::​eof(), returns traits​::​eof().
Otherwise, returns the value of traits​::​to_int_type(*gptr()) and increments the value of the next pointer for the input sequence.
Returns: traits​::​eof() to indicate failure.
276)276)
The morphemes of showmanyc are β€œes-how-many-see”, not β€œshow-manic”.
277)277)
underflow or uflow can fail by throwing an exception prematurely.
The intention is not only that the calls will not return eof() but that they will return β€œimmediately”.
278)278)
Classes derived from basic_streambuf can provide more efficient ways to implement xsgetn() and xsputn() by overriding these definitions from the base class.

31.6.3.5.4 Putback [streambuf.virt.pback]

int_type pbackfail(int_type c = traits::eof());
The pending sequence is defined as for underflow(), with the modifications that
  • If traits​::​eq_int_type(c, traits​::​eof()) returns true, then the input sequence is backed up one character before the pending sequence is determined.
  • If traits​::​eq_int_type(c, traits​::​eof()) returns false, then c is prepended.
    Whether the input sequence is backed up or modified in any other way is unspecified.
Postconditions: On return, the constraints of gptr(), eback(), and pptr() are the same as for underflow().
Returns: traits​::​eof() to indicate failure.
Failure may occur because the input sequence could not be backed up, or if for some other reason the pointers cannot be set consistent with the constraints.
pbackfail() is called only when put back has really failed.
Returns some value other than traits​::​eof() to indicate success.
Default behavior: Returns traits​::​eof().
Remarks: The public functions of basic_streambuf call this virtual function only when gptr() is null, gptr() == eback(), or traits​::​eq(traits​::​to_char_type(c), gptr()[-1]) returns false.
Other calls shall also satisfy that constraint.

31.6.3.5.5 Put area [streambuf.virt.put]

streamsize xsputn(const char_type* s, streamsize n);
Effects: Writes up to n characters to the output sequence as if by repeated calls to sputc(c).
The characters written are obtained from successive elements of the array whose first element is designated by s.
Writing stops when either n characters have been written or a call to sputc(c) would return traits​::​eof().
It is unspecified whether the function calls overflow() when pptr() == epptr() becomes true or whether it achieves the same effects by other means.
Returns: The number of characters written.
int_type overflow(int_type c = traits::eof());
Effects: Consumes some initial subsequence of the characters of the pending sequence.
The pending sequence is defined as the concatenation of
  • the empty sequence if pbase() is null, otherwise the pptr() - pbase() characters beginning at pbase(), followed by
  • the empty sequence if traits​::​eq_int_type(c, traits​::​eof()) returns true, otherwise the sequence consisting of c.
Preconditions: Every overriding definition of this virtual function obeys the following constraints:
  • The effect of consuming a character on the associated output sequence is specified.279
  • Let r be the number of characters in the pending sequence not consumed.
    If r is nonzero then pbase() and pptr() are set so that: pptr() - pbase() == r and the r characters starting at pbase() are the associated output stream.
    In case r is zero (all characters of the pending sequence have been consumed) then either pbase() is set to nullptr, or pbase() and pptr() are both set to the same non-null value.
  • The function may fail if either appending some character to the associated output stream fails or if it is unable to establish pbase() and pptr() according to the above rules.
Returns: traits​::​eof() or throws an exception if the function fails.
Otherwise, returns some value other than traits​::​eof() to indicate success.280
Default behavior: Returns traits​::​eof().
Remarks: The member functions sputc() and sputn() call this function in case that no room can be found in the put buffer enough to accommodate the argument character sequence.
279)279)
That is, for each class derived from a specialization of basic_streambuf in this Clause ([stringbuf], [filebuf]), a specification of how consuming a character effects the associated output sequence is given.
There is no requirement on a program-defined class.
280)280)
Typically, overflow returns c to indicate success, except when traits​::​eq_int_type(c, traits​::​eof()) returns true, in which case it returns traits​::​not_eof(c).

31.7 Formatting and manipulators [iostream.format]

31.7.1 Header <istream> synopsis [istream.syn]

namespace std { template<class charT, class traits = char_traits<charT>> class basic_istream; using istream = basic_istream<char>; using wistream = basic_istream<wchar_t>; template<class charT, class traits = char_traits<charT>> class basic_iostream; using iostream = basic_iostream<char>; using wiostream = basic_iostream<wchar_t>; template<class charT, class traits> basic_istream<charT, traits>& ws(basic_istream<charT, traits>& is); template<class Istream, class T> Istream&& operator>>(Istream&& is, T&& x); }

31.7.2 Header <ostream> synopsis [ostream.syn]

namespace std { template<class charT, class traits = char_traits<charT>> class basic_ostream; using ostream = basic_ostream<char>; using wostream = basic_ostream<wchar_t>; template<class charT, class traits> basic_ostream<charT, traits>& endl(basic_ostream<charT, traits>& os); template<class charT, class traits> basic_ostream<charT, traits>& ends(basic_ostream<charT, traits>& os); template<class charT, class traits> basic_ostream<charT, traits>& flush(basic_ostream<charT, traits>& os); template<class charT, class traits> basic_ostream<charT, traits>& emit_on_flush(basic_ostream<charT, traits>& os); template<class charT, class traits> basic_ostream<charT, traits>& noemit_on_flush(basic_ostream<charT, traits>& os); template<class charT, class traits> basic_ostream<charT, traits>& flush_emit(basic_ostream<charT, traits>& os); template<class Ostream, class T> Ostream&& operator<<(Ostream&& os, const T& x); // [ostream.formatted.print], print functions template<class... Args> void print(ostream& os, format_string<Args...> fmt, Args&&... args); template<class... Args> void println(ostream& os, format_string<Args...> fmt, Args&&... args); void vprint_unicode(ostream& os, string_view fmt, format_args args); void vprint_nonunicode(ostream& os, string_view fmt, format_args args); }

31.7.3 Header <iomanip> synopsis [iomanip.syn]

namespace std { unspecified resetiosflags(ios_base::fmtflags mask); unspecified setiosflags (ios_base::fmtflags mask); unspecified setbase(int base); template<class charT> unspecified setfill(charT c); unspecified setprecision(int n); unspecified setw(int n); template<class moneyT> unspecified get_money(moneyT& mon, bool intl = false); template<class moneyT> unspecified put_money(const moneyT& mon, bool intl = false); template<class charT> unspecified get_time(tm* tmb, const charT* fmt); template<class charT> unspecified put_time(const tm* tmb, const charT* fmt); template<class charT> unspecified quoted(const charT* s, charT delim = charT('"'), charT escape = charT('\\')); template<class charT, class traits, class Allocator> unspecified quoted(const basic_string<charT, traits, Allocator>& s, charT delim = charT('"'), charT escape = charT('\\')); template<class charT, class traits, class Allocator> unspecified quoted(basic_string<charT, traits, Allocator>& s, charT delim = charT('"'), charT escape = charT('\\')); template<class charT, class traits> unspecified quoted(basic_string_view<charT, traits> s, charT delim = charT('"'), charT escape = charT('\\')); }

31.7.4 Header <print> synopsis [print.syn]

namespace std { // [print.fun], print functions template<class... Args> void print(format_string<Args...> fmt, Args&&... args); template<class... Args> void print(FILE* stream, format_string<Args...> fmt, Args&&... args); template<class... Args> void println(format_string<Args...> fmt, Args&&... args); template<class... Args> void println(FILE* stream, format_string<Args...> fmt, Args&&... args); void vprint_unicode(string_view fmt, format_args args); void vprint_unicode(FILE* stream, string_view fmt, format_args args); void vprint_nonunicode(string_view fmt, format_args args); void vprint_nonunicode(FILE* stream, string_view fmt, format_args args); }

31.7.5 Input streams [input.streams]

31.7.5.1 General [input.streams.general]

The header <istream> defines two class templates and a function template that control input from a stream buffer, along with a function template that extracts from stream rvalues.

31.7.5.2 Class template basic_istream [istream]

31.7.5.2.1 General [istream.general]

When a function is specified with a type placeholder of extended-floating-point-type, the implementation provides overloads for all cv-unqualified extended floating-point types ([basic.fundamental]) in lieu of extended-floating-​point-type.
namespace std { template<class charT, class traits = char_traits<charT>> class basic_istream : virtual public basic_ios<charT, traits> { public: // types (inherited from basic_ios) using char_type = charT; using int_type = typename traits::int_type; using pos_type = typename traits::pos_type; using off_type = typename traits::off_type; using traits_type = traits; // [istream.cons], constructor/destructor explicit basic_istream(basic_streambuf<charT, traits>* sb); virtual ~basic_istream(); // [istream.sentry], prefix/suffix class sentry; // [istream.formatted], formatted input basic_istream& operator>>(basic_istream& (*pf)(basic_istream&)); basic_istream& operator>>(basic_ios<charT, traits>& (*pf)(basic_ios<charT, traits>&)); basic_istream& operator>>(ios_base& (*pf)(ios_base&)); basic_istream& operator>>(bool& n); basic_istream& operator>>(short& n); basic_istream& operator>>(unsigned short& n); basic_istream& operator>>(int& n); basic_istream& operator>>(unsigned int& n); basic_istream& operator>>(long& n); basic_istream& operator>>(unsigned long& n); basic_istream& operator>>(long long& n); basic_istream& operator>>(unsigned long long& n); basic_istream& operator>>(float& f); basic_istream& operator>>(double& f); basic_istream& operator>>(long double& f); basic_istream& operator>>(extended-floating-point-type& f); basic_istream& operator>>(void*& p); basic_istream& operator>>(basic_streambuf<char_type, traits>* sb); // [istream.unformatted], unformatted input streamsize gcount() const; int_type get(); basic_istream& get(char_type& c); basic_istream& get(char_type* s, streamsize n); basic_istream& get(char_type* s, streamsize n, char_type delim); basic_istream& get(basic_streambuf<char_type, traits>& sb); basic_istream& get(basic_streambuf<char_type, traits>& sb, char_type delim); basic_istream& getline(char_type* s, streamsize n); basic_istream& getline(char_type* s, streamsize n, char_type delim); basic_istream& ignore(streamsize n = 1, int_type delim = traits::eof()); int_type peek(); basic_istream& read (char_type* s, streamsize n); streamsize readsome(char_type* s, streamsize n); basic_istream& putback(char_type c); basic_istream& unget(); int sync(); pos_type tellg(); basic_istream& seekg(pos_type); basic_istream& seekg(off_type, ios_base::seekdir); protected: // [istream.cons], copy/move constructor basic_istream(const basic_istream&) = delete; basic_istream(basic_istream&& rhs); // [istream.assign], assignment and swap basic_istream& operator=(const basic_istream&) = delete; basic_istream& operator=(basic_istream&& rhs); void swap(basic_istream& rhs); }; // [istream.extractors], character extraction templates template<class charT, class traits> basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>&, charT&); template<class traits> basic_istream<char, traits>& operator>>(basic_istream<char, traits>&, unsigned char&); template<class traits> basic_istream<char, traits>& operator>>(basic_istream<char, traits>&, signed char&); template<class charT, class traits, size_t N> basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>&, charT(&)[N]); template<class traits, size_t N> basic_istream<char, traits>& operator>>(basic_istream<char, traits>&, unsigned char(&)[N]); template<class traits, size_t N> basic_istream<char, traits>& operator>>(basic_istream<char, traits>&, signed char(&)[N]); }
The class template basic_istream defines a number of member function signatures that assist in reading and interpreting input from sequences controlled by a stream buffer.
Two groups of member function signatures share common properties: the formatted input functions (or extractors) and the unformatted input functions. Both groups of input functions are described as if they obtain (or extract) input characters by calling rdbuf()->sbumpc() or rdbuf()->sgetc().
They may use other public members of istream.

31.7.5.2.2 Constructors [istream.cons]

explicit basic_istream(basic_streambuf<charT, traits>* sb);
Effects: Initializes the base class subobject with basic_ios​::​init(sb) ([basic.ios.cons]).
Postconditions: gcount() == 0.
basic_istream(basic_istream&& rhs);
Effects: Default constructs the base class, copies the gcount() from rhs, calls basic_ios<charT, traits>​::​move(rhs) to initialize the base class, and sets the gcount() for rhs to 0.
virtual ~basic_istream();
Remarks: Does not perform any operations of rdbuf().

31.7.5.2.3 Assignment and swap [istream.assign]

basic_istream& operator=(basic_istream&& rhs);
Effects: Equivalent to swap(rhs).
Returns: *this.
void swap(basic_istream& rhs);
Effects: Calls basic_ios<charT, traits>​::​swap(rhs).
Exchanges the values returned by gcount() and rhs.gcount().

31.7.5.2.4 Class basic_istream​::​sentry [istream.sentry]

namespace std { template<class charT, class traits> class basic_istream<charT, traits>::sentry { bool ok_; // exposition only public: explicit sentry(basic_istream& is, bool noskipws = false); ~sentry(); explicit operator bool() const { return ok_; } sentry(const sentry&) = delete; sentry& operator=(const sentry&) = delete; }; }
The class sentry defines a class that is responsible for doing exception safe prefix and suffix operations.
explicit sentry(basic_istream& is, bool noskipws = false);
Effects: If is.good() is false, calls is.setstate(failbit).
Otherwise, prepares for formatted or unformatted input.
First, if is.tie() is not a null pointer, the function calls is.tie()->flush() to synchronize the output sequence with any associated external C stream.
Except that this call can be suppressed if the put area of is.tie() is empty.
Further an implementation is allowed to defer the call to flush until a call of is.rdbuf()->underflow() occurs.
If no such call occurs before the sentry object is destroyed, the call to flush may be eliminated entirely.281
If noskipws is zero and is.flags() & ios_base​::​skipws is nonzero, the function extracts and discards each character as long as the next available input character c is a whitespace character.
If is.rdbuf()->sbumpc() or is.rdbuf()->sgetc() returns traits​::​eof(), the function calls setstate(failbit | eofbit) (which may throw ios_base​::​failure).
Remarks: The constructor explicit sentry(basic_istream& is, bool noskipws = false) uses the currently imbued locale in is, to determine whether the next input character is whitespace or not.
To decide if the character c is a whitespace character, the constructor performs as if it executes the following code fragment: const ctype<charT>& ctype = use_facet<ctype<charT>>(is.getloc()); if (ctype.is(ctype.space, c) != 0) // c is a whitespace character.
If, after any preparation is completed, is.good() is true, ok_ != false otherwise, ok_ == false.
During preparation, the constructor may call setstate(failbit) (which may throw ios_base​::​​failure ([iostate.flags])).282
~sentry();
Effects: None.
explicit operator bool() const;
Returns: ok_.
281)281)
This will be possible only in functions that are part of the library.
The semantics of the constructor used in user code is as specified.
282)282)
The sentry constructor and destructor can also perform additional implementation-dependent operations.

31.7.5.3 Formatted input functions [istream.formatted]

31.7.5.3.1 Common requirements [istream.formatted.reqmts]

Each formatted input function begins execution by constructing an object of type ios_base​::​iostate, termed the local error state, and initializing it to ios_base​::​goodbit.
It then creates an object of class sentry with the noskipws (second) argument false.
If the sentry object returns true, when converted to a value of type bool, the function endeavors to obtain the requested input.
Otherwise, if the sentry constructor exits by throwing an exception or if the sentry object produces false when converted to a value of type bool, the function returns without attempting to obtain any input.
If rdbuf()->sbumpc() or rdbuf()->sgetc() returns traits​::​eof(), then ios_base​::​eofbit is set in the local error state and the input function stops trying to obtain the requested input.
If an exception is thrown during input then ios_base​::​badbit is set in the local error state, *this's error state is set to the local error state, and the exception is rethrown if (exceptions() & badbit) != 0.
After extraction is done, the input function calls setstate, which sets *this's error state to the local error state, and may throw an exception.
In any case, the formatted input function destroys the sentry object.
If no exception has been thrown, it returns *this.

31.7.5.3.2 Arithmetic extractors [istream.formatted.arithmetic]

basic_istream& operator>>(unsigned short& val); basic_istream& operator>>(unsigned int& val); basic_istream& operator>>(long& val); basic_istream& operator>>(unsigned long& val); basic_istream& operator>>(long long& val); basic_istream& operator>>(unsigned long long& val); basic_istream& operator>>(float& val); basic_istream& operator>>(double& val); basic_istream& operator>>(long double& val); basic_istream& operator>>(bool& val); basic_istream& operator>>(void*& val);
As in the case of the inserters, these extractors depend on the locale's num_get<> object to perform parsing the input stream data.
These extractors behave as formatted input functions (as described in [istream.formatted.reqmts]).
After a sentry object is constructed, the conversion occurs as if performed by the following code fragment, where state represents the input function's local error state: using numget = num_get<charT, istreambuf_iterator<charT, traits>>; use_facet<numget>(loc).get(*this, 0, *this, state, val);
In the above fragment, loc stands for the private member of the basic_ios class.
[Note 1: 
The first argument provides an object of the istreambuf_iterator class which is an iterator pointed to an input stream.
It bypasses istreams and uses streambufs directly.
β€” end note]
Class locale relies on this type as its interface to istream, so that it does not need to depend directly on istream.
basic_istream& operator>>(short& val);
The conversion occurs as if performed by the following code fragment (using the same notation as for the preceding code fragment): using numget = num_get<charT, istreambuf_iterator<charT, traits>>; long lval; use_facet<numget>(loc).get(*this, 0, *this, state, lval); if (lval < numeric_limits<short>::min()) { state |= ios_base::failbit; val = numeric_limits<short>::min(); } else if (numeric_limits<short>::max() < lval) { state |= ios_base::failbit; val = numeric_limits<short>::max(); } else val = static_cast<short>(lval);
basic_istream& operator>>(int& val);
The conversion occurs as if performed by the following code fragment (using the same notation as for the preceding code fragment): using numget = num_get<charT, istreambuf_iterator<charT, traits>>; long lval; use_facet<numget>(loc).get(*this, 0, *this, state, lval); if (lval < numeric_limits<int>::min()) { state |= ios_base::failbit; val = numeric_limits<int>::min(); } else if (numeric_limits<int>::max() < lval) { state |= ios_base::failbit; val = numeric_limits<int>::max(); } else val = static_cast<int>(lval);
basic_istream& operator>>(extended-floating-point-type& val);
If the floating-point conversion rank of extended-floating-point-type is not less than or equal to that of long double, then an invocation of the operator function is conditionally supported with implementation-defined semantics.
Otherwise, let FP be a standard floating-point type:
  • if the floating-point conversion rank of extended-floating-point-type is less than or equal to that of float, then FP is float,
  • otherwise, if the floating-point conversion rank of extended-floating-point-type is less than or equal to that of double, then FP is double,
  • otherwise, FP is long double.
The conversion occurs as if performed by the following code fragment (using the same notation as for the preceding code fragment): using numget = num_get<charT, istreambuf_iterator<charT, traits>>; FP fval; use_facet<numget>(loc).get(*this, 0, *this, state, fval); if (fval < -numeric_limits<extended-floating-point-type>::max()) { state |= ios_base::failbit; val = -numeric_limits<extended-floating-point-type>::max(); } else if (numeric_limits<extended-floating-point-type>::max() < fval) { state |= ios_base::failbit; val = numeric_limits<extended-floating-point-type>::max(); } else { val = static_cast<extended-floating-point-type>(fval); }
[Note 2: 
When the extended floating-point type has a floating-point conversion rank that is not equal to the rank of any standard floating-point type, then double rounding during the conversion can result in inaccurate results.
from_chars can be used in situations where maximum accuracy is important.
β€” end note]

31.7.5.3.3 basic_istream​::​operator>> [istream.extractors]

basic_istream& operator>>(basic_istream& (*pf)(basic_istream&));
Effects: None.
This extractor does not behave as a formatted input function (as described in [istream.formatted.reqmts]).
Returns: pf(*this).
283
basic_istream& operator>>(basic_ios<charT, traits>& (*pf)(basic_ios<charT, traits>&));
Effects: Calls pf(*this).
This extractor does not behave as a formatted input function (as described in [istream.formatted.reqmts]).
Returns: *this.
basic_istream& operator>>(ios_base& (*pf)(ios_base&));
Effects: Calls pf(*this).284
This extractor does not behave as a formatted input function (as described in [istream.formatted.reqmts]).
Returns: *this.
template<class charT, class traits, size_t N> basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& in, charT (&s)[N]); template<class traits, size_t N> basic_istream<char, traits>& operator>>(basic_istream<char, traits>& in, unsigned char (&s)[N]); template<class traits, size_t N> basic_istream<char, traits>& operator>>(basic_istream<char, traits>& in, signed char (&s)[N]);
Effects: Behaves like a formatted input member (as described in [istream.formatted.reqmts]) of in.
After a sentry object is constructed, operator>> extracts characters and stores them into s.
If width() is greater than zero, n is min(size_t(width()), N).
Otherwise n is N.
n is the maximum number of characters stored.
Characters are extracted and stored until any of the following occurs:
  • n-1 characters are stored;
  • end of file occurs on the input sequence;
  • letting ct be use_facet<ctype<charT>>(in.getloc()), ct.is(ct.space, c) is true.
operator>> then stores a null byte (charT()) in the next position, which may be the first position if no characters were extracted.
operator>> then calls width(0).
If the function extracted no characters, ios_base​::​failbit is set in the input function's local error state before setstate is called.
Returns: in.
template<class charT, class traits> basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& in, charT& c); template<class traits> basic_istream<char, traits>& operator>>(basic_istream<char, traits>& in, unsigned char& c); template<class traits> basic_istream<char, traits>& operator>>(basic_istream<char, traits>& in, signed char& c);
Effects: Behaves like a formatted input member (as described in [istream.formatted.reqmts]) of in.
A character is extracted from in, if one is available, and stored in c.
Otherwise, ios_base​::​failbit is set in the input function's local error state before setstate is called.
Returns: in.
basic_istream& operator>>(basic_streambuf<charT, traits>* sb);
Effects: Behaves as an unformatted input function.
If sb is null, calls setstate(failbit), which may throw ios_base​::​failure ([iostate.flags]).
After a sentry object is constructed, extracts characters from *this and inserts them in the output sequence controlled by sb.
Characters are extracted and inserted until any of the following occurs:
  • end-of-file occurs on the input sequence;
  • inserting in the output sequence fails (in which case the character to be inserted is not extracted);
  • an exception occurs (in which case the exception is caught).
If the function inserts no characters, ios_base​::​failbit is set in the input function's local error state before setstate is called.
Returns: *this.
283)283)
See, for example, the function signature ws(basic_istream&) ([istream.manip]).
284)284)
See, for example, the function signature dec(ios_base&) ([basefield.manip]).

31.7.5.4 Unformatted input functions [istream.unformatted]

Each unformatted input function begins execution by constructing an object of type ios_base​::​iostate, termed the local error state, and initializing it to ios_base​::​goodbit.
It then creates an object of class sentry with the default argument noskipws (second) argument true.
If the sentry object returns true, when converted to a value of type bool, the function endeavors to obtain the requested input.
Otherwise, if the sentry constructor exits by throwing an exception or if the sentry object produces false, when converted to a value of type bool, the function returns without attempting to obtain any input.
In either case the number of extracted characters is set to 0; unformatted input functions taking a character array of nonzero size as an argument shall also store a null character (using charT()) in the first location of the array.
If rdbuf()->sbumpc() or rdbuf()->sgetc() returns traits​::​eof(), then ios_base​::​eofbit is set in the local error state and the input function stops trying to obtain the requested input.
If an exception is thrown during input then ios_base​::​badbit is set in the local error state, *this's error state is set to the local error state, and the exception is rethrown if (exceptions() & badbit) != 0.
If no exception has been thrown it stores the number of characters extracted in a member object.
After extraction is done, the input function calls setstate, which sets *this's error state to the local error state, and may throw an exception.
In any event the sentry object is destroyed before leaving the unformatted input function.
streamsize gcount() const;
Effects: None.
This member function does not behave as an unformatted input function (as described above).
Returns: The number of characters extracted by the last unformatted input member function called for the object.
If the number cannot be represented, returns numeric_limits<streamsize>​::​max().
int_type get();
Effects: Behaves as an unformatted input function (as described above).
After constructing a sentry object, extracts a character c, if one is available.
Otherwise, ios_base​::​failbit is set in the input function's local error state before setstate is called.
Returns: c if available, otherwise traits​::​eof().
basic_istream& get(char_type& c);
Effects: Behaves as an unformatted input function (as described above).
After constructing a sentry object, extracts a character, if one is available, and assigns it to c.285
Otherwise, ios_base​::​failbit is set in the input function's local error state before setstate is called.
Returns: *this.
basic_istream& get(char_type* s, streamsize n, char_type delim);
Effects: Behaves as an unformatted input function (as described above).
After constructing a sentry object, extracts characters and stores them into successive locations of an array whose first element is designated by s.286
Characters are extracted and stored until any of the following occurs:
  • n is less than one or n - 1 characters are stored;
  • end-of-file occurs on the input sequence;
  • traits​::​eq(c, delim) for the next available input character c (in which case c is not extracted).
If the function stores no characters, ios_base​::​failbit is set in the input function's local error state before setstate is called.
In any case, if n is greater than zero it then stores a null character into the next successive location of the array.
Returns: *this.
basic_istream& get(char_type* s, streamsize n);
Effects: Calls get(s, n, widen('\n')).
Returns: Value returned by the call.
basic_istream& get(basic_streambuf<char_type, traits>& sb, char_type delim);
Effects: Behaves as an unformatted input function (as described above).
After constructing a sentry object, extracts characters and inserts them in the output sequence controlled by sb.
Characters are extracted and inserted until any of the following occurs:
  • end-of-file occurs on the input sequence;
  • inserting in the output sequence fails (in which case the character to be inserted is not extracted);
  • traits​::​eq(c, delim) for the next available input character c (in which case c is not extracted);
  • an exception occurs (in which case, the exception is caught but not rethrown).
If the function inserts no characters, ios_base​::​failbit is set in the input function's local error state before setstate is called.
Returns: *this.
basic_istream& get(basic_streambuf<char_type, traits>& sb);
Effects: Calls get(sb, widen('\n')).
Returns: Value returned by the call.
basic_istream& getline(char_type* s, streamsize n, char_type delim);
Effects: Behaves as an unformatted input function (as described above).
After constructing a sentry object, extracts characters and stores them into successive locations of an array whose first element is designated by s.287
Characters are extracted and stored until one of the following occurs:
1.end-of-file occurs on the input sequence;
2.traits​::​eq(c, delim) for the next available input character c (in which case the input character is extracted but not stored);288
3.n is less than one or n - 1 characters are stored (in which case the function calls setstate(​failbit)).
These conditions are tested in the order shown.289
If the function extracts no characters, ios_base​::​failbit is set in the input function's local error state before setstate is called.290
In any case, if n is greater than zero, it then stores a null character (using charT()) into the next successive location of the array.
Returns: *this.
[Example 1: #include <iostream> int main() { using namespace std; const int line_buffer_size = 100; char buffer[line_buffer_size]; int line_number = 0; while (cin.getline(buffer, line_buffer_size, '\n') || cin.gcount()) { int count = cin.gcount(); if (cin.eof()) cout << "Partial final line"; // cin.fail() is false else if (cin.fail()) { cout << "Partial long line"; cin.clear(cin.rdstate() & ~ios_base::failbit); } else { count--; // Don't include newline in count cout << "Line " << ++line_number; } cout << " (" << count << " chars): " << buffer << endl; } } β€” end example]
basic_istream& getline(char_type* s, streamsize n);
Returns: getline(s, n, widen('\n'))
basic_istream& ignore(streamsize n = 1, int_type delim = traits::eof());
Effects: Behaves as an unformatted input function (as described above).
After constructing a sentry object, extracts characters and discards them.
Characters are extracted until any of the following occurs:
  • n != numeric_limits<streamsize>​::​max()numeric.limits and n characters have been extracted so far
  • end-of-file occurs on the input sequence (in which case the function calls setstate(eofbit), which may throw ios_base​::​failure ([iostate.flags]));
  • traits​::​eq_int_type(traits​::​to_int_type(c), delim) for the next available input character c (in which case c is extracted).
[Note 1: 
The last condition will never occur if traits​::​eq_int_type(delim, traits​::​eof()).
β€” end note]
Returns: *this.
int_type peek();
Effects: Behaves as an unformatted input function (as described above).
After constructing a sentry object, reads but does not extract the current input character.
Returns: traits​::​eof() if good() is false.
Otherwise, returns rdbuf()->sgetc().
basic_istream& read(char_type* s, streamsize n);
Effects: Behaves as an unformatted input function (as described above).
After constructing a sentry object, if !good() calls setstate(failbit) which may throw an exception, and return.
Otherwise extracts characters and stores them into successive locations of an array whose first element is designated by s.291
Characters are extracted and stored until either of the following occurs:
  • n characters are stored;
  • end-of-file occurs on the input sequence (in which case the function calls setstate(failbit | eofbit), which may throw ios_base​::​failure ([iostate.flags])).
Returns: *this.
streamsize readsome(char_type* s, streamsize n);
Effects: Behaves as an unformatted input function (as described above).
After constructing a sentry object, if !good() calls setstate(failbit) which may throw an exception, and return.
Otherwise extracts characters and stores them into successive locations of an array whose first element is designated by s.
If rdbuf()->in_avail() == -1, calls setstate(eofbit) (which may throw ios_base​::​failure ([iostate.flags])), and extracts no characters;
  • If rdbuf()->in_avail() == 0, extracts no characters
  • If rdbuf()->in_avail() > 0, extracts min(rdbuf()->in_avail(), n)).
Returns: The number of characters extracted.
basic_istream& putback(char_type c);
Effects: Behaves as an unformatted input function (as described above), except that the function first clears eofbit.
After constructing a sentry object, if !good() calls setstate(failbit) which may throw an exception, and return.
If rdbuf() is not null, calls rdbuf()->sputbackc(c).
If rdbuf() is null, or if sputbackc returns traits​::​eof(), calls setstate(badbit) (which may throw ios_base​::​failure ([iostate.flags])).
[Note 2: 
This function extracts no characters, so the value returned by the next call to gcount() is 0.
β€” end note]
Returns: *this.
basic_istream& unget();
Effects: Behaves as an unformatted input function (as described above), except that the function first clears eofbit.
After constructing a sentry object, if !good() calls setstate(failbit) which may throw an exception, and return.
If rdbuf() is not null, calls rdbuf()->sungetc().
If rdbuf() is null, or if sungetc returns traits​::​eof(), calls setstate(badbit) (which may throw ios_base​::​failure ([iostate.flags])).
[Note 3: 
This function extracts no characters, so the value returned by the next call to gcount() is 0.
β€” end note]
Returns: *this.
int sync();
Effects: Behaves as an unformatted input function (as described above), except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to gcount().
After constructing a sentry object, if rdbuf() is a null pointer, returns -1.
Otherwise, calls rdbuf()->pubsync() and, if that function returns -1 calls setstate(badbit) (which may throw ios_base​::​failure ([iostate.flags]), and returns -1.
Otherwise, returns zero.
pos_type tellg();
Effects: Behaves as an unformatted input function (as described above), except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to gcount().
Returns: After constructing a sentry object, if fail() != false, returns pos_type(-1) to indicate failure.
Otherwise, returns rdbuf()->pubseekoff(0, cur, in).
basic_istream& seekg(pos_type pos);
Effects: Behaves as an unformatted input function (as described above), except that the function first clears eofbit, it does not count the number of characters extracted, and it does not affect the value returned by subsequent calls to gcount().
After constructing a sentry object, if fail() != true, executes rdbuf()->pubseekpos(pos, ios_base​::​in).
In case of failure, the function calls setstate(failbit) (which may throw ios_base​::​failure).
Returns: *this.
basic_istream& seekg(off_type off, ios_base::seekdir dir);
Effects: Behaves as an unformatted input function (as described above), except that the function first clears eofbit, does not count the number of characters extracted, and does not affect the value returned by subsequent calls to gcount().
After constructing a sentry object, if fail() != true, executes rdbuf()->pubseekoff(off, dir, ios_base​::​in).
In case of failure, the function calls setstate(​failbit) (which may throw ios_base​::​failure).
Returns: *this.
285)285)
Note that this function is not overloaded on types signed char and unsigned char.
286)286)
Note that this function is not overloaded on types signed char and unsigned char.
287)287)
Note that this function is not overloaded on types signed char and unsigned char.
288)288)
Since the final input character is β€œextracted”, it is counted in the gcount(), even though it is not stored.
289)289)
This allows an input line which exactly fills the buffer, without setting failbit.
This is different behavior than the historical AT&T implementation.
290)290)
This implies an empty input line will not cause failbit to be set.
291)291)
Note that this function is not overloaded on types signed char and unsigned char.

31.7.5.5 Standard basic_istream manipulators [istream.manip]

Each instantiation of the function template specified in this subclause is a designated addressable function ([namespace.std]).
template<class charT, class traits> basic_istream<charT, traits>& ws(basic_istream<charT, traits>& is);
Effects: Behaves as an unformatted input function, except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to is.gcount().
After constructing a sentry object extracts characters as long as the next available character c is whitespace or until there are no more characters in the sequence.
Whitespace characters are distinguished with the same criterion as used by sentry​::​sentry.
If ws stops extracting characters because there are no more available it sets eofbit, but not failbit.
Returns: is.

31.7.5.6 Rvalue stream extraction [istream.rvalue]

template<class Istream, class T> Istream&& operator>>(Istream&& is, T&& x);
Constraints: The expression is >> std​::​forward<T>(x) is well-formed when treated as an unevaluated operand and Istream is publicly and unambiguously derived from ios_base.
Effects: Equivalent to: is >> std::forward<T>(x); return std::move(is);

31.7.5.7 Class template basic_iostream [iostreamclass]

31.7.5.7.1 General [iostreamclass.general]

namespace std { template<class charT, class traits = char_traits<charT>> class basic_iostream : public basic_istream<charT, traits>, public basic_ostream<charT, traits> { public: using char_type = charT; using int_type = typename traits::int_type; using pos_type = typename traits::pos_type; using off_type = typename traits::off_type; using traits_type = traits; // [iostream.cons], constructor explicit basic_iostream(basic_streambuf<charT, traits>* sb); // [iostream.dest], destructor virtual ~basic_iostream(); protected: // [iostream.cons], constructor basic_iostream(const basic_iostream&) = delete; basic_iostream(basic_iostream&& rhs); // [iostream.assign], assignment and swap basic_iostream& operator=(const basic_iostream&) = delete; basic_iostream& operator=(basic_iostream&& rhs); void swap(basic_iostream& rhs); }; }
The class template basic_iostream inherits a number of functions that allow reading input and writing output to sequences controlled by a stream buffer.

31.7.5.7.2 Constructors [iostream.cons]

explicit basic_iostream(basic_streambuf<charT, traits>* sb);
Effects: Initializes the base class subobjects with basic_istream<charT, traits>(sb) ([istream]) and basic_ostream<charT, traits>(sb).
Postconditions: rdbuf() == sb and gcount() == 0.
basic_iostream(basic_iostream&& rhs);
Effects: Move constructs from the rvalue rhs by constructing the basic_istream base class with std​::​move(rhs).

31.7.5.7.3 Destructor [iostream.dest]

virtual ~basic_iostream();
Remarks: Does not perform any operations on rdbuf().

31.7.5.7.4 Assignment and swap [iostream.assign]

basic_iostream& operator=(basic_iostream&& rhs);
Effects: Equivalent to swap(rhs).
void swap(basic_iostream& rhs);
Effects: Calls basic_istream<charT, traits>​::​swap(rhs).

31.7.6 Output streams [output.streams]

31.7.6.1 General [output.streams.general]

The header <ostream> defines a class template and several function templates that control output to a stream buffer, along with a function template that inserts into stream rvalues.

31.7.6.2 Class template basic_ostream [ostream]

31.7.6.2.1 General [ostream.general]

When a function has a parameter type extended-floating-point-type, the implementation provides overloads for all cv-unqualified extended floating-point types ([basic.fundamental]).
namespace std { template<class charT, class traits = char_traits<charT>> class basic_ostream : virtual public basic_ios<charT, traits> { public: // types (inherited from basic_ios) using char_type = charT; using int_type = typename traits::int_type; using pos_type = typename traits::pos_type; using off_type = typename traits::off_type; using traits_type = traits; // [ostream.cons], constructor/destructor explicit basic_ostream(basic_streambuf<char_type, traits>* sb); virtual ~basic_ostream(); // [ostream.sentry], prefix/suffix class sentry; // [ostream.formatted], formatted output basic_ostream& operator<<(basic_ostream& (*pf)(basic_ostream&)); basic_ostream& operator<<(basic_ios<charT, traits>& (*pf)(basic_ios<charT, traits>&)); basic_ostream& operator<<(ios_base& (*pf)(ios_base&)); basic_ostream& operator<<(bool n); basic_ostream& operator<<(short n); basic_ostream& operator<<(unsigned short n); basic_ostream& operator<<(int n); basic_ostream& operator<<(unsigned int n); basic_ostream& operator<<(long n); basic_ostream& operator<<(unsigned long n); basic_ostream& operator<<(long long n); basic_ostream& operator<<(unsigned long long n); basic_ostream& operator<<(float f); basic_ostream& operator<<(double f); basic_ostream& operator<<(long double f); basic_ostream& operator<<(extended-floating-point-type f); basic_ostream& operator<<(const void* p); basic_ostream& operator<<(const volatile void* p); basic_ostream& operator<<(nullptr_t); basic_ostream& operator<<(basic_streambuf<char_type, traits>* sb); // [ostream.unformatted], unformatted output basic_ostream& put(char_type c); basic_ostream& write(const char_type* s, streamsize n); basic_ostream& flush(); // [ostream.seeks], seeks pos_type tellp(); basic_ostream& seekp(pos_type); basic_ostream& seekp(off_type, ios_base::seekdir); protected: // [ostream.cons], copy/move constructor basic_ostream(const basic_ostream&) = delete; basic_ostream(basic_ostream&& rhs); // [ostream.assign], assignment and swap basic_ostream& operator=(const basic_ostream&) = delete; basic_ostream& operator=(basic_ostream&& rhs); void swap(basic_ostream& rhs); }; // [ostream.inserters.character], character inserters template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>&, charT); template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>&, char); template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>&, char); template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>&, signed char); template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>&, unsigned char); template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>&, wchar_t) = delete; template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>&, char8_t) = delete; template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>&, char16_t) = delete; template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>&, char32_t) = delete; template<class traits> basic_ostream<wchar_t, traits>& operator<<(basic_ostream<wchar_t, traits>&, char8_t) = delete; template<class traits> basic_ostream<wchar_t, traits>& operator<<(basic_ostream<wchar_t, traits>&, char16_t) = delete; template<class traits> basic_ostream<wchar_t, traits>& operator<<(basic_ostream<wchar_t, traits>&, char32_t) = delete; template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>&, const charT*); template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>&, const char*); template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>&, const char*); template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>&, const signed char*); template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>&, const unsigned char*); template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>&, const wchar_t*) = delete; template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>&, const char8_t*) = delete; template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>&, const char16_t*) = delete; template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>&, const char32_t*) = delete; template<class traits> basic_ostream<wchar_t, traits>& operator<<(basic_ostream<wchar_t, traits>&, const char8_t*) = delete; template<class traits> basic_ostream<wchar_t, traits>& operator<<(basic_ostream<wchar_t, traits>&, const char16_t*) = delete; template<class traits> basic_ostream<wchar_t, traits>& operator<<(basic_ostream<wchar_t, traits>&, const char32_t*) = delete; }
The class template basic_ostream defines a number of member function signatures that assist in formatting and writing output to output sequences controlled by a stream buffer.
Two groups of member function signatures share common properties: the formatted output functions (or inserters) and the unformatted output functions. Both groups of output functions generate (or insert) output characters by actions equivalent to calling rdbuf()->sputc(int_type).
They may use other public members of basic_ostream except that they shall not invoke any virtual members of rdbuf() except overflow(), xsputn(), and sync().
If one of these called functions throws an exception, then unless explicitly noted otherwise the output function sets badbit in the error state.
If badbit is set in exceptions(), the output function rethrows the exception without completing its actions, otherwise it does not throw anything and proceeds as if the called function had returned a failure indication.
[Note 1: 
The deleted overloads of operator<< prevent formatting characters as integers and strings as pointers.
β€” end note]

31.7.6.2.2 Constructors [ostream.cons]

explicit basic_ostream(basic_streambuf<charT, traits>* sb);
Effects: Initializes the base class subobject with basic_ios<charT, traits>​::​init(sb) ([basic.ios.cons]).
Postconditions: rdbuf() == sb.
basic_ostream(basic_ostream&& rhs);
Effects: Move constructs from the rvalue rhs.
This is accomplished by default constructing the base class and calling basic_ios<charT, traits>​::​move(rhs) to initialize the base class.
virtual ~basic_ostream();
Remarks: Does not perform any operations on rdbuf().

31.7.6.2.3 Assignment and swap [ostream.assign]

basic_ostream& operator=(basic_ostream&& rhs);
Effects: Equivalent to swap(rhs).
Returns: *this.
void swap(basic_ostream& rhs);
Effects: Calls basic_ios<charT, traits>​::​swap(rhs).

31.7.6.2.4 Class basic_ostream​::​sentry [ostream.sentry]

namespace std { template<class charT, class traits> class basic_ostream<charT, traits>::sentry { bool ok_; // exposition only public: explicit sentry(basic_ostream& os); ~sentry(); explicit operator bool() const { return ok_; } sentry(const sentry&) = delete; sentry& operator=(const sentry&) = delete; }; }
The class sentry defines a class that is responsible for doing exception safe prefix and suffix operations.
explicit sentry(basic_ostream& os);
If os.good() is nonzero, prepares for formatted or unformatted output.
If os.tie() is not a null pointer, calls os.tie()->flush().292
If, after any preparation is completed, os.good() is true, ok_ == true otherwise, ok_ == false.
During preparation, the constructor may call setstate(failbit) (which may throw ios_base​::​​failure ([iostate.flags])).293
~sentry();
If (os.flags() & ios_base​::​unitbuf) && !uncaught_exceptions() && os.good() is true, calls os.rdbuf()->pubsync().
If that function returns , sets badbit in os.rdstate() without propagating an exception.
explicit operator bool() const;
Effects: Returns ok_.
292)292)
The call os.tie()->flush() does not necessarily occur if the function can determine that no synchronization is necessary.
293)293)
The sentry constructor and destructor can also perform additional implementation-dependent operations.

31.7.6.2.5 Seek members [ostream.seeks]

Each seek member function begins execution by constructing an object of class sentry.
It returns by destroying the sentry object.
pos_type tellp();
Returns: If fail() != false, returns pos_type(-1) to indicate failure.
Otherwise, returns rdbuf()->​pubseekoff(0, cur, out).
basic_ostream& seekp(pos_type pos);
Effects: If fail() != true, executes rdbuf()->pubseekpos(pos, ios_base​::​out).
In case of failure, the function calls setstate(failbit) (which may throw ios_base​::​failure).
Returns: *this.
basic_ostream& seekp(off_type off, ios_base::seekdir dir);
Effects: If fail() != true, executes rdbuf()->pubseekoff(off, dir, ios_base​::​out).
In case of failure, the function calls setstate(failbit) (which may throw ios_base​::​failure).
Returns: *this.

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 set294 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 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.
294)294)
This is done without causing an ios_base​::​failure to be thrown.

31.7.6.3.2 Arithmetic inserters [ostream.inserters.arithmetic]

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.
basic_ostream& operator<<(const volatile void* p);
Effects: Equivalent to: return operator<<(const_cast<const void*>(p));
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]

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).295
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.296
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.
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:
  • end-of-file occurs on the input sequence;
  • inserting in the output sequence fails (in which case the character to be inserted is not extracted);
  • 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.
basic_ostream& operator<<(nullptr_t);
Effects: Equivalent to: return *this << s; where s is an implementation-defined NTCTS.
295)295)
See, for example, the function signature endl(basic_ostream&) ([ostream.manip]).
296)296)
See, for example, the function signature dec(ios_base&) ([basefield.manip]).

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

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 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.
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:
  • 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*,
  • 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*,
  • 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]

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(std::forward<Args>(args)...));
Otherwise, equivalent to: vprint_nonunicode(os, fmt.str, make_format_args(std::forward<Args>(args)...));
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)...));
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:
  • failure to generate output is reported as specified below, and
  • 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 an automatic variable 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.

31.7.6.4 Unformatted output functions [ostream.unformatted]

Each unformatted output function begins execution by constructing an object of class sentry.
If that object returns true, while converting to a value of type bool, the function endeavors to generate the requested output.
If an exception is thrown during output, then ios_base​::​badbit is set297 in *this's error state.
If (exceptions() & badbit) != 0 then the exception is rethrown.
In any case, the unformatted output function ends by destroying the sentry object, then, if no exception was thrown, returning the value specified for the unformatted output function.
basic_ostream& put(char_type c);
Effects: Behaves as an unformatted output function (as described above).
After constructing a sentry object, inserts the character c, if possible.298
Otherwise, calls setstate(badbit) (which may throw ios_base​::​failure ([iostate.flags])).
Returns: *this.
basic_ostream& write(const char_type* s, streamsize n);
Effects: Behaves as an unformatted output function (as described above).
After constructing a sentry object, obtains characters to insert from successive locations of an array whose first element is designated by s.299
Characters are inserted until either of the following occurs:
  • n characters are inserted;
  • inserting in the output sequence fails (in which case the function calls setstate(badbit), which may throw ios_base​::​failure ([iostate.flags])).
Returns: *this.
basic_ostream& flush();
Effects: Behaves as an unformatted output function (as described above).
If rdbuf() is not a null pointer, constructs a sentry object.
If that object returns true when converted to a value of type bool the function calls rdbuf()->pubsync().
If that function returns calls setstate(badbit) (which may throw ios_base​::​failure ([iostate.flags])).
Otherwise, if the sentry object returns false, does nothing.
Returns: *this.
297)297)
This is done without causing an ios_base​::​failure to be thrown.
298)298)
Note that this function is not overloaded on types signed char and unsigned char.
299)299)
Note that this function is not overloaded on types signed char and unsigned char.

31.7.6.5 Standard manipulators [ostream.manip]

Each instantiation of any of the function templates specified in this subclause is a designated addressable function ([namespace.std]).
template<class charT, class traits> basic_ostream<charT, traits>& endl(basic_ostream<charT, traits>& os);
Effects: Calls os.put(os.widen('\n')), then os.flush().
Returns: os.
template<class charT, class traits> basic_ostream<charT, traits>& ends(basic_ostream<charT, traits>& os);
Effects: Inserts a null character into the output sequence: calls os.put(charT()).
Returns: os.
template<class charT, class traits> basic_ostream<charT, traits>& flush(basic_ostream<charT, traits>& os);
Effects: Calls os.flush().
Returns: os.
template<class charT, class traits> basic_ostream<charT, traits>& emit_on_flush(basic_ostream<charT, traits>& os);
Effects: If os.rdbuf() is a basic_syncbuf<charT, traits, Allocator>*, called buf for the purpose of exposition, calls buf->set_emit_on_sync(true).
Otherwise this manipulator has no effect.
[Note 1: 
To work around the issue that the Allocator template argument cannot be deduced, implementations can introduce an intermediate base class to basic_syncbuf that manages its emit_on_sync flag.
β€” end note]
Returns: os.
template<class charT, class traits> basic_ostream<charT, traits>& noemit_on_flush(basic_ostream<charT, traits>& os);
Effects: If os.rdbuf() is a basic_syncbuf<charT, traits, Allocator>*, called buf for the purpose of exposition, calls buf->set_emit_on_sync(false).
Otherwise this manipulator has no effect.
Returns: os.
template<class charT, class traits> basic_ostream<charT, traits>& flush_emit(basic_ostream<charT, traits>& os);
Effects: Calls os.flush().
Then, if os.rdbuf() is a basic_syncbuf<charT, traits, Allocator>*, called buf for the purpose of exposition, behaves as an unformatted output function ([ostream.unformatted]) of os.
After constructing a sentry object, calls buf->emit().
If that call returns false, calls os.setstate(ios_base​::​badbit).
Returns: os.

31.7.6.6 Rvalue stream insertion [ostream.rvalue]

template<class Ostream, class T> Ostream&& operator<<(Ostream&& os, const T& x);
Constraints: The expression os << x is well-formed when treated as an unevaluated operand and Ostream is publicly and unambiguously derived from ios_base.
Effects: As if by: os << x;
Returns: std​::​move(os).

31.7.7 Standard manipulators [std.manip]

The header <iomanip> defines several functions that support extractors and inserters that alter information maintained by class ios_base and its derived classes.
unspecified resetiosflags(ios_base::fmtflags mask);
Returns: An object of unspecified type such that if out is an object of type basic_ostream<charT, traits> then the expression out << resetiosflags(mask) behaves as if it called f(out, mask), or if in is an object of type basic_istream<charT, traits> then the expression in >> resetiosflags(​mask) behaves as if it called f(in, mask), where the function f is defined as:300 void f(ios_base& str, ios_base::fmtflags mask) { // reset specified flags str.setf(ios_base::fmtflags(0), mask); }
The expression out << resetiosflags(mask) has type basic_ostream<charT, traits>& and value out.
The expression in >> resetiosflags(mask) has type basic_istream<charT, traits>& and value in.
unspecified setiosflags(ios_base::fmtflags mask);
Returns: An object of unspecified type such that if out is an object of type basic_ostream<charT, traits> then the expression out << setiosflags(mask) behaves as if it called f(out, mask), or if in is an object of type basic_istream<charT, traits> then the expression in >> setiosflags(mask) behaves as if it called f(in, mask), where the function f is defined as: void f(ios_base& str, ios_base::fmtflags mask) { // set specified flags str.setf(mask); }
The expression out << setiosflags(mask) has type basic_ostream<charT, traits>& and value out.
The expression in >> setiosflags(mask) has type basic_istream<charT, traits>& and value in.
unspecified setbase(int base);
Returns: An object of unspecified type such that if out is an object of type basic_ostream<charT, traits> then the expression out << setbase(base) behaves as if it called f(out, base), or if in is an object of type basic_istream<charT, traits> then the expression in >> setbase(base) behaves as if it called f(in, base), where the function f is defined as: void f(ios_base& str, int base) { // set basefield str.setf(base == 8 ? ios_base::oct : base == 10 ? ios_base::dec : base == 16 ? ios_base::hex : ios_base::fmtflags(0), ios_base::basefield); }
The expression out << setbase(base) has type basic_ostream<charT, traits>& and value out.
The expression in >> setbase(base) has type basic_istream<charT, traits>& and value in.
unspecified setfill(char_type c);
Returns: An object of unspecified type such that if out is an object of type basic_ostream<charT, traits> and c has type charT then the expression out << setfill(c) behaves as if it called f(out, c), where the function f is defined as: template<class charT, class traits> void f(basic_ios<charT, traits>& str, charT c) { // set fill character str.fill(c); }
The expression out << setfill(c) has type basic_ostream<charT, traits>& and value out.
unspecified setprecision(int n);
Returns: An object of unspecified type such that if out is an object of type basic_ostream<charT, traits> then the expression out << setprecision(n) behaves as if it called f(out, n), or if in is an object of type basic_istream<charT, traits> then the expression in >> setprecision(n) behaves as if it called f(in, n), where the function f is defined as: void f(ios_base& str, int n) { // set precision str.precision(n); }
The expression out << setprecision(n) has type basic_ostream<charT, traits>& and value out.
The expression in >> setprecision(n) has type basic_istream<charT, traits>& and value in.
unspecified setw(int n);
Returns: An object of unspecified type such that if out is an instance of basic_ostream<charT, traits> then the expression out << setw(n) behaves as if it called f(out, n), or if in is an object of type basic_istream<charT, traits> then the expression in >> setw(n) behaves as if it called f(in, n), where the function f is defined as: void f(ios_base& str, int n) { // set width str.width(n); }
The expression out << setw(n) has type basic_ostream<charT, traits>& and value out.
The expression in >> setw(n) has type basic_istream<charT, traits>& and value in.
300)300)
The expression cin >> resetiosflags(ios_base​::​skipws) clears ios_base​::​skipws in the format flags stored in the basic_istream<charT, traits> object cin (the same as cin >> noskipws), and the expression cout << resetiosflags(ios_base​::​showbase) clears ios_base​::​showbase in the format flags stored in the basic_ostream<charT, traits> object cout (the same as cout << noshowbase).

31.7.8 Extended manipulators [ext.manip]

The header <iomanip> defines several functions that support extractors and inserters that allow for the parsing and formatting of sequences and values for money and time.
template<class moneyT> unspecified get_money(moneyT& mon, bool intl = false);
Mandates: The type moneyT is either long double or a specialization of the basic_string template ([strings]).
Effects: The expression in >> get_money(mon, intl) described below behaves as a formatted input function.
Returns: An object of unspecified type such that if in is an object of type basic_istream<charT, traits> then the expression in >> get_money(mon, intl) behaves as if it called f(in, mon, intl), where the function f is defined as: template<class charT, class traits, class moneyT> void f(basic_ios<charT, traits>& str, moneyT& mon, bool intl) { using Iter = istreambuf_iterator<charT, traits>; using MoneyGet = money_get<charT, Iter>; ios_base::iostate err = ios_base::goodbit; const MoneyGet& mg = use_facet<MoneyGet>(str.getloc()); mg.get(Iter(str.rdbuf()), Iter(), intl, str, err, mon); if (ios_base::goodbit != err) str.setstate(err); }
The expression in >> get_money(mon, intl) has type basic_istream<charT, traits>& and value in.
template<class moneyT> unspecified put_money(const moneyT& mon, bool intl = false);
Mandates: The type moneyT is either long double or a specialization of the basic_string template ([strings]).
Returns: An object of unspecified type such that if out is an object of type basic_ostream<charT, traits> then the expression out << put_money(mon, intl) behaves as a formatted output function that calls f(out, mon, intl), where the function f is defined as: template<class charT, class traits, class moneyT> void f(basic_ios<charT, traits>& str, const moneyT& mon, bool intl) { using Iter = ostreambuf_iterator<charT, traits>; using MoneyPut = money_put<charT, Iter>; const MoneyPut& mp = use_facet<MoneyPut>(str.getloc()); const Iter end = mp.put(Iter(str.rdbuf()), intl, str, str.fill(), mon); if (end.failed()) str.setstate(ios_base::badbit); }
The expression out << put_money(mon, intl) has type basic_ostream<charT, traits>& and value out.
template<class charT> unspecified get_time(tm* tmb, const charT* fmt);
Preconditions: The argument tmb is a valid pointer to an object of type tm, and [fmt, fmt + char_traits<charT>​::​length(fmt)) is a valid range.
Returns: An object of unspecified type such that if in is an object of type basic_istream<charT, traits> then the expression in >> get_time(tmb, fmt) behaves as if it called f(in, tmb, fmt), where the function f is defined as: template<class charT, class traits> void f(basic_ios<charT, traits>& str, tm* tmb, const charT* fmt) { using Iter = istreambuf_iterator<charT, traits>; using TimeGet = time_get<charT, Iter>; ios_base::iostate err = ios_base::goodbit; const TimeGet& tg = use_facet<TimeGet>(str.getloc()); tg.get(Iter(str.rdbuf()), Iter(), str, err, tmb, fmt, fmt + traits::length(fmt)); if (err != ios_base::goodbit) str.setstate(err); }
The expression in >> get_time(tmb, fmt) has type basic_istream<charT, traits>& and value in.
template<class charT> unspecified put_time(const tm* tmb, const charT* fmt);
Preconditions: The argument tmb is a valid pointer to an object of type tm, and [fmt, fmt + char_traits<charT>​::​length(fmt)) is a valid range.
Returns: An object of unspecified type such that if out is an object of type basic_ostream<charT, traits> then the expression out << put_time(tmb, fmt) behaves as if it called f(out, tmb, fmt), where the function f is defined as: template<class charT, class traits> void f(basic_ios<charT, traits>& str, const tm* tmb, const charT* fmt) { using Iter = ostreambuf_iterator<charT, traits>; using TimePut = time_put<charT, Iter>; const TimePut& tp = use_facet<TimePut>(str.getloc()); const Iter end = tp.put(Iter(str.rdbuf()), str, str.fill(), tmb, fmt, fmt + traits::length(fmt)); if (end.failed()) str.setstate(ios_base::badbit); }
The expression out << put_time(tmb, fmt) has type basic_ostream<charT, traits>& and value out.

31.7.9 Quoted manipulators [quoted.manip]

[Note 1: 
Quoted manipulators provide string insertion and extraction of quoted strings (for example, XML and CSV formats).
Quoted manipulators are useful in ensuring that the content of a string with embedded spaces remains unchanged if inserted and then extracted via stream I/O.
β€” end note]
template<class charT> unspecified quoted(const charT* s, charT delim = charT('"'), charT escape = charT('\\')); template<class charT, class traits, class Allocator> unspecified quoted(const basic_string<charT, traits, Allocator>& s, charT delim = charT('"'), charT escape = charT('\\')); template<class charT, class traits> unspecified quoted(basic_string_view<charT, traits> s, charT delim = charT('"'), charT escape = charT('\\'));
Returns: An object of unspecified type such that if out is an instance of basic_ostream with member type char_type the same as charT and with member type traits_type, which in the second and third forms is the same as traits, then the expression out << quoted(s, delim, escape) behaves as a formatted output function of out.
This forms a character sequence seq, initially consisting of the following elements:
  • delim.
  • Each character in s.
    If the character to be output is equal to escape or delim, as determined by traits_type​::​eq, first output escape.
  • delim.
Let x be the number of elements initially in seq.
Then padding is determined for seq as described in [ostream.formatted.reqmts], seq is inserted as if by calling out.rdbuf()->sputn(seq, n), where n is the larger of out.width() and x, and out.width(0) is called.
The expression out << quoted(s, delim, escape) has type basic_ostream<charT, traits>& and value out.
template<class charT, class traits, class Allocator> unspecified quoted(basic_string<charT, traits, Allocator>& s, charT delim = charT('"'), charT escape = charT('\\'));
Returns: An object of unspecified type such that:
  • If in is an instance of basic_istream with member types char_type and traits_type the same as charT and traits, respectively, then the expression in >> quoted(s, delim, escape) behaves as if it extracts the following characters from in using operator>>(basic_istream<charT, traits>&, charT&) ([istream.extractors]) which may throw ios_base​::​failure ([ios.failure]):
    • If the first character extracted is equal to delim, as determined by traits_type​::​eq, then:
      • Turn off the skipws flag.
      • s.clear()
      • Until an unescaped delim character is reached or !in, extract characters from in and append them to s, except that if an escape is reached, ignore it and append the next character to s.
      • Discard the final delim character.
      • Restore the skipws flag to its original value.
    • Otherwise, in >> s.
  • If out is an instance of basic_ostream with member types char_type and traits_type the same as charT and traits, respectively, then the expression out << quoted(s, delim, escape) behaves as specified for the const basic_string<charT, traits, Allocator>& overload of the quoted function.
  • The expression in >> quoted(s, delim, escape) has type basic_istream<charT, traits>& and value in.
  • The expression out << quoted(s, delim, escape) has type basic_ostream​<charT, traits>& and value out.

31.7.10 Print functions [print.fun]

template<class... Args> void print(format_string<Args...> fmt, Args&&... args);
Effects: Equivalent to: print(stdout, fmt, std::forward<Args>(args)...);
template<class... Args> void print(FILE* stream, format_string<Args...> fmt, Args&&... args);
Effects: If the ordinary literal encoding ([lex.charset]) is UTF-8, equivalent to: vprint_unicode(stream, fmt.str, make_format_args(std::forward<Args>(args)...));
Otherwise, equivalent to: vprint_nonunicode(stream, fmt.str, make_format_args(std::forward<Args>(args)...));
template<class... Args> void println(format_string<Args...> fmt, Args&&... args);
Effects: Equivalent to: println(stdout, fmt, std::forward<Args>(args)...);
template<class... Args> void println(FILE* stream, format_string<Args...> fmt, Args&&... args);
Effects: Equivalent to: print(stream, "{}\n", format(fmt, std::forward<Args>(args)...));
void vprint_unicode(string_view fmt, format_args args);
Effects: Equivalent to: vprint_unicode(stdout, fmt, args);
void vprint_unicode(FILE* stream, string_view fmt, format_args args);
Preconditions: stream is a valid pointer to an output C stream.
Effects: The function initializes an automatic variable via string out = vformat(fmt, args);
If stream refers to a terminal capable of displaying Unicode, 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.
Otherwise writes out to stream unchanged.
If the native Unicode API is used, the function flushes stream before writing out.
[Note 1: 
On POSIX and Windows, stream referring to a terminal means that, respectively, isatty(fileno(
stream))
and GetConsoleMode(_get_osfhandle(_fileno(stream)), ...) return nonzero.
β€” end note]
[Note 2: 
On Windows, the native Unicode API is WriteConsoleW.
β€” end note]
Throws: Any exception thrown by the call to vformat ([format.err.report]).
system_error if writing to the terminal or stream fails.
May throw bad_alloc.
Recommended practice: 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.
void vprint_nonunicode(string_view fmt, format_args args);
Effects: Equivalent to: vprint_nonunicode(stdout, fmt, args);
void vprint_nonunicode(FILE* stream, string_view fmt, format_args args);
Preconditions: stream is a valid pointer to an output C stream.
Effects: Writes the result of vformat(fmt, args) to stream.
Throws: Any exception thrown by the call to vformat ([format.err.report]).
system_error if writing to stream fails.
May throw bad_alloc.

31.8 String-based streams [string.streams]

31.8.1 Header <sstream> synopsis [sstream.syn]

namespace std { template<class charT, class traits = char_traits<charT>, class Allocator = allocator<charT>> class basic_stringbuf; template<class charT, class traits, class Allocator> void swap(basic_stringbuf<charT, traits, Allocator>& x, basic_stringbuf<charT, traits, Allocator>& y) noexcept(noexcept(x.swap(y))); using stringbuf = basic_stringbuf<char>; using wstringbuf = basic_stringbuf<wchar_t>; template<class charT, class traits = char_traits<charT>, class Allocator = allocator<charT>> class basic_istringstream; template<class charT, class traits, class Allocator> void swap(basic_istringstream<charT, traits, Allocator>& x, basic_istringstream<charT, traits, Allocator>& y); using istringstream = basic_istringstream<char>; using wistringstream = basic_istringstream<wchar_t>; template<class charT, class traits = char_traits<charT>, class Allocator = allocator<charT>> class basic_ostringstream; template<class charT, class traits, class Allocator> void swap(basic_ostringstream<charT, traits, Allocator>& x, basic_ostringstream<charT, traits, Allocator>& y); using ostringstream = basic_ostringstream<char>; using wostringstream = basic_ostringstream<wchar_t>; template<class charT, class traits = char_traits<charT>, class Allocator = allocator<charT>> class basic_stringstream; template<class charT, class traits, class Allocator> void swap(basic_stringstream<charT, traits, Allocator>& x, basic_stringstream<charT, traits, Allocator>& y); using stringstream = basic_stringstream<char>; using wstringstream = basic_stringstream<wchar_t>; }
The header <sstream> defines four class templates and eight types that associate stream buffers with objects of class basic_string, as described in [string.classes].

31.8.2 Class template basic_stringbuf [stringbuf]

31.8.2.1 General [stringbuf.general]

namespace std { template<class charT, class traits = char_traits<charT>, class Allocator = allocator<charT>> class basic_stringbuf : public basic_streambuf<charT, traits> { public: using char_type = charT; using int_type = typename traits::int_type; using pos_type = typename traits::pos_type; using off_type = typename traits::off_type; using traits_type = traits; using allocator_type = Allocator; // [stringbuf.cons], constructors basic_stringbuf() : basic_stringbuf(ios_base::in | ios_base::out) {} explicit basic_stringbuf(ios_base::openmode which); explicit basic_stringbuf( const basic_string<charT, traits, Allocator>& s, ios_base::openmode which = ios_base::in | ios_base::out); explicit basic_stringbuf(const Allocator& a) : basic_stringbuf(ios_base::in | ios_base::out, a) {} basic_stringbuf(ios_base::openmode which, const Allocator& a); explicit basic_stringbuf( basic_string<charT, traits, Allocator>&& s, ios_base::openmode which = ios_base::in | ios_base::out); template<class SAlloc> basic_stringbuf( const basic_string<charT, traits, SAlloc>& s, const Allocator& a) : basic_stringbuf(s, ios_base::in | ios_base::out, a) {} template<class SAlloc> basic_stringbuf( const basic_string<charT, traits, SAlloc>& s, ios_base::openmode which, const Allocator& a); template<class SAlloc> explicit basic_stringbuf( const basic_string<charT, traits, SAlloc>& s, ios_base::openmode which = ios_base::in | ios_base::out); template<class T> explicit basic_stringbuf(const T& t, ios_base::openmode which = ios_base::in | ios_base::out); template<class T> basic_stringbuf(const T& t, const Allocator& a); template<class T> basic_stringbuf(const T& t, ios_base::openmode which, const Allocator& a); basic_stringbuf(const basic_stringbuf&) = delete; basic_stringbuf(basic_stringbuf&& rhs); basic_stringbuf(basic_stringbuf&& rhs, const Allocator& a); // [stringbuf.assign], assignment and swap basic_stringbuf& operator=(const basic_stringbuf&) = delete; basic_stringbuf& operator=(basic_stringbuf&& rhs); void swap(basic_stringbuf& rhs) noexcept(see below); // [stringbuf.members], getters and setters allocator_type get_allocator() const noexcept; basic_string<charT, traits, Allocator> str() const &; template<class SAlloc> basic_string<charT,traits,SAlloc> str(const SAlloc& sa) const; basic_string<charT, traits, Allocator> str() &&; basic_string_view<charT, traits> view() const noexcept; void str(const basic_string<charT, traits, Allocator>& s); template<class SAlloc> void str(const basic_string<charT, traits, SAlloc>& s); void str(basic_string<charT, traits, Allocator>&& s); template<class T> void str(const T& t); protected: // [stringbuf.virtuals], overridden virtual functions int_type underflow() override; int_type pbackfail(int_type c = traits::eof()) override; int_type overflow (int_type c = traits::eof()) override; basic_streambuf<charT, traits>* setbuf(charT*, streamsize) override; pos_type seekoff(off_type off, ios_base::seekdir way, ios_base::openmode which = ios_base::in | ios_base::out) override; pos_type seekpos(pos_type sp, ios_base::openmode which = ios_base::in | ios_base::out) override; private: ios_base::openmode mode; // exposition only basic_string<charT, traits, Allocator> buf; // exposition only void init_buf_ptrs(); // exposition only }; }
The class basic_stringbuf is derived from basic_streambuf to associate possibly the input sequence and possibly the output sequence with a sequence of arbitrary characters.
The sequence can be initialized from, or made available as, an object of class basic_string.
For the sake of exposition, the maintained data and internal pointer initialization is presented here as:
  • ios_base​::​openmode mode, has in set if the input sequence can be read, and out set if the output sequence can be written.
  • basic_string<charT, traits, Allocator> buf contains the underlying character sequence.
  • init_buf_ptrs() sets the base class' get area ([streambuf.get.area]) and put area ([streambuf.put.area]) pointers after initializing, moving from, or assigning to buf accordingly.

31.8.2.2 Constructors [stringbuf.cons]

explicit basic_stringbuf(ios_base::openmode which);
Effects: Initializes the base class with basic_streambuf() ([streambuf.cons]), and mode with which.
It is implementation-defined whether the sequence pointers (eback(), gptr(), egptr(), pbase(), pptr(), epptr()) are initialized to null pointers.
Postconditions: str().empty() is true.
explicit basic_stringbuf( const basic_string<charT, traits, Allocator>& s, ios_base::openmode which = ios_base::in | ios_base::out);
Effects: Initializes the base class with basic_streambuf() ([streambuf.cons]), mode with which, and buf with s, then calls init_buf_ptrs().
basic_stringbuf(ios_base::openmode which, const Allocator &a);
Effects: Initializes the base class with basic_streambuf() ([streambuf.cons]), mode with which, and buf with a, then calls init_buf_ptrs().
Postconditions: str().empty() is true.
explicit basic_stringbuf( basic_string<charT, traits, Allocator>&& s, ios_base::openmode which = ios_base::in | ios_base::out);
Effects: Initializes the base class with basic_streambuf() ([streambuf.cons]), mode with which, and buf with std​::​move(s), then calls init_buf_ptrs().
template<class SAlloc> basic_stringbuf( const basic_string<charT, traits, SAlloc>& s, ios_base::openmode which, const Allocator &a);
Effects: Initializes the base class with basic_streambuf() ([streambuf.cons]), mode with which, and buf with {s,a}, then calls init_buf_ptrs().
template<class SAlloc> explicit basic_stringbuf( const basic_string<charT, traits, SAlloc>& s, ios_base::openmode which = ios_base::in | ios_base::out);
Constraints: is_same_v<SAlloc, Allocator> is false.
Effects: Initializes the base class with basic_streambuf() ([streambuf.cons]), mode with which, and buf with s, then calls init_buf_ptrs().
template<class T> explicit basic_stringbuf(const T& t, ios_base::openmode which = ios_base::in | ios_base::out); template<class T> basic_stringbuf(const T& t, const Allocator& a); template<class T> basic_stringbuf(const T& t, ios_base::openmode which, const Allocator& a);
Let which be ios_base​::​in | ios_base​::​out for the overload with no parameter which, and a be Allocator() for the overload with no parameter a.
Constraints: is_convertible_v<const T&, basic_string_view<charT, traits>> is true.
Effects: Creates a variable sv as if by basic_string_view<charT, traits> sv = t, then value-initializes the base class, initializes mode with which, and direct-non-list-initializes buf with sv, a, then calls init_buf_ptrs().
basic_stringbuf(basic_stringbuf&& rhs); basic_stringbuf(basic_stringbuf&& rhs, const Allocator& a);
Effects: Copy constructs the base class from rhs and initializes mode with rhs.mode.
In the first form buf is initialized from std​::​move(rhs).str().
In the second form buf is initialized from {std​::​move(rhs).str(), a}.
It is implementation-defined whether the sequence pointers in *this (eback(), gptr(), egptr(), pbase(), pptr(), epptr()) obtain the values which rhs had.
Postconditions: Let rhs_p refer to the state of rhs just prior to this construction and let rhs_a refer to the state of rhs just after this construction.
  • str() == rhs_p.str()
  • gptr() - eback() == rhs_p.gptr() - rhs_p.eback()
  • egptr() - eback() == rhs_p.egptr() - rhs_p.eback()
  • pptr() - pbase() == rhs_p.pptr() - rhs_p.pbase()
  • epptr() - pbase() == rhs_p.epptr() - rhs_p.pbase()
  • if (eback()) eback() != rhs_a.eback()
  • if (gptr()) gptr() != rhs_a.gptr()
  • if (egptr()) egptr() != rhs_a.egptr()
  • if (pbase()) pbase() != rhs_a.pbase()
  • if (pptr()) pptr() != rhs_a.pptr()
  • if (epptr()) epptr() != rhs_a.epptr()
  • getloc() == rhs_p.getloc()
  • rhs is empty but usable, as if std​::​move(rhs).str() was called.

31.8.2.3 Assignment and swap [stringbuf.assign]

basic_stringbuf& operator=(basic_stringbuf&& rhs);