30 Input/output library [input.output]

30.1 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 106.
Table 106 — Input/output library summary
Subclause
Header(s)
Requirements
Forward declarations
<iosfwd>
Standard iostream objects
<iostream>
Iostreams base classes
<ios>
Stream buffers
<streambuf>
Formatting and manipulators
<istream>
<ostream>
<iomanip>
String streams
<sstream>
File streams
<fstream>
File systems
<filesystem>
C library files
<cstdio>
<cinttypes>
Figure [fig:streampos] illustrates relationships among various types described in this clause.
A line from A to B indicates that A is an alias (e.g. a typedef) for B or that A is defined in terms of B.
figstreampos traits_pos_type_char char_traits<char> ::pos_type streampos streampos traits_pos_type_char->streampos iostreams.limits.pos traits_pos_type_wchar_t char_traits<wchar_t> ::pos_type wstreampos wstreampos traits_pos_type_wchar_t->wstreampos iostreams.limits.pos fpos fpos<mbstate_t> streampos->fpos iostream.forward wstreampos->fpos iostream.forward traits_off_type_char char_traits<char> ::off_type streamoff streamoff traits_off_type_char->streamoff iostreams.limits.pos traits_off_type_wchar_t char_traits<wchar_t> ::off_type traits_off_type_wchar_t->streamoff iostreams.limits.pos streamoff_type signed integer type sufficient for O/S maximum file size streamoff->streamoff_type stream.types streamsize streamsize streamsize_type signed integer type represents characters xfered or buffer sizes streamsize->streamsize_type stream.types
Figure 7 — Stream position, offset, and size types [non-normative]

30.2 Iostreams requirements [iostreams.requirements]

30.2.1 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.

30.2.2 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.
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 satisfy the requirements for a character on which any of the iostream components can be instantiated.

30.2.3 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
:
Data races result in undefined behavior ([intro.multithread]).
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.

30.3 Forward declarations [iostream.forward]

30.3.1 Header <iosfwd> synopsis [iosfwd.syn]

namespace std {
  template<class charT> class char_traits;
  template<> class char_traits<char>;
  template<> class char_traits<char16_t>;
  template<> class char_traits<char32_t>;
  template<> class 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_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 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 filebuf  = basic_filebuf<char>;
  using ifstream = basic_ifstream<char>;
  using ofstream = basic_ofstream<char>;
  using fstream  = basic_fstream<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 wfilebuf  = basic_filebuf<wchar_t>;
  using wifstream = basic_ifstream<wchar_t>;
  using wofstream = basic_ofstream<wchar_t>;
  using wfstream  = basic_fstream<wchar_t>;

  template <class state> class fpos;
  using streampos  = fpos<char_traits<char>::state_type>;
  using wstreampos = fpos<char_traits<wchar_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.288
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.

30.3.2 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 and basic_­filebuf.
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 and basic_­ofstream.
The class template specialization basic_­iostream<charT, traits> serves as a base class for class templates basic_­stringstream and basic_­fstream.
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.
The types streampos and wstreampos are used for positioning streams specialized on char and wchar_­t respectively.
[Note
:
This synopsis suggests a circularity between streampos and char_­traits<char>.
An implementation can avoid this circularity by substituting equivalent types.
One way to do this might be
template<class stateT> class fpos { ... };      // depends on nothing
using _STATE = ... ;             // implementation private declaration of stateT

using streampos = fpos<_STATE>;

template<> struct char_traits<char> {
  using pos_type = streampos;
}
end note
]

30.4 Standard iostream objects [iostream.objects]

30.4.1 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;
}

30.4.2 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> ([c.files]), 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 begins execution.289
The objects are not destroyed during program execution.290
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.
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 and output functions or a standard C stream by multiple threads shall not result in a data race.
[Note
:
Users must still synchronize concurrent use of these objects and streams by multiple threads if they wish to avoid interleaved characters.
end note
]
See also: ISO C 7.21.2
If it is possible for them to do so, implementations should initialize the objects earlier than required.
Constructors and destructors for static objects can access these objects to read input from stdin or write output to stdout or stderr.

30.4.3 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>.

30.4.4 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>.

30.5 Iostreams base classes [iostreams.base]

30.5.1 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;
}

30.5.2 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.291
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.292
Typically long long.
streamsize is used in most places where ISO C would use size_­t.
Most of the uses of streamsize could use size_­t, except for the strstreambuf constructors, which require negative values.
It should probably be the signed type corresponding to size_­t (which is what Posix.
2 calls ssize_­t).

30.5.3 Class ios_­base [ios.base]

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 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 index);
    void*& pword(int index);

    // destructor:
    virtual ~ios_base();

    // [ios.base.callback], callbacks;
    enum event { erase_event, imbue_event, copyfmt_event };
    using event_callback = void (*)(event, ios_base&, int index);
    void register_callback(event_callback fn, int index);

    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
:
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.
end note
]

30.5.3.1 Types [ios.types]

30.5.3.1.1 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
:
When ios_­base​::​failure is a synonym for another type it shall provide a nested type failure, to emulate the injected class name.
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
:
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()).
end note
]
explicit failure(const string& msg, const error_code& ec = io_errc::stream);
Effects: Constructs an object of class failure by constructing the base class with msg and ec.
explicit failure(const char* msg, const error_code& ec = io_errc::stream);
Effects: Constructs an object of class failure by constructing the base class with msg and ec.

30.5.3.1.2 Type ios_­base​::​fmtflags [ios::fmtflags]

using fmtflags = T1;
The type fmtflags is a bitmask type.
Setting its elements has the effects indicated in Table 107.
Table 107fmtflags effects
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 108.
Table 108fmtflags constants
Constant
Allowable values
adjustfield
left | right | internal
basefield
dec | oct | hex
floatfield
scientific | fixed

30.5.3.1.3 Type ios_­base​::​iostate [ios::iostate]

using iostate = T2;
The type iostate is a bitmask type that contains the elements indicated in Table 109.
Table 109iostate effects
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:

30.5.3.1.4 Type ios_­base​::​openmode [ios::openmode]

using openmode = T3;
The type openmode is a bitmask type.
It contains the elements indicated in Table 110.
Table 110openmode effects
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
out
open for output
trunc
truncate an existing stream when opening

30.5.3.1.5 Type ios_­base​::​seekdir [ios::seekdir]

using seekdir = T4;
The type seekdir is an enumerated type that contains the elements indicated in Table 111.
Table 111seekdir effects
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

30.5.3.1.6 Class ios_­base​::​Init [ios::Init]

namespace std {
  class ios_base::Init {
  public:
    Init();
    ~Init();
  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 an object of class Init.
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: Destroys an object of class Init.
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().

30.5.3.2 ios_­base 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().

30.5.3.3 ios_­base functions [ios.base.locales]

locale imbue(const locale& loc);
Effects: Calls each registered callback pair (fn, index) ([ios.base.callback]) as (*fn)(imbue_­event, *this, index) at such a time that a call to ios_­base​::​getloc() from within fn returns the new locale value loc.
Returns: The previous value of getloc().
Postconditions: loc == 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.

30.5.3.4 ios_­base static members [ios.members.static]

bool sync_with_stdio(bool sync = true);
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.
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.
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.293
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.

30.5.3.5 ios_­base storage functions [ios.base.storage]

static int xalloc();
Returns: index ++.
Remarks: Concurrent access to this function by multiple threads shall not result in a data race.
long& iword(int idx);
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 operations on the object.294
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 fails295 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);
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 operations 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 fails296 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.
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.
For example, because it cannot allocate space.
For example, because it cannot allocate space.

30.5.3.6 ios_­base callbacks [ios.base.callback]

void register_callback(event_callback fn, int index);
Effects: Registers the pair (fn, index) such that during calls to imbue() ([ios.base.locales]), copyfmt(), or ~ios_­base() ([ios.base.cons]), the function fn is called with argument index.
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.
Requires: The function fn shall not throw exceptions.
Remarks: Identical pairs are not merged.
A function registered twice will be called twice.

30.5.3.7 ios_­base constructors/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: Destroys an object of class ios_­base.
Calls each registered callback pair (fn, index) ([ios.base.callback]) as (*fn)(​erase_­event, *this, index) at such time that any ios_­base member function called from within fn has well-defined results.

30.5.4 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
  };
}

30.5.4.1 fpos members [fpos.members]

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

30.5.4.2 fpos requirements [fpos.operations]

Operations specified in Table 112 are permitted.
In that table,
  • P refers to an instance of fpos,
  • p and q refer to values of type P,
  • O refers to type streamoff,
  • o refers to a value of type streamoff,
  • sz refers to a value of type streamsize and
  • i refers to a value of type int.
Table 112 — Position type requirements
Expression
Return type
Operational
Assertion/note
semantics
pre-/post-condition
P(i)
p == P(i)
note: a destructor is assumed.
P p(i);
P p = i;
Postconditions: p == P(i).
P(o)
fpos
converts from offset
O(p)
streamoff
converts to offset
P(O(p)) == p
p == q
convertible to bool
== is an equivalence relation
p != q
convertible to bool
!(p == q)
q = p + o
p += o
fpos
+ offset
q - o == p
q = p - o
p -= o
fpos
- offset
q + o == p
o = p - q
streamoff
distance
q + o == p
streamsize(o)
O(sz)
streamsize
streamoff
converts
converts
streamsize(O(sz)) == sz
streamsize(O(sz)) == sz
[Note
:
Every implementation is required to supply overloaded operators on fpos objects to satisfy the requirements of [fpos.operations].
It is unspecified whether these operators are members of fpos, global operators, or provided in some other way.
end note
]
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.

30.5.5 Class template basic_­ios [ios]

30.5.5.1 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);

  };
}

30.5.5.2 basic_­ios constructors [basic.ios.cons]

explicit basic_ios(basic_streambuf<charT, traits>* sb);
Effects: Constructs an object of class basic_­ios, assigning initial values to its member objects by calling init(sb).
basic_ios();
Effects: Constructs an object of class basic_­ios ([ios.base.cons]) leaving 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 113.
Table 113basic_­ios​::​init() effects
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

30.5.5.3 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);
Requires: If tiestr is not null, tiestr shall not be 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);
Postconditions: sb == rdbuf().
Effects: Calls clear().
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 == &rhs) does nothing.
Otherwise assigns to the member objects of *this the corresponding member objects of rhs as follows:
  1. 1.
    calls each registered callback pair (fn, index) as (*fn)(erase_­event, *this, index);
  2. 2.
    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;297 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;
  3. 3.
    calls each callback pair that was copied from rhs as (*fn)(copyfmt_­event, *this, index);
  4. 4.
    calls exceptions(rhs.exceptions()).
[Note
:
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.
end note
]
Postconditions: The postconditions of this function are indicated in Table 114.
Table 114basic_­ios​::​copyfmt() effects
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 shall have the state that rhs had before the function call, except that rdbuf() shall return 0.
rhs shall be in a valid but unspecified state, except that rhs.rdbuf() shall return the same value as it returned before the function call, and rhs.tie() shall return 0.
void swap(basic_ios& rhs) noexcept;
Effects: The states of *this and rhs shall be exchanged, except that rdbuf() shall return the same value as it returned before the function call, and rhs.rdbuf() shall return the same value as it returned before the function call.
void set_rdbuf(basic_streambuf<charT, traits>* sb);
Requires: sb != nullptr.
Effects: Associates the basic_­streambuf object pointed to by sb with this stream without calling clear().
Postconditions: rdbuf() == sb.
Throws: Nothing.
This suggests an infinite amount of copying, but the implementation can keep track of the maximum element of the arrays that is nonzero.

30.5.5.4 basic_­ios 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);
Postconditions: If rdbuf() != 0 then state == rdstate(); otherwise rdstate() == (state | ios_­base​::​badbit).
Effects: If ((state | (rdbuf() ? goodbit : badbit)) & exceptions()) == 0, returns.
Otherwise, the function throws an object of class basic_­ios​::​failure, constructed with implementation-defined argument values.
void setstate(iostate state);
Effects: Calls clear(rdstate() | state) (which may throw basic_­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().298
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);
Postconditions: except == exceptions().
Effects: Calls clear(rdstate()).
Checking badbit also for fail() is historical practice.

30.5.6 ios_­base manipulators [std.ios.manip]

30.5.6.1 fmtflags manipulators [fmtflags.manip]

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.

30.5.6.2 adjustfield manipulators [adjustfield.manip]

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.

30.5.6.3 basefield manipulators [basefield.manip]

ios_base& dec(ios_base& str);
Effects: Calls str.setf(ios_­base​::​dec, ios_­base​::​basefield).
Returns: str299.
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.
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.

30.5.6.4 floatfield manipulators [floatfield.manip]

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
:
The more obvious use of ios_­base​::​hex to specify hexadecimal floating-point format would change the meaning of existing well-defined programs.
C++ 2003 gives no meaning to the combination of fixed and scientific.
end note
]
ios_base& defaultfloat(ios_base& str);
Effects: Calls str.unsetf(ios_­base​::​floatfield).
Returns: str.

30.5.6.5 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".

30.6 Stream buffers [stream.buffers]

30.6.1 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.

30.6.2 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 “the 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).

30.6.3 Class template basic_­streambuf [streambuf]

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 an abstract base class for deriving various stream buffers whose objects each control two character sequences:
  • a character input sequence;
  • a character output sequence.

30.6.3.1 basic_­streambuf constructors [streambuf.cons]

basic_streambuf();
Effects: Constructs an object of class basic_­streambuf<charT, traits> and initializes:300
  • all its pointer member objects to null pointers,
  • the getloc() member to a copy 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);
Effects: Constructs a copy of 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.
The default constructor is protected for class basic_­streambuf to assure that only objects for classes derived from this class may be constructed.

30.6.3.2 basic_­streambuf public member functions [streambuf.members]

30.6.3.2.1 Locales [streambuf.locales]

locale pubimbue(const locale& loc);
Postconditions: loc == getloc().
Effects: Calls imbue(loc).
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.

30.6.3.2.2 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().

30.6.3.2.3 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();
Returns: 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).

30.6.3.2.4 Putback [streambuf.pub.pback]

int_type sputbackc(char_type c);
Returns: 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();
Returns: 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()).

30.6.3.2.5 Put area [streambuf.pub.put]

int_type sputc(char_type c);
Returns: 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).

30.6.3.3 basic_­streambuf protected member functions [streambuf.protected]

30.6.3.3.1 Assignment [streambuf.assign]

basic_streambuf& operator=(const basic_streambuf& rhs);
Effects: Assigns the data members of rhs to *this.
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.

30.6.3.3.2 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().

30.6.3.3.3 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().

30.6.3.4 basic_­streambuf virtual functions [streambuf.virtuals]

30.6.3.4.1 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.

30.6.3.4.2 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.

30.6.3.4.3 Get area [streambuf.virt.get]

streamsize showmanyc();301
Returns: An estimate of the number of characters available in the sequence, or -1.
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 -1, then calls to underflow() or uflow() will fail.302
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.303
Remarks: Uses traits​::​eof().
int_type underflow();
Remarks: The public members of basic_­streambuf call this virtual function only if gptr() is null or gptr() >= egptr()
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.
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 “usual 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).
Default behavior: Returns traits​::​eof().
int_type uflow();
Requires: The constraints are the same as for underflow(), except that the result character shall be transferred from the pending sequence to the backup sequence, and the pending sequence shall not be 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 increment the value of the next pointer for the input sequence.
Returns: traits​::​eof() to indicate failure.
The morphemes of showmanyc are “es-how-many-see”, not “show-manic”.
underflow or uflow might fail by throwing an exception prematurely.
The intention is not only that the calls will not return eof() but that they will return “immediately”.
Classes derived from basic_­streambuf can provide more efficient ways to implement xsgetn() and xsputn() by overriding these definitions from the base class.

30.6.3.4.4 Putback [streambuf.virt.pback]

int_type pbackfail(int_type c = 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.
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 could not 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().

30.6.3.4.5 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.
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.
Requires: Every overriding definition of this virtual function shall obey the following constraints:
  1. 1.
    The effect of consuming a character on the associated output sequence is specified304
  2. 2.
    Let r be the number of characters in the pending sequence not consumed.
    If r is nonzero then pbase() and pptr() shall be 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.
  3. 3.
    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.305
Default behavior: Returns traits​::​eof().
That is, for each class derived from an instance 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.
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).

30.7 Formatting and manipulators [iostream.format]

30.7.1 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 charT, class traits, class T>
    basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>&& is, T&& x);
}

30.7.2 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, class T>
    basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>&& os, const T& x);
}

30.7.3 Header <iomanip> synopsis [iomanip.syn]

namespace std {
  // types T1, T2, ... are unspecified implementation types
  T1 resetiosflags(ios_base::fmtflags mask);
  T2 setiosflags  (ios_base::fmtflags mask);
  T3 setbase(int base);
  template<class charT> T4 setfill(charT c);
  T5 setprecision(int n);
  T6 setw(int n);
  template <class moneyT> T7 get_money(moneyT& mon, bool intl = false);
  template <class moneyT> T8 put_money(const moneyT& mon, bool intl = false);
  template <class charT> T9 get_time(struct tm* tmb, const charT* fmt);
  template <class charT> T10 put_time(const struct tm* tmb, const charT* fmt);

  template <class charT>
    T11 quoted(const charT* s, charT delim = charT('"'), charT escape = charT('\\'));

  template <class charT, class traits, class Allocator>
    T12 quoted(const basic_string<charT, traits, Allocator>& s,
               charT delim = charT('"'), charT escape = charT('\\'));

  template <class charT, class traits, class Allocator>
    T13 quoted(basic_string<charT, traits, Allocator>& s,
               charT delim = charT('"'), charT escape = charT('\\'));

  template <class charT, class traits>
    T14 quoted(basic_string_view<charT, traits> s,
               charT delim = charT('"'), charT escape = charT('\\'));
}

30.7.4 Input streams [input.streams]

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

30.7.4.1 Class template basic_­istream [istream]

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

    basic_istream<charT, traits>& operator>>(bool& n);
    basic_istream<charT, traits>& operator>>(short& n);
    basic_istream<charT, traits>& operator>>(unsigned short& n);
    basic_istream<charT, traits>& operator>>(int& n);
    basic_istream<charT, traits>& operator>>(unsigned int& n);
    basic_istream<charT, traits>& operator>>(long& n);
    basic_istream<charT, traits>& operator>>(unsigned long& n);
    basic_istream<charT, traits>& operator>>(long long& n);
    basic_istream<charT, traits>& operator>>(unsigned long long& n);
    basic_istream<charT, traits>& operator>>(float& f);
    basic_istream<charT, traits>& operator>>(double& f);
    basic_istream<charT, traits>& operator>>(long double& f);

    basic_istream<charT, traits>& operator>>(void*& p);
    basic_istream<charT, traits>& operator>>(basic_streambuf<char_type, traits>* sb);

    // [istream.unformatted], unformatted input
    streamsize gcount() const;
    int_type get();
    basic_istream<charT, traits>& get(char_type& c);
    basic_istream<charT, traits>& get(char_type* s, streamsize n);
    basic_istream<charT, traits>& get(char_type* s, streamsize n, char_type delim);
    basic_istream<charT, traits>& get(basic_streambuf<char_type, traits>& sb);
    basic_istream<charT, traits>& get(basic_streambuf<char_type, traits>& sb, char_type delim);

    basic_istream<charT, traits>& getline(char_type* s, streamsize n);
    basic_istream<charT, traits>& getline(char_type* s, streamsize n, char_type delim);

    basic_istream<charT, traits>& ignore(streamsize n = 1, int_type delim = traits::eof());
    int_type                      peek();
    basic_istream<charT, traits>& read    (char_type* s, streamsize n);
    streamsize                    readsome(char_type* s, streamsize n);

    basic_istream<charT, traits>& putback(char_type c);
    basic_istream<charT, traits>& unget();
    int sync();

    pos_type tellg();
    basic_istream<charT, traits>& seekg(pos_type);
    basic_istream<charT, traits>& seekg(off_type, ios_base::seekdir);

  protected:
    // [istream.cons], copy/move constructor
    basic_istream(const basic_istream& rhs) = delete;
    basic_istream(basic_istream&& rhs);

    // [istream.assign], assign and swap
    basic_istream& operator=(const basic_istream& rhs) = 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>
    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*);
}
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.
If rdbuf()->sbumpc() or rdbuf()->sgetc() returns traits​::​eof(), then the input function, except as explicitly noted otherwise, completes its actions and does setstate(eofbit), which may throw ios_­base​::​failure ([iostate.flags]), before returning.
If one of these called functions throws an exception, then unless explicitly noted otherwise, the input function sets badbit in error state.
If badbit is on in exceptions(), the input 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.

30.7.4.1.1 basic_­istream constructors [istream.cons]

explicit basic_istream(basic_streambuf<charT, traits>* sb);
Effects: Constructs an object of class basic_­istream, initializing the base class subobject with basic_­ios​::​init(sb) ([basic.ios.cons]).
Postconditions: gcount() == 0.
basic_istream(basic_istream&& rhs);
Effects: Move constructs from the rvalue rhs.
This is accomplished by default constructing the base class, copying the gcount() from rhs, calling basic_­ios<charT, traits>​::​move(rhs) to initialize the base class, and setting the gcount() for rhs to 0.
virtual ~basic_istream();
Effects: Destroys an object of class basic_­istream.
Remarks: Does not perform any operations of rdbuf().

30.7.4.1.2 Class basic_­istream assign and swap [istream.assign]

basic_istream& operator=(basic_istream&& rhs);
Effects: As if by 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().

30.7.4.1.3 Class basic_­istream​::​sentry [istream::sentry]

namespace std {
  template <class charT, class traits = char_traits<charT>>
  class basic_istream<charT, traits>::sentry {
    using traits_type = traits;
    bool ok_; // exposition only
  public:
    explicit sentry(basic_istream<charT, traits>& 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<charT, traits>& 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.306
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<charT, traits>& 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]))307
~sentry();
Effects: None.
explicit operator bool() const;
Effects: Returns ok_­.
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.
The sentry constructor and destructor can also perform additional implementation-dependent operations.

30.7.4.2 Formatted input functions [istream.formatted]

30.7.4.2.1 Common requirements [istream.formatted.reqmts]

Each formatted input function begins execution by constructing 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.
If an exception is thrown during input then ios​::​badbit is turned on308 in *this's error state.
If (exceptions()&badbit) != 0 then the exception is rethrown.
In any case, the formatted input function destroys the sentry object.
If no exception has been thrown, it returns *this.
This is done without causing an ios​::​failure to be thrown.

30.7.4.2.2 Arithmetic extractors [istream.formatted.arithmetic]

operator>>(unsigned short& val); operator>>(unsigned int& val); operator>>(long& val); operator>>(unsigned long& val); operator>>(long long& val); operator>>(unsigned long long& val); operator>>(float& val); operator>>(double& val); operator>>(long double& val); operator>>(bool& val); 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:
using numget = num_get<charT, istreambuf_iterator<charT, traits>>;
iostate err = iostate::goodbit;
use_facet<numget>(loc).get(*this, 0, *this, err, val);
setstate(err);
In the above fragment, loc stands for the private member of the basic_­ios class.
[Note
:
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.
end note
]
Class locale relies on this type as its interface to istream, so that it does not need to depend directly on 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>>;
iostate err = ios_base::goodbit;
long lval;
use_facet<numget>(loc).get(*this, 0, *this, err, lval);
if (lval < numeric_limits<short>::min()) {
  err |= ios_base::failbit;
  val = numeric_limits<short>::min();
} else if (numeric_limits<short>::max() < lval) {
  err |= ios_base::failbit;
  val = numeric_limits<short>::max();
}  else
  val = static_cast<short>(lval);
setstate(err);
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>>;
iostate err = ios_base::goodbit;
long lval;
use_facet<numget>(loc).get(*this, 0, *this, err, lval);
if (lval < numeric_limits<int>::min()) {
  err |= ios_base::failbit;
  val = numeric_limits<int>::min();
} else if (numeric_limits<int>::max() < lval) {
  err |= ios_base::failbit;
  val = numeric_limits<int>::max();
}  else
  val = static_cast<int>(lval);
setstate(err);

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

basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& (*pf)(basic_istream<charT, traits>&));
Effects: None.
This extractor does not behave as a formatted input function (as described in [istream.formatted.reqmts]).
Returns: pf(*this).309
basic_istream<charT, traits>& 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<charT, traits>& operator>>(ios_base& (*pf)(ios_base&));
Effects: Calls pf(*this).310
This extractor does not behave as a formatted input function (as described in [istream.formatted.reqmts]).
Returns: *this.
template<class charT, class traits> basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& in, charT* s); template<class traits> basic_istream<char, traits>& operator>>(basic_istream<char, traits>& in, unsigned char* s); template<class traits> basic_istream<char, traits>& operator>>(basic_istream<char, traits>& in, signed char* s);
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 successive locations of an array whose first element is designated by s.
If width() is greater than zero, n is width().
Otherwise n is the number of elements of the largest array of char_­type that can store a terminating charT().
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, it calls setstate(failbit), which may throw ios_­base​::​​failure ([iostate.flags]).
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.
After a sentry object is constructed a character is extracted from in, if one is available, and stored in c.
Otherwise, the function calls in.setstate(failbit).
Returns: in.
basic_istream<charT, traits>& 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, it calls setstate(failbit), which may throw ios_­base​::​​failure ([iostate.flags]).
If it inserted no characters because it caught an exception thrown while extracting characters from *this and failbit is on in exceptions() ([iostate.flags]), then the caught exception is rethrown.
Returns: *this.
See, for example, the function signature ws(basic_­istream&).
See, for example, the function signature dec(ios_­base&).

30.7.4.3 Unformatted input functions [istream.unformatted]

Each unformatted input function begins execution by constructing 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 returns 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 an exception is thrown during input then ios​::​badbit is turned on311 in *this's error state.
(Exceptions thrown from basic_­ios<>​::​clear() are not caught or rethrown.)
If (exceptions()&badbit) != 0 then the exception is rethrown.
It also counts the number of characters extracted.
If no exception has been thrown it ends by storing the count in a member object and returning the value specified.
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.
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, the function calls setstate(failbit), which may throw ios_­base​::​failure ([iostate.flags]),
Returns: c if available, otherwise traits​::​eof().
basic_istream<charT, traits>& 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.312
Otherwise, the function calls setstate(failbit) (which may throw ios_­base​::​failure ([iostate.flags])).
Returns: *this.
basic_istream<charT, traits>& 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.313
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 (in which case the function calls setstate(eofbit));
  • traits​::​eq(c, delim) for the next available input character c (in which case c is not extracted).
If the function stores no characters, it calls setstate(failbit) (which may throw ios_­base​::​​failure ([iostate.flags])).
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<charT, traits>& get(char_type* s, streamsize n);
Effects: Calls get(s, n, widen('\n')).
Returns: Value returned by the call.
basic_istream<charT, traits>& 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, it calls setstate(failbit), which may throw ios_­base​::​​failure ([iostate.flags]).
Returns: *this.
basic_istream<charT, traits>& get(basic_streambuf<char_type, traits>& sb);
Effects: Calls get(sb, widen('\n')).
Returns: Value returned by the call.
basic_istream<charT, traits>& 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.314
Characters are extracted and stored until one of the following occurs:
  1. 1.
    end-of-file occurs on the input sequence (in which case the function calls setstate(eofbit));
  2. 2.
    traits​::​eq(c, delim) for the next available input character c (in which case the input character is extracted but not stored);315
  3. 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.316
If the function extracts no characters, it calls setstate(failbit) (which may throw ios_­base​::​​failure ([iostate.flags])).317
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
:
#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;
  }
}
end example
]
basic_istream<charT, traits>& getline(char_type* s, streamsize n);
Returns: getline(s, n, widen('\n'))
basic_istream<charT, traits>& 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() 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).
Remarks: The last condition will never occur if traits​::​eq_­int_­type(delim, traits​::​eof()).
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<charT, traits>& 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.318
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<charT, traits>& 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().
If rdbuf() is null, or if sputbackc() returns traits​::​eof(), calls setstate(badbit) (which may throw ios_­base​::​failure ([iostate.flags])).
[Note
:
This function extracts no characters, so the value returned by the next call to gcount() is 0.
end note
]
Returns: *this.
basic_istream<charT, traits>& 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
:
This function extracts no characters, so the value returned by the next call to gcount() is 0.
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<charT, traits>& 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<charT, traits>& 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.
This is done without causing an ios​::​failure to be thrown.
Note that this function is not overloaded on types signed char and unsigned char.
Note that this function is not overloaded on types signed char and unsigned char.
Note that this function is not overloaded on types signed char and unsigned char.
Since the final input character is “extracted”, it is counted in the gcount(), even though it is not stored.
This allows an input line which exactly fills the buffer, without setting failbit.
This is different behavior than the historical AT&T implementation.
This implies an empty input line will not cause failbit to be set.
Note that this function is not overloaded on types signed char and unsigned char.

30.7.4.4 Standard basic_­istream manipulators [istream.manip]

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.

30.7.4.5 Rvalue stream extraction [istream.rvalue]

template <class charT, class traits, class T> basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>&& is, T&& x);
Effects: Equivalent to:
is >> std::forward<T>(x);
return is;
Remarks: This function shall not participate in overload resolution unless the expression is >> std​::​forward<T>(x) is well-formed.

30.7.4.6 Class template basic_­iostream [iostreamclass]

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& rhs) = delete;
    basic_iostream(basic_iostream&& rhs);

    // [iostream.assign], assign and swap
    basic_iostream& operator=(const basic_iostream& rhs) = 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.

30.7.4.6.1 basic_­iostream constructors [iostream.cons]

explicit basic_iostream(basic_streambuf<charT, traits>* sb);
Effects: Constructs an object of class basic_­iostream, initializing the base class subobjects with basic_­istream<charT, traits>(sb) 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 move(rhs).

30.7.4.6.2 basic_­iostream destructor [iostream.dest]

virtual ~basic_iostream();
Effects: Destroys an object of class basic_­iostream.
Remarks: Does not perform any operations on rdbuf().

30.7.4.6.3 basic_­iostream assign and swap [iostream.assign]

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

30.7.5 Output streams [output.streams]

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

30.7.5.1 Class template basic_­ostream [ostream]

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

    basic_ostream<charT, traits>& operator<<(bool n);
    basic_ostream<charT, traits>& operator<<(short n);
    basic_ostream<charT, traits>& operator<<(unsigned short n);
    basic_ostream<charT, traits>& operator<<(int n);
    basic_ostream<charT, traits>& operator<<(unsigned int n);
    basic_ostream<charT, traits>& operator<<(long n);
    basic_ostream<charT, traits>& operator<<(unsigned long n);
    basic_ostream<charT, traits>& operator<<(long long n);
    basic_ostream<charT, traits>& operator<<(unsigned long long n);
    basic_ostream<charT, traits>& operator<<(float f);
    basic_ostream<charT, traits>& operator<<(double f);
    basic_ostream<charT, traits>& operator<<(long double f);

    basic_ostream<charT, traits>& operator<<(const void* p);
    basic_ostream<charT, traits>& operator<<(nullptr_t);
    basic_ostream<charT, traits>& operator<<(basic_streambuf<char_type, traits>* sb);

    // [ostream.unformatted], unformatted output
    basic_ostream<charT, traits>& put(char_type c);
    basic_ostream<charT, traits>& write(const char_type* s, streamsize n);

    basic_ostream<charT, traits>& flush();

    // [ostream.seeks], seeks
    pos_type tellp();
    basic_ostream<charT, traits>& seekp(pos_type);
    basic_ostream<charT, traits>& seekp(off_type, ios_base::seekdir);

  protected:
    // [ostream.cons], copy/move constructor
    basic_ostream(const basic_ostream& rhs) = delete;
    basic_ostream(basic_ostream&& rhs);

    // [ostream.assign], assign and swap
    basic_ostream& operator=(const basic_ostream& rhs) = 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 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*);
}
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 error state.
If badbit is on in exceptions(), the output function rethrows the exception without completing its actions, otherwise it does not throw anything and treat as an error.

30.7.5.1.1 basic_­ostream constructors [ostream.cons]

explicit basic_ostream(basic_streambuf<charT, traits>* sb);
Effects: Constructs an object of class basic_­ostream, initializing 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();
Effects: Destroys an object of class basic_­ostream.
Remarks: Does not perform any operations on rdbuf().

30.7.5.1.2 Class basic_­ostream assign and swap [ostream.assign]

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

30.7.5.1.3 Class basic_­ostream​::​sentry [ostream::sentry]

namespace std {
  template <class charT, class traits = char_traits<charT>>
  class basic_ostream<charT, traits>::sentry {
    bool ok_; // exposition only
  public:
    explicit sentry(basic_ostream<charT, traits>& 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<charT, traits>& os);
If os.good() is nonzero, prepares for formatted or unformatted output.
If os.tie() is not a null pointer, calls os.tie()->flush().319
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]))320
~sentry();
If (os.flags() & ios_­base​::​unitbuf) && !uncaught_­exceptions() && os.good() is true, calls os.rdbuf()->pubsync().
If that function returns -1, sets badbit in os.rdstate() without propagating an exception.
explicit operator bool() const;
Effects: Returns ok_­.
The call os.tie()->flush() does not necessarily occur if the function can determine that no synchronization is necessary.
The sentry constructor and destructor can also perform additional implementation-dependent operations.

30.7.5.1.4 basic_­ostream 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<charT, traits>& 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<charT, traits>& 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.

30.7.5.2 Formatted output functions [ostream.formatted]

30.7.5.2.1 Common requirements [ostream.formatted.reqmts]

Each formatted output function begins execution by constructing an object of class sentry.
If this 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 might throw an exception.
If an exception is thrown during output, then ios​::​badbit is turned on321 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.
without causing an ios​::​failure to be thrown.

30.7.5.2.2 Arithmetic inserters [ostream.inserters.arithmetic]

operator<<(bool val); operator<<(short val); operator<<(unsigned short val); operator<<(int val); operator<<(unsigned int val); operator<<(long val); operator<<(unsigned long val); operator<<(long long val); operator<<(unsigned long long val); operator<<(float val); operator<<(double val); operator<<(long double val); 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.

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

basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& (*pf)(basic_ostream<charT, traits>&));
Effects: None.
Does not behave as a formatted output function (as described in [ostream.formatted.reqmts]).
Returns: pf(*this).322
basic_ostream<charT, traits>& 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.323
basic_ostream<charT, traits>& 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<charT, traits>& operator<<(basic_streambuf<charT, traits>* sb);
Effects: Behaves as an unformatted output function.
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 error state, and if failbit is on in exceptions() the caught exception is rethrown.
Returns: *this.
basic_ostream<charT, traits>& operator<<(nullptr_t);
Effects: Equivalent to:
return *this << s;
where s is an implementation-defined NTCTS.
See, for example, the function signature endl(basic_­ostream&).
See, for example, the function signature dec(ios_­base&).

30.7.5.2.4 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);
Requires: s shall not be 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.

30.7.5.3 Unformatted output functions [ostream.unformatted]

Each unformatted output function begins execution by constructing an object of class sentry.
If this 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​::​badbit is turned on324 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<charT, traits>& 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.325
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.326
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 this object returns true when converted to a value of type bool the function calls rdbuf()->pubsync().
If that function returns -1 calls setstate(badbit) (which may throw ios_­base​::​failure ([iostate.flags])).
Otherwise, if the sentry object returns false, does nothing.
Returns: *this.
without causing an ios​::​failure to be thrown.
Note that this function is not overloaded on types signed char and unsigned char.
Note that this function is not overloaded on types signed char and unsigned char.

30.7.5.4 Standard basic_­ostream manipulators [ostream.manip]

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.

30.7.5.5 Rvalue stream insertion [ostream.rvalue]

template <class charT, class traits, class T> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>&& os, const T& x);
Effects: As if by: os << x;
Returns: os.
Remarks: This function shall not participate in overload resolution unless the expression os << x is well-formed.

30.7.6 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:327
void f(ios_base& str, ios_base::fmtflags mask) {
  // reset specified flags
  str.setf(ios_base::fmtflags(0), mask);
}
The expression out << resetiosflags(mask) shall have type basic_­ostream<charT, traits>& and value out.
The expression in >> resetiosflags(mask) shall have 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) shall have type basic_­ostream<charT, traits>& and value out.
The expression in >> setiosflags(mask) shall have 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) shall have type basic_­ostream<charT, traits>& and value out.
The expression in >> setbase(base) shall have 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) shall have 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) shall have type basic_­ostream<charT, traits>& and value out.
The expression in >> setprecision(n) shall have 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) shall have type basic_­ostream<charT, traits>& and value out.
The expression in >> setw(n) shall have type basic_­istream<charT, traits>& and value in.
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).

30.7.7 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);
Requires: The type moneyT shall be 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) shall have type basic_­istream<charT, traits>& and value in.
template <class moneyT> unspecified put_money(const moneyT& mon, bool intl = false);
Requires: The type moneyT shall be 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::badbit);
}
The expression out << put_­money(mon, intl) shall have type basic_­ostream<charT, traits>& and value out.
template <class charT> unspecified get_time(struct tm* tmb, const charT* fmt);
Requires: The argument tmb shall be a valid pointer to an object of type struct tm.
The argument fmt shall be a valid pointer to an array of objects of type charT with char_­traits<charT>​::​length(fmt) elements.
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, struct 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) shall have type basic_­istream<charT, traits>& and value in.
template <class charT> unspecified put_time(const struct tm* tmb, const charT* fmt);
Requires: The argument tmb shall be a valid pointer to an object of type struct tm, and the argument fmt shall be a valid pointer to an array of objects of type charT with char_­traits<charT>​::​length(​fmt) elements.
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 struct 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) shall have type basic_­ostream<charT, traits>& and value out.

30.7.8 Quoted manipulators [quoted.manip]

[Note
:
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.
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) shall have 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) shall have type basic_­istream<charT, traits>& and value in.
The expression out << quoted(s, delim, escape) shall have type basic_­ostream​<charT, traits>& and value out.

30.8 String-based streams [string.streams]

30.8.1 Header <sstream> synopsis [sstream.syn]

namespace std {
  template <class charT, class traits = char_traits<charT>,
            class Allocator = allocator<charT>>
    class basic_stringbuf;

  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;

  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;
  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;
  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].

30.8.2 Class template basic_­stringbuf [stringbuf]

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
    explicit basic_stringbuf(
      ios_base::openmode which = ios_base::in | ios_base::out);
    explicit basic_stringbuf(
      const basic_string<charT, traits, Allocator>& str,
      ios_base::openmode which = ios_base::in | ios_base::out);
    basic_stringbuf(const basic_stringbuf& rhs) = delete;
    basic_stringbuf(basic_stringbuf&& rhs);

    // [stringbuf.assign], assign and swap
    basic_stringbuf& operator=(const basic_stringbuf& rhs) = delete;
    basic_stringbuf& operator=(basic_stringbuf&& rhs);
    void swap(basic_stringbuf& rhs);

    // [stringbuf.members], get and set
    basic_string<charT, traits, Allocator> str() const;
    void str(const basic_string<charT, traits, Allocator>& s);

  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
  };

  template <class charT, class traits, class Allocator>
    void swap(basic_stringbuf<charT, traits, Allocator>& x,
              basic_stringbuf<charT, traits, Allocator>& y);
}
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 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.

30.8.2.1 basic_­stringbuf constructors [stringbuf.cons]

explicit basic_stringbuf( ios_base::openmode which = ios_base::in | ios_base::out);
Effects: Constructs an object of class basic_­stringbuf, initializing the base class with basic_­streambuf() ([streambuf.cons]), and initializing mode with which.
Postconditions: str() == "".
explicit basic_stringbuf( const basic_string<charT, traits, Allocator>& s, ios_base::openmode which = ios_base::in | ios_base::out);
Effects: Constructs an object of class basic_­stringbuf, initializing the base class with basic_­streambuf() ([streambuf.cons]), and initializing mode with which.
Then calls str(s).
basic_stringbuf(basic_stringbuf&& rhs);
Effects: Move constructs from the rvalue rhs.
It is implementation-defined whether the sequence pointers in *this (eback(), gptr(), egptr(), pbase(), pptr(), epptr()) obtain the values which rhs had.
Whether they do or not, *this and rhs reference separate buffers (if any at all) after the construction.
The openmode, locale and any other state of rhs is also copied.
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()

30.8.2.2 Assign and swap [stringbuf.assign]

basic_stringbuf& operator=(basic_stringbuf&& rhs);
Effects: After the move assignment *this has the observable state it would have had if it had been move constructed from rhs (see [stringbuf.cons]).
Returns: *this.
void swap(basic_stringbuf& rhs);
Effects: Exchanges the state of *this and rhs.
template <class charT, class traits, class Allocator> void swap(basic_stringbuf<charT, traits, Allocator>& x, basic_stringbuf<charT, traits, Allocator>& y);
Effects: As if by x.swap(y).

30.8.2.3 Member functions [stringbuf.members]

basic_string<charT, traits, Allocator> str() const;
Returns: A basic_­string object whose content is equal to the basic_­stringbuf underlying character sequence.
If the basic_­stringbuf was created only in input mode, the resultant basic_­string contains the character sequence in the range [eback(), egptr()).
If the basic_­stringbuf was created with which & ios_­base​::​out being nonzero then the resultant basic_­string contains the character sequence in the range [pbase(), high_­mark), where high_­mark represents the position one past the highest initialized character in the buffer.
Characters can be initialized by writing to the stream, by constructing the basic_­stringbuf with a basic_­string, or by calling the str(basic_­string) member function.
In the case of calling the str(basic_­string) member function, all characters initialized prior to the call are now considered uninitialized (except for those characters re-initialized by the new basic_­string).
Otherwise the basic_­stringbuf has been created in neither input nor output mode and a zero length basic_­string is returned.
void str(const basic_string<charT, traits, Allocator>& s);
Effects: Copies the content of s into the basic_­stringbuf underlying character sequence and initializes the input and output sequences according to mode.
Postconditions: If mode & ios_­base​::​out is nonzero, pbase() points to the first underlying character and epptr() >= pbase() + s.size() holds; in addition, if mode & ios_­base​::​ate is nonzero, pptr() == pbase() + s.size() holds, otherwise pptr() == pbase() is true.
If mode & ios_­base​::​in is nonzero, eback() points to the first underlying character, and both gptr() == eback() and egptr() == eback() + s.size() hold.

30.8.2.4 Overridden virtual functions [stringbuf.virtuals]

int_type underflow() override;
Returns: If the input sequence has a read position available, returns traits​::​to_­int_­type(*gptr()).
Otherwise, returns traits​::​eof().
Any character in the underlying buffer which has been initialized is considered to be part of the input sequence.
int_type pbackfail(int_type c = traits::eof()) override;
Effects: Puts back the character designated by c to the input sequence, if possible, in one of three ways:
  • If traits​::​eq_­int_­type(c, traits​::​eof()) returns false and if the input sequence has a putback position available, and if traits​::​eq(to_­char_­type(c), gptr()[-1]) returns true, assigns gptr() - 1 to gptr().
    Returns: c.
  • If traits​::​eq_­int_­type(c, traits​::​eof()) returns false and if the input sequence has a putback position available, and if mode & ios_­base​::​out is nonzero, assigns c to *--gptr().
    Returns: c.
  • If traits​::​eq_­int_­type(c, traits​::​eof()) returns true and if the input sequence has a putback position available, assigns gptr() - 1 to gptr().
    Returns: traits​::​not_­eof(c).
Returns: As specified above, or traits​::​eof() to indicate failure.
Remarks: If the function can succeed in more than one of these ways, it is unspecified which way is chosen.
int_type overflow(int_type c = traits::eof()) override;
Effects: Appends the character designated by c to the output sequence, if possible, in one of two ways:
  • If traits​::​eq_­int_­type(c, traits​::​eof()) returns false and if either the output sequence has a write position available or the function makes a write position available (as described below), the function calls sputc(c).
    Signals success by returning c.
  • If traits​::​eq_­int_­type(c, traits​::​eof()) returns true, there is no character to append.
    Signals success by returning a value other than traits​::​eof().
Remarks: The function can alter the number of write positions available as a result of any call.
Returns: As specified above, or traits​::​eof() to indicate failure.
The function can make a write position available only if (mode & ios_­base​::​out) != 0.
To make a write position available, the function reallocates (or initially allocates) an array object with a sufficient number of elements to hold the current array object (if any), plus at least one additional write position.
If (mode & ios_­base​::​in) != 0, the function alters the read end pointer egptr() to point just past the new write position.
pos_type seekoff(off_type off, ios_base::seekdir way, ios_base::openmode which = ios_base::in | ios_base::out) override;
Effects: Alters the stream position within one of the controlled sequences, if possible, as indicated in Table 115.
Table 115seekoff positioning
Conditions
Result
(which & ios_­base​::​in) == ios_­base​::​in
positions the input sequence
(which & ios_­base​::​out) == ios_­base​::​out
positions the output sequence
(which & (ios_­base​::​in |
ios_­base​::​out)) ==
(ios_­base​::​in |
ios_­base​::​out)
and way == either
ios_­base​::​beg or
ios_­base​::​end
positions both the input and the output sequences
Otherwise
the positioning operation fails.
For a sequence to be positioned, the function determines newoff as indicated in Table 116.
If the sequence's next pointer (either gptr() or pptr()) is a null pointer and newoff is nonzero, the positioning operation fails.
Table 116newoff values
Condition
newoff Value
way == ios_­base​::​beg
0
way == ios_­base​::​cur
the next pointer minus the beginning pointer (xnext - xbeg).
way == ios_­base​::​end
the high mark pointer minus the beginning pointer (high_­mark - xbeg).
If (newoff + off) < 0, or if newoff + off refers to an uninitialized character ([stringbuf.members]), the positioning operation fails.
Otherwise, the function assigns xbeg + newoff + off to the next pointer xnext.
Returns: pos_­type(newoff), constructed from the resultant offset newoff (of type off_­type), that stores the resultant stream position, if possible.
If the positioning operation fails, or if the constructed object cannot represent the resultant stream position, the return value is pos_­type(off_­type(-1)).
pos_type seekpos(pos_type sp, ios_base::openmode which = ios_base::in | ios_base::out) override;
Effects: Equivalent to seekoff(off_­type(sp), ios_­base​::​beg, which).
Returns: sp to indicate success, or pos_­type(off_­type(-1)) to indicate failure.
basic_streambuf<charT, traits>* setbuf(charT* s, streamsize n);
Effects: implementation-defined, except that setbuf(0, 0) has no effect.
Returns: this.

30.8.3 Class template basic_­istringstream [istringstream]

namespace std {
  template <class charT, class traits = char_traits<charT>,
            class Allocator = allocator<charT>>
  class basic_istringstream : public basic_istream<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;

    // [istringstream.cons], constructors
    explicit basic_istringstream(
      ios_base::openmode which = ios_base::in);
    explicit basic_istringstream(
      const basic_string<charT, traits, Allocator>& str,
      ios_base::openmode which = ios_base::in);
    basic_istringstream(const basic_istringstream& rhs) = delete;
    basic_istringstream(basic_istringstream&& rhs);

    // [istringstream.assign], assign and swap
    basic_istringstream& operator=(const basic_istringstream& rhs) = delete;
    basic_istringstream& operator=(basic_istringstream&& rhs);
    void swap(basic_istringstream& rhs);

    // [istringstream.members], members
    basic_stringbuf<charT, traits, Allocator>* rdbuf() const;

    basic_string<charT, traits, Allocator> str() const;
    void str(const basic_string<charT, traits, Allocator>& s);
  private:
    basic_stringbuf<charT, traits, Allocator> sb; // exposition only
  };

  template <class charT, class traits, class Allocator>
    void swap(basic_istringstream<charT, traits, Allocator>& x,
              basic_istringstream<charT, traits, Allocator>& y);
}
The class basic_­istringstream<charT, traits, Allocator> supports reading objects of class basic_­string<​charT, traits, Allocator>.
It uses a basic_­stringbuf<charT, traits, Allocator> object to control the associated storage.
For the sake of exposition, the maintained data is presented here as:
  • sb, the stringbuf object.

30.8.3.1 basic_­istringstream constructors [istringstream.cons]

explicit basic_istringstream(ios_base::openmode which = ios_base::in);
Effects: Constructs an object of class basic_­istringstream<charT, traits>, initializing the base class with basic_­istream<charT, traits>(&sb) ([istream]) and initializing sb with basic_­stringbuf<charT, traits, Allocator>(which | ios_­base​::​in)) ([stringbuf.cons]).
explicit basic_istringstream( const basic_string<charT, traits, Allocator>& str, ios_base::openmode which = ios_base::in);
Effects: Constructs an object of class basic_­istringstream<charT, traits>, initializing the base class with basic_­istream<charT, traits>(&sb) ([istream]) and initializing sb with basic_­stringbuf<charT, traits, Allocator>(str, which | ios_­base​::​in)) ([stringbuf.cons]).
basic_istringstream(basic_istringstream&& rhs);
Effects: Move constructs from the rvalue rhs.
This is accomplished by move constructing the base class, and the contained basic_­stringbuf.
Next basic_­istream<charT, traits>​::​set_­rdbuf(&sb) is called to install the contained basic_­stringbuf.

30.8.3.2 Assign and swap [istringstream.assign]

basic_istringstream& operator=(basic_istringstream&& rhs);
Effects: Move assigns the base and members of *this from the base and corresponding members of rhs.
Returns: *this.
void swap(basic_istringstream& rhs);
Effects: Exchanges the state of *this and rhs by calling basic_­istream<charT, traits>​::​swap(rhs) and sb.swap(rhs.sb).
template <class charT, class traits, class Allocator> void swap(basic_istringstream<charT, traits, Allocator>& x, basic_istringstream<charT, traits, Allocator>& y);
Effects: As if by x.swap(y).

30.8.3.3 Member functions [istringstream.members]

basic_stringbuf<charT, traits, Allocator>* rdbuf() const;
Returns: const_­cast<basic_­stringbuf<charT, traits, Allocator>*>(&sb).
basic_string<charT, traits, Allocator> str() const;
Returns: rdbuf()->str().
void str(const basic_string<charT, traits, Allocator>& s);
Effects: Calls rdbuf()->str(s).

30.8.4 Class template basic_­ostringstream [ostringstream]

namespace std {
  template <class charT, class traits = char_traits<charT>,
            class Allocator = allocator<charT>>
  class basic_ostringstream : 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;
    using allocator_type = Allocator;

    // [ostringstream.cons], constructors
    explicit basic_ostringstream(
      ios_base::openmode which = ios_base::out);
    explicit basic_ostringstream(
      const basic_string<charT, traits, Allocator>& str,
      ios_base::openmode which = ios_base::out);
    basic_ostringstream(const basic_ostringstream& rhs) = delete;
    basic_ostringstream(basic_ostringstream&& rhs);

    // [ostringstream.assign], assign and swap
    basic_ostringstream& operator=(const basic_ostringstream& rhs) = delete;
    basic_ostringstream& operator=(basic_ostringstream&& rhs);
    void swap(basic_ostringstream& rhs);

    // [ostringstream.members], members
    basic_stringbuf<charT, traits, Allocator>* rdbuf() const;

    basic_string<charT, traits, Allocator> str() const;
    void str(const basic_string<charT, traits, Allocator>& s);
   private:
    basic_stringbuf<charT, traits, Allocator> sb; // exposition only
  };

  template <class charT, class traits, class Allocator>
    void swap(basic_ostringstream<charT, traits, Allocator>& x,
              basic_ostringstream<charT, traits, Allocator>& y);
}
The class basic_­ostringstream<charT, traits, Allocator> supports writing objects of class basic_­string<​charT, traits, Allocator>.
It uses a basic_­stringbuf object to control the associated storage.
For the sake of exposition, the maintained data is presented here as:
  • sb, the stringbuf object.

30.8.4.1 basic_­ostringstream constructors [ostringstream.cons]

explicit basic_ostringstream( ios_base::openmode which = ios_base::out);
Effects: Constructs an object of class basic_­ostringstream<charT, traits>, initializing the base class with basic_­ostream<charT, traits>(&sb) ([ostream]) and initializing sb with basic_­stringbuf<charT, traits, Allocator>(which | ios_­base​::​out)) ([stringbuf.cons]).
explicit basic_ostringstream( const basic_string<charT, traits, Allocator>& str, ios_base::openmode which = ios_base::out);
Effects: Constructs an object of class basic_­ostringstream<charT, traits>, initializing the base class with basic_­ostream<charT, traits>(&sb) ([ostream]) and initializing sb with basic_­stringbuf<charT, traits, Allocator>(str, which | ios_­base​::​out)) ([stringbuf.cons]).
basic_ostringstream(basic_ostringstream&& rhs);
Effects: Move constructs from the rvalue rhs.
This is accomplished by move constructing the base class, and the contained basic_­stringbuf.
Next basic_­ostream<charT, traits>​::​set_­rdbuf(&sb) is called to install the contained basic_­stringbuf.

30.8.4.2 Assign and swap [ostringstream.assign]

basic_ostringstream& operator=(basic_ostringstream&& rhs);
Effects: Move assigns the base and members of *this from the base and corresponding members of rhs.
Returns: *this.
void swap(basic_ostringstream& rhs);
Effects: Exchanges the state of *this and rhs by calling basic_­ostream<charT, traits>​::​swap(rhs) and sb.swap(rhs.sb).
template <class charT, class traits, class Allocator> void swap(basic_ostringstream<charT, traits, Allocator>& x, basic_ostringstream<charT, traits, Allocator>& y);
Effects: As if by x.swap(y).

30.8.4.3 Member functions [ostringstream.members]

basic_stringbuf<charT, traits, Allocator>* rdbuf() const;
Returns: const_­cast<basic_­stringbuf<charT, traits, Allocator>*>(&sb).
basic_string<charT, traits, Allocator> str() const;
Returns: rdbuf()->str().
void str(const basic_string<charT, traits, Allocator>& s);
Effects: Calls rdbuf()->str(s).

30.8.5 Class template basic_­stringstream [stringstream]

namespace std {
  template <class charT, class traits = char_traits<charT>,
            class Allocator = allocator<charT>>
  class basic_stringstream : public basic_iostream<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;

    // [stringstream.cons], constructors
    explicit basic_stringstream(
      ios_base::openmode which = ios_base::out | ios_base::in);
    explicit basic_stringstream(
      const basic_string<charT, traits, Allocator>& str,
      ios_base::openmode which = ios_base::out | ios_base::in);
    basic_stringstream(const basic_stringstream& rhs) = delete;
    basic_stringstream(basic_stringstream&& rhs);

    // [stringstream.assign], assign and swap
    basic_stringstream& operator=(const basic_stringstream& rhs) = delete;
    basic_stringstream& operator=(basic_stringstream&& rhs);
    void swap(basic_stringstream& rhs);

    // [stringstream.members], members
    basic_stringbuf<charT, traits, Allocator>* rdbuf() const;
    basic_string<charT, traits, Allocator> str() const;
    void str(const basic_string<charT, traits, Allocator>& str);

  private:
    basic_stringbuf<charT, traits> sb;  // exposition only
  };

  template <class charT, class traits, class Allocator>
    void swap(basic_stringstream<charT, traits, Allocator>& x,
              basic_stringstream<charT, traits, Allocator>& y);
}
The class template basic_­stringstream<charT, traits> supports reading and writing from objects of class basic_­string<charT, traits, Allocator>.
It uses a basic_­stringbuf<charT, traits, Allocator> object to control the associated sequence.
For the sake of exposition, the maintained data is presented here as
  • sb, the stringbuf object.

30.8.5.1 basic_­stringstream constructors [stringstream.cons]

explicit basic_stringstream( ios_base::openmode which = ios_base::out | ios_base::in);
Effects: Constructs an object of class basic_­stringstream<charT, traits>, initializing the base class with basic_­iostream<charT, traits>(&sb) ([iostream.cons]) and initializing sb with basic_­stringbuf<charT, traits, Allocator>(which).
explicit basic_stringstream( const basic_string<charT, traits, Allocator>& str, ios_base::openmode which = ios_base::out | ios_base::in);
Effects: Constructs an object of class basic_­stringstream<charT, traits>, initializing the base class with basic_­iostream<charT, traits>(&sb) ([iostream.cons]) and initializing sb with basic_­stringbuf<charT, traits, Allocator>(str, which).
basic_stringstream(basic_stringstream&& rhs);
Effects: Move constructs from the rvalue rhs.
This is accomplished by move constructing the base class, and the contained basic_­stringbuf.
Next basic_­istream<charT, traits>​::​set_­rdbuf(&sb) is called to install the contained basic_­stringbuf.

30.8.5.2 Assign and swap [stringstream.assign]

basic_stringstream& operator=(basic_stringstream&& rhs);
Effects: Move assigns the base and members of *this from the base and corresponding members of rhs.
Returns: *this.
void swap(basic_stringstream& rhs);
Effects: Exchanges the state of *this and rhs by calling basic_­iostream<charT,traits>​::​swap(rhs) and sb.swap(rhs.sb).
template <class charT, class traits, class Allocator> void swap(basic_stringstream<charT, traits, Allocator>& x, basic_stringstream<charT, traits, Allocator>& y);
Effects: As if by x.swap(y).

30.8.5.3 Member functions [stringstream.members]

basic_stringbuf<charT, traits, Allocator>* rdbuf() const;
Returns: const_­cast<basic_­stringbuf<charT, traits, Allocator>*>(&sb)
basic_string<charT, traits, Allocator> str() const;
Returns: rdbuf()->str().
void str(const basic_string<charT, traits, Allocator>& str);
Effects: Calls rdbuf()->str(str).

30.9 File-based streams [file.streams]

30.9.1 Header <fstream> synopsis [fstream.syn]

namespace std {
  template <class charT, class traits = char_traits<charT>>
    class basic_filebuf;
  using filebuf  = basic_filebuf<char>;
  using wfilebuf = basic_filebuf<wchar_t>;

  template <class charT, class traits = char_traits<charT>>
    class basic_ifstream;
  using ifstream  = basic_ifstream<char>;
  using wifstream = basic_ifstream<wchar_t>;

  template <class charT, class traits = char_traits<charT>>
    class basic_ofstream;
  using ofstream  = basic_ofstream<char>;
  using wofstream = basic_ofstream<wchar_t>;

  template <class charT, class traits = char_traits<charT>>
    class basic_fstream;
  using fstream  = basic_fstream<char>;
  using wfstream = basic_fstream<wchar_t>;
}
The header <fstream> defines four class templates and eight types that associate stream buffers with files and assist reading and writing files.
[Note
:
The class template basic_­filebuf treats a file as a source or sink of bytes.
In an environment that uses a large character set, the file typically holds multibyte character sequences and the basic_­filebuf object converts those multibyte sequences into wide character sequences.
end note
]
In this subclause, member functions taking arguments of const filesystem​::​path​::​value_­type* are only be provided on systems where filesystem​::​path​::​value_­type ([fs.class.path]) is not char.
[Note
:
These functions enable class path support for systems with a wide native path character type, such as wchar_­t.
end note
]

30.9.2 Class template basic_­filebuf [filebuf]

namespace std {
  template <class charT, class traits = char_traits<charT>>
  class basic_filebuf : 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;

    // [filebuf.cons], constructors/destructor
    basic_filebuf();
    basic_filebuf(const basic_filebuf& rhs) = delete;
    basic_filebuf(basic_filebuf&& rhs);
    virtual ~basic_filebuf();

    // [filebuf.assign], assign and swap
    basic_filebuf& operator=(const basic_filebuf& rhs) = delete;
    basic_filebuf& operator=(basic_filebuf&& rhs);
    void swap(basic_filebuf& rhs);

    // [filebuf.members], members
    bool is_open() const;
    basic_filebuf* open(const char* s, ios_base::openmode mode);
    basic_filebuf* open(const filesystem::path::value_type* s,
                        ios_base::openmode mode);  // wide systems only; see [fstream.syn]
    basic_filebuf* open(const string& s,
                        ios_base::openmode mode);
    basic_filebuf* open(const filesystem::path& s,
                        ios_base::openmode mode);
    basic_filebuf* close();

  protected:
    // [filebuf.virtuals], overridden virtual functions
    streamsize showmanyc() override;
    int_type underflow() override;
    int_type uflow() override;
    int_type pbackfail(int_type c = traits::eof()) override;
    int_type overflow (int_type c = traits::eof()) override;

    basic_streambuf<charT, traits>* setbuf(char_type* s,
                                           streamsize n) 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;
    int      sync() override;
    void     imbue(const locale& loc) override;
  };

  template <class charT, class traits>
    void swap(basic_filebuf<charT, traits>& x,
              basic_filebuf<charT, traits>& y);
}
The class basic_­filebuf<charT, traits> associates both the input sequence and the output sequence with a file.
The restrictions on reading and writing a sequence controlled by an object of class basic_­filebuf<charT, traits> are the same as for reading and writing with the C standard library FILEs.
In particular:
  • If the file is not open for reading the input sequence cannot be read.
  • If the file is not open for writing the output sequence cannot be written.
  • A joint file position is maintained for both the input sequence and the output sequence.
An instance of basic_­filebuf behaves as described in [filebuf] provided traits​::​pos_­type is fpos<traits​::​​state_­type>.
Otherwise the behavior is undefined.
In order to support file I/O and multibyte/wide character conversion, conversions are performed using members of a facet, referred to as a_­codecvt in following sections, obtained as if by
const codecvt<charT, char, typename traits::state_type>& a_codecvt =
  use_facet<codecvt<charT, char, typename traits::state_type>>(getloc());

30.9.2.1 basic_­filebuf constructors [filebuf.cons]

basic_filebuf();
Effects: Constructs an object of class basic_­filebuf<charT, traits>, initializing the base class with basic_­streambuf<charT, traits>() ([streambuf.cons]).
Postconditions: is_­open() == false.
basic_filebuf(basic_filebuf&& rhs);
Effects: Move constructs from the rvalue rhs.
It is implementation-defined whether the sequence pointers in *this (eback(), gptr(), egptr(), pbase(), pptr(), epptr()) obtain the values which rhs had.
Whether they do or not, *this and rhs reference separate buffers (if any at all) after the construction.
Additionally *this references the file which rhs did before the construction, and rhs references no file after the construction.
The openmode, locale and any other state of rhs is also copied.
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.
  • is_­open() == rhs_­p.is_­open()
  • rhs_­a.is_­open() == false
  • 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()
virtual ~basic_filebuf();
Effects: Destroys an object of class basic_­filebuf<charT, traits>.
Calls close().
If an exception occurs during the destruction of the object, including the call to close(), the exception is caught but not rethrown (see [res.on.exception.handling]).

30.9.2.2 Assign and swap [filebuf.assign]

basic_filebuf& operator=(basic_filebuf&& rhs);
Effects: Calls close() then move assigns from rhs.
After the move assignment *this has the observable state it would have had if it had been move constructed from rhs (see [filebuf.cons]).
Returns: *this.
void swap(basic_filebuf& rhs);
Effects: Exchanges the state of *this and rhs.
template <class charT, class traits> void swap(basic_filebuf<charT, traits>& x, basic_filebuf<charT, traits>& y);
Effects: As if by x.swap(y).

30.9.2.3 Member functions [filebuf.members]

bool is_open() const;
Returns: true if a previous call to open succeeded (returned a non-null value) and there has been no intervening call to close.
basic_filebuf* open(const char* s, ios_base::openmode mode); basic_filebuf* open(const filesystem::path::value_type* s, ios_base::openmode mode); // wide systems only; see [fstream.syn]
Effects: If is_­open() != false, returns a null pointer.
Otherwise, initializes the filebuf as required.
It then opens a file, if possible, whose name is the ntbs s (as if by calling fopen(s, modstr)).
The ntbs modstr is determined from mode & ~ios_­base​::​ate as indicated in Table 117.
If mode is not some combination of flags shown in the table then the open fails.
Table 117 — File open modes
ios_­base flag combination
stdio equivalent
binary
in
out
trunc
app
+
"w"
+
+
"a"
+
"a"
+
+
"w"
+
"r"
+
+
"r+"
+
+
+
"w+"
+
+
+
"a+"
+
+
"a+"
+
+
"wb"
+
+
+
"ab"
+
+
"ab"
+
+
+
"wb"
+
+
"rb"
+
+
+
"r+b"
+
+
+
+
"w+b"
+
+
+
+
"a+b"
+
+
+
"a+b"
If the open operation succeeds and (mode & ios_­base​::​ate) != 0, positions the file to the end (as if by calling fseek(file, 0, SEEK_­END)).328
If the repositioning operation fails, calls close() and returns a null pointer to indicate failure.
Returns: this if successful, a null pointer otherwise.
basic_filebuf* open(const string& s, ios_base::openmode mode); basic_filebuf* open(const filesystem::path& s, ios_base::openmode mode);
Returns: open(s.c_­str(), mode);
basic_filebuf* close();
Effects: If is_­open() == false, returns a null pointer.
If a put area exists, calls overflow(traits​::​​eof()) to flush characters.
If the last virtual member function called on *this (between underflow, overflow, seekoff, and seekpos) was overflow then calls a_­codecvt.unshift (possibly several times) to determine a termination sequence, inserts those characters and calls overflow(traits​::​​eof()) again.
Finally, regardless of whether any of the preceding calls fails or throws an exception, the function closes the file (as if by calling fclose(file)).
If any of the calls made by the function, including fclose, fails, close fails by returning a null pointer.
If one of these calls throws an exception, the exception is caught and rethrown after closing the file.
Returns: this on success, a null pointer otherwise.
Postconditions: is_­open() == false.
The macro SEEK_­END is defined, and the function signatures fopen(const char*, const char*) and fseek(FILE*, long, int) are declared, in <cstdio>.

30.9.2.4 Overridden virtual functions [filebuf.virtuals]

streamsize showmanyc() override;
Effects: Behaves the same as basic_­streambuf​::​showmanyc() ([streambuf.virtuals]).
Remarks: An implementation might well provide an overriding definition for this function signature if it can determine that more characters can be read from the input sequence.
int_type underflow() override;
Effects: Behaves according to the description of basic_­streambuf<charT, traits>​::​underflow(), with the specialization that a sequence of characters is read from the input sequence as if by reading from the associated file into an internal buffer (extern_­buf) and then as if by doing:
char   extern_buf[XSIZE];
char*  extern_end;
charT  intern_buf[ISIZE];
charT* intern_end;
codecvt_base::result r =
  a_codecvt.in(state, extern_buf, extern_buf+XSIZE, extern_end,
               intern_buf, intern_buf+ISIZE, intern_end);
This shall be done in such a way that the class can recover the position (fpos_­t) corresponding to each character between intern_­buf and intern_­end.
If the value of r indicates that a_­codecvt.in() ran out of space in intern_­buf, retry with a larger intern_­buf.
int_type uflow() override;
Effects: Behaves according to the description of basic_­streambuf<charT, traits>​::​uflow(), with the specialization that a sequence of characters is read from the input with the same method as used by underflow.
int_type pbackfail(int_type c = traits::eof()) override;
Effects: Puts back the character designated by c to the input sequence, if possible, in one of three ways:
  • If traits​::​eq_­int_­type(c, traits​::​eof()) returns false and if the function makes a putback position available and if traits​::​eq(to_­char_­type(c), gptr()[-1]) returns true, decrements the next pointer for the input sequence, gptr().
    Returns: c.
  • If traits​::​eq_­int_­type(c, traits​::​eof()) returns false and if the function makes a putback position available and if the function is permitted to assign to the putback position, decrements the next pointer for the input sequence, and stores c there.
    Returns: c.
  • If traits​::​eq_­int_­type(c, traits​::​eof()) returns true, and if either the input sequence has a putback position available or the function makes a putback position available, decrements the next pointer for the input sequence, gptr().
    Returns: traits​::​not_­eof(c).
Returns: As specified above, or traits​::​eof() to indicate failure.
Remarks: If is_­open() == false, the function always fails.
The function does not put back a character directly to the input sequence.
If the function can succeed in more than one of these ways, it is unspecified which way is chosen.
The function can alter the number of putback positions available as a result of any call.
int_type overflow(int_type c = traits::eof()) override;
Effects: Behaves according to the description of basic_­streambuf<charT, traits>​::​overflow(c), except that the behavior of “consuming characters” is performed by first converting as if by:
charT* b = pbase();
charT* p = pptr();
charT* end;
char   xbuf[XSIZE];
char*  xbuf_end;
codecvt_base::result r =
  a_codecvt.out(state, b, p, end, xbuf, xbuf+XSIZE, xbuf_end);
and then
  • If r == codecvt_­base​::​error then fail.
  • If r == codecvt_­base​::​noconv then output characters from b up to (and not including) p.
  • If r == codecvt_­base​::​partial then output to the file characters from xbuf up to xbuf_­end, and repeat using characters from end to p.
    If output fails, fail (without repeating).
  • Otherwise output from xbuf to xbuf_­end, and fail if output fails.
    At this point if b != p and b == end (xbuf isn't large enough) then increase XSIZE and repeat from the beginning.
Returns: traits​::​not_­eof(c) to indicate success, and traits​::​eof() to indicate failure.
If is_­open() == false, the function always fails.
basic_streambuf* setbuf(char_type* s, streamsize n) override;
Effects: If setbuf(0, 0) is called on a stream before any I/O has occurred on that stream, the stream becomes unbuffered.
Otherwise the results are implementation-defined.
“Unbuffered” means that pbase() and pptr() always return null and output to the file should appear as soon as possible.
pos_type seekoff(off_type off, ios_base::seekdir way, ios_base::openmode which = ios_base::in | ios_base::out) override;
Effects: Let width denote a_­codecvt.encoding().
If is_­open() == false, or off != 0 && width <= 0, then the positioning operation fails.
Otherwise, if way != basic_­ios​::​cur or off != 0, and if the last operation was output, then update the output sequence and write any unshift sequence.
Next, seek to the new position: if width > 0, call fseek(file, width * off, whence), otherwise call fseek(file, 0, whence).
Remarks: “The last operation was output” means either the last virtual operation was overflow or the put buffer is non-empty.
“Write any unshift sequence” means, if width if less than zero then call a_­codecvt.unshift(state, xbuf, xbuf+XSIZE, xbuf_­end) and output the resulting unshift sequence.
The function determines one of three values for the argument whence, of type int, as indicated in Table 118.
Table 118seekoff effects
way Value
stdio Equivalent
basic_­ios​::​beg
SEEK_­SET
basic_­ios​::​cur
SEEK_­CUR
basic_­ios​::​end
SEEK_­END
Returns: A newly constructed pos_­type object that stores the resultant stream position, if possible.
If the positioning operation fails, or if the object cannot represent the resultant stream position, returns pos_­type(off_­type(-1)).
pos_type seekpos(pos_type sp, ios_base::openmode which = ios_base::in | ios_base::out) override;
Alters the file position, if possible, to correspond to the position stored in sp (as described below).
Altering the file position performs as follows:
  1. 1.
    if (om & ios_­base​::​out) != 0, then update the output sequence and write any unshift sequence;
  2. 2.
    set the file position to sp as if by a call to fsetpos;
  3. 3.
    if (om & ios_­base​::​in) != 0, then update the input sequence;
where om is the open mode passed to the last call to open().
The operation fails if is_­open() returns false.
If sp is an invalid stream position, or if the function positions neither sequence, the positioning operation fails.
If sp has not been obtained by a previous successful call to one of the positioning functions (seekoff or seekpos) on the same file the effects are undefined.
Returns: sp on success.
Otherwise returns pos_­type(off_­type(-1)).
int sync() override;
Effects: If a put area exists, calls filebuf​::​overflow to write the characters to the file, then flushes the file as if by calling fflush(file).
If a get area exists, the effect is implementation-defined.
void imbue(const locale& loc) override;
Requires: If the file is not positioned at its beginning and the encoding of the current locale as determined by a_­codecvt.encoding() is state-dependent ([locale.codecvt.virtuals]) then that facet is the same as the corresponding facet of loc.
Effects: Causes characters inserted or extracted after this call to be converted according to loc until another call of imbue.
Remarks: This may require reconversion of previously converted characters.
This in turn may require the implementation to be able to reconstruct the original contents of the file.

30.9.3 Class template basic_­ifstream [ifstream]

namespace std {
  template <class charT, class traits = char_traits<charT>>
  class basic_ifstream : public basic_istream<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;

    // [ifstream.cons], constructors
    basic_ifstream();
    explicit basic_ifstream(const char* s,
                            ios_base::openmode mode = ios_base::in);
    explicit basic_ifstream(const filesystem::path::value_type* s,
                            ios_base::openmode mode = ios_base::in);  // wide systems only; see [fstream.syn]
    explicit basic_ifstream(const string& s,
                            ios_base::openmode mode = ios_base::in);
    explicit basic_ifstream(const filesystem::path& s,
                            ios_base::openmode mode = ios_base::in);
    basic_ifstream(const basic_ifstream& rhs) = delete;
    basic_ifstream(basic_ifstream&& rhs);

    // [ifstream.assign], assign and swap
    basic_ifstream& operator=(const basic_ifstream& rhs) = delete;
    basic_ifstream& operator=(basic_ifstream&& rhs);
    void swap(basic_ifstream& rhs);

    // [ifstream.members], members
    basic_filebuf<charT, traits>* rdbuf() const;

    bool is_open() const;
    void open(const char* s, ios_base::openmode mode = ios_base::in);
    void open(const filesystem::path::value_type* s,
              ios_base::openmode mode = ios_base::in);  // wide systems only; see [fstream.syn]
    void open(const string& s, ios_base::openmode mode = ios_base::in);
    void open(const filesystem::path& s, ios_base::openmode mode = ios_base::in);
    void close();
  private:
    basic_filebuf<charT, traits> sb; // exposition only
  };

  template <class charT, class traits>
    void swap(basic_ifstream<charT, traits>& x,
              basic_ifstream<charT, traits>& y);
}
The class basic_­ifstream<charT, traits> supports reading from named files.
It uses a basic_­filebuf<​charT, traits> object to control the associated sequence.
For the sake of exposition, the maintained data is presented here as:

30.9.3.1 basic_­ifstream constructors [ifstream.cons]

basic_ifstream();
Effects: Constructs an object of class basic_­ifstream<charT, traits>, initializing the base class with basic_­istream<charT, traits>(&sb) ([istream.cons]) and initializing sb with basic_­filebuf<charT, traits>()) ([filebuf.cons]).
explicit basic_ifstream(const char* s, ios_base::openmode mode = ios_base::in); explicit basic_ifstream(const filesystem::path::value_type* s, ios_base::openmode mode = ios_base::in); // wide systems only; see [fstream.syn]
Effects: Constructs an object of class basic_­ifstream<charT, traits>, initializing the base class with basic_­istream<charT, traits>(&sb) ([istream.cons]) and initializing sb with basic_­filebuf<charT, traits>()) ([filebuf.cons]), then calls rdbuf()->open(s, mode | ios_­base​::​in).
If that function returns a null pointer, calls setstate(failbit).
explicit basic_ifstream(const string& s, ios_base::openmode mode = ios_base::in); explicit basic_ifstream(const filesystem::path& s, ios_base::openmode mode = ios_base::in);
Effects: The same as basic_­ifstream(s.c_­str(), mode).
basic_ifstream(basic_ifstream&& rhs);
Effects: Move constructs from the rvalue rhs.
This is accomplished by move constructing the base class, and the contained basic_­filebuf.
Next basic_­istream<charT, traits>​::​set_­rdbuf(&sb) is called to install the contained basic_­filebuf.

30.9.3.2 Assign and swap [ifstream.assign]

basic_ifstream& operator=(basic_ifstream&& rhs);
Effects: Move assigns the base and members of *this from the base and corresponding members of rhs.
Returns: *this.
void swap(basic_ifstream& rhs);
Effects: Exchanges the state of *this and rhs by calling basic_­istream<charT, traits>​::​swap(rhs) and sb.swap(rhs.sb).
template <class charT, class traits> void swap(basic_ifstream<charT, traits>& x, basic_ifstream<charT, traits>& y);
Effects: As if by x.swap(y).

30.9.3.3 Member functions [ifstream.members]

basic_filebuf<charT, traits>* rdbuf() const;
Returns: const_­cast<basic_­filebuf<charT, traits>*>(&sb).
bool is_open() const;
Returns: rdbuf()->is_­open().
void open(const char* s, ios_base::openmode mode = ios_base::in); void open(const filesystem::path::value_type* s, ios_base::openmode mode = ios_base::in); // wide systems only; see [fstream.syn]
Effects: Calls rdbuf()->open(s, mode | ios_­base​::​in).
If that function does not return a null pointer calls clear(), otherwise calls setstate(failbit) (which may throw ios_­base​::​failure) ([iostate.flags]).
void open(const string& s, ios_base::openmode mode = ios_base::in); void open(const filesystem::path& s, ios_base::openmode mode = ios_base::in);
Effects: Calls open(s.c_­str(), mode).
void close();
Effects: Calls rdbuf()->close() and, if that function returns a null pointer, calls setstate(failbit) (which may throw ios_­base​::​failure) ([iostate.flags]).

30.9.4 Class template basic_­ofstream [ofstream]

namespace std {
  template <class charT, class traits = char_traits<charT>>
  class basic_ofstream : 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;

    // [ofstream.cons], constructors
    basic_ofstream();
    explicit basic_ofstream(const char* s,
                            ios_base::openmode mode = ios_base::out);
    explicit basic_ofstream(const filesystem::path::value_type* s,
                            ios_base::openmode mode = ios_base::out);  // wide systems only; see [fstream.syn]
    explicit basic_ofstream(const string& s,
                            ios_base::openmode mode = ios_base::out);
    explicit basic_ofstream(const filesystem::path& s,
                            ios_base::openmode mode = ios_base::out);
    basic_ofstream(const basic_ofstream& rhs) = delete;
    basic_ofstream(basic_ofstream&& rhs);

    // [ofstream.assign], assign and swap
    basic_ofstream& operator=(const basic_ofstream& rhs) = delete;
    basic_ofstream& operator=(basic_ofstream&& rhs);
    void swap(basic_ofstream& rhs);

    // [ofstream.members], members
    basic_filebuf<charT, traits>* rdbuf() const;

    bool is_open() const;
    void open(const char* s, ios_base::openmode mode = ios_base::out);
    void open(const filesystem::path::value_type* s,
              ios_base::openmode mode = ios_base::out);  // wide systems only; see [fstream.syn]
    void open(const string& s, ios_base::openmode mode = ios_base::out);
    void open(const filesystem::path& s, ios_base::openmode mode = ios_base::out);
    void close();
  private:
    basic_filebuf<charT, traits> sb; // exposition only
  };

  template <class charT, class traits>
    void swap(basic_ofstream<charT, traits>& x,
              basic_ofstream<charT, traits>& y);
}
The class basic_­ofstream<charT, traits> supports writing to named files.
It uses a basic_­filebuf<​charT, traits> object to control the associated sequence.
For the sake of exposition, the maintained data is presented here as:

30.9.4.1 basic_­ofstream constructors [ofstream.cons]

basic_ofstream();
Effects: Constructs an object of class basic_­ofstream<charT, traits>, initializing the base class with basic_­ostream<charT, traits>(&sb) ([ostream.cons]) and initializing sb with basic_­filebuf<charT, traits>()) ([filebuf.cons]).
explicit basic_ofstream(const char* s, ios_base::openmode mode = ios_base::out); explicit basic_ofstream(const filesystem::path::value_type* s, ios_base::openmode mode = ios_base::out); // wide systems only; see [fstream.syn]
Effects: Constructs an object of class basic_­ofstream<charT, traits>, initializing the base class with basic_­ostream<charT, traits>(&sb) ([ostream.cons]) and initializing sb with basic_­filebuf<charT, traits>()) ([filebuf.cons]), then calls rdbuf()->open(s, mode | ios_­base​::​out).
If that function returns a null pointer, calls setstate(​failbit).
explicit basic_ofstream(const string& s, ios_base::openmode mode = ios_base::out); explicit basic_ofstream(const filesystem::path& s, ios_base::openmode mode = ios_base::out);
Effects: The same as basic_­ofstream(s.c_­str(), mode).
basic_ofstream(basic_ofstream&& rhs);
Effects: Move constructs from the rvalue rhs.
This is accomplished by move constructing the base class, and the contained basic_­filebuf.
Next basic_­ostream<charT, traits>​::​set_­rdbuf(&sb) is called to install the contained basic_­filebuf.

30.9.4.2 Assign and swap [ofstream.assign]

basic_ofstream& operator=(basic_ofstream&& rhs);
Effects: Move assigns the base and members of *this from the base and corresponding members of rhs.
Returns: *this.
void swap(basic_ofstream& rhs);
Effects: Exchanges the state of *this and rhs by calling basic_­ostream<charT, traits>​::​swap(rhs) and sb.swap(rhs.sb).
template <class charT, class traits> void swap(basic_ofstream<charT, traits>& x, basic_ofstream<charT, traits>& y);
Effects: As if by x.swap(y).

30.9.4.3 Member functions [ofstream.members]

basic_filebuf<charT, traits>* rdbuf() const;
Returns: const_­cast<basic_­filebuf<charT, traits>*>(&sb).
bool is_open() const;
Returns: rdbuf()->is_­open().
void open(const char* s, ios_base::openmode mode = ios_base::out); void open(const filesystem::path::value_type* s, ios_base::openmode mode = ios_base::out); // wide systems only; see [fstream.syn]
Effects: Calls rdbuf()->open(s, mode | ios_­base​::​out).
If that function does not return a null pointer calls clear(), otherwise calls setstate(​failbit) (which may throw ios_­base​::​failure) ([iostate.flags]).
void close();
Effects: Calls rdbuf()->close() and, if that function fails (returns a null pointer), calls setstate(​failbit) (which may throw ios_­base​::​failure) ([iostate.flags]).
void open(const string& s, ios_base::openmode mode = ios_base::out); void open(const filesystem::path& s, ios_base::openmode mode = ios_base::out);
Effects: Calls open(s.c_­str(), mode).

30.9.5 Class template basic_­fstream [fstream]

namespace std {
  template <class charT, class traits = char_traits<charT>>
  class basic_fstream : public basic_iostream<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;

    // [fstream.cons], constructors
    basic_fstream();
    explicit basic_fstream(
      const char* s,
      ios_base::openmode mode = ios_base::in | ios_base::out);
    explicit basic_fstream(
      const std::filesystem::path::value_type* s,
      ios_base::openmode mode = ios_base::in|ios_base::out);  // wide systems only; see [fstream.syn]
    explicit basic_fstream(
      const string& s,
      ios_base::openmode mode = ios_base::in | ios_base::out);
    explicit basic_fstream(
      const filesystem::path& s,
      ios_base::openmode mode = ios_base::in | ios_base::out);
    basic_fstream(const basic_fstream& rhs) = delete;
    basic_fstream(basic_fstream&& rhs);

    // [fstream.assign], assign and swap
    basic_fstream& operator=(const basic_fstream& rhs) = delete;
    basic_fstream& operator=(basic_fstream&& rhs);
    void swap(basic_fstream& rhs);

    // [fstream.members], members
    basic_filebuf<charT, traits>* rdbuf() const;
    bool is_open() const;
    void open(
      const char* s,
      ios_base::openmode mode = ios_base::in | ios_base::out);
    void open(
      const std::filesystem::path::value_type* s,
      ios_base::openmode mode = ios_base::in|ios_base::out);  // wide systems only; see [fstream.syn]
    void open(
      const string& s,
      ios_base::openmode mode = ios_base::in | ios_base::out);
    void open(
      const filesystem::path& s,
      ios_base::openmode mode = ios_base::in | ios_base::out);
    void close();

  private:
    basic_filebuf<charT, traits> sb; // exposition only
  };

  template <class charT, class traits>
    void swap(basic_fstream<charT, traits>& x,
              basic_fstream<charT, traits>& y);
}
The class template basic_­fstream<charT, traits> supports reading and writing from named files.
It uses a basic_­filebuf<charT, traits> object to control the associated sequences.
For the sake of exposition, the maintained data is presented here as:
  • sb, the basic_­filebuf object.

30.9.5.1 basic_­fstream constructors [fstream.cons]

basic_fstream();
Effects: Constructs an object of class basic_­fstream<charT, traits>, initializing the base class with basic_­iostream<charT, traits>(&sb) ([iostream.cons]) and initializing sb with basic_­filebuf<charT, traits>().
explicit basic_fstream( const char* s, ios_base::openmode mode = ios_base::in | ios_base::out); explicit basic_fstream( const filesystem::path::value_type* s, ios_base::openmode mode = ios_base::in | ios_base::out); // wide systems only; see [fstream.syn]
Effects: Constructs an object of class basic_­fstream<charT, traits>, initializing the base class with basic_­iostream<charT, traits>(&sb) ([iostream.cons]) and initializing sb with basic_­filebuf<charT, traits>().
Then calls rdbuf()->open(s, mode).
If that function returns a null pointer, calls setstate(failbit).
explicit basic_fstream( const string& s, ios_base::openmode mode = ios_base::in | ios_base::out); explicit basic_fstream( const filesystem::path& s, ios_base::openmode mode = ios_base::in | ios_base::out);
Effects: The same as basic_­fstream(s.c_­str(), mode).
basic_fstream(basic_fstream&& rhs);
Effects: Move constructs from the rvalue rhs.
This is accomplished by move constructing the base class, and the contained basic_­filebuf.
Next basic_­istream<charT, traits>​::​set_­rdbuf(&sb) is called to install the contained basic_­filebuf.

30.9.5.2 Assign and swap [fstream.assign]

basic_fstream& operator=(basic_fstream&& rhs);
Effects: Move assigns the base and members of *this from the base and corresponding members of rhs.
Returns: *this.
void swap(basic_fstream& rhs);
Effects: Exchanges the state of *this and rhs by calling basic_­iostream<charT,traits>​::​swap(rhs) and sb.swap(rhs.sb).
template <class charT, class traits> void swap(basic_fstream<charT, traits>& x, basic_fstream<charT, traits>& y);
Effects: As if by x.swap(y).

30.9.5.3 Member functions [fstream.members]

basic_filebuf<charT, traits>* rdbuf() const;
Returns: const_­cast<basic_­filebuf<charT, traits>*>(&sb).
bool is_open() const;
Returns: rdbuf()->is_­open().
void open( const char* s, ios_base::openmode mode = ios_base::in | ios_base::out); void open( const filesystem::path::value_type* s, ios_base::openmode mode = ios_base::in | ios_base::out); // wide systems only; see [fstream.syn]
Effects: Calls rdbuf()->open(s, mode).
If that function does not return a null pointer calls clear(), otherwise calls setstate(failbit) (which may throw ios_­base​::​failure) ([iostate.flags]).
void open( const string& s, ios_base::openmode mode = ios_base::in | ios_base::out); void open( const filesystem::path& s, ios_base::openmode mode = ios_base::in | ios_base::out);
Effects: Calls open(s.c_­str(), mode).
void close();
Effects: Calls rdbuf()->close() and, if that function returns a null pointer, calls setstate(failbit) (which may throw ios_­base​::​failure) ([iostate.flags]).

30.10 File systems [filesystems]

30.10.1 General [fs.general]

This subclause describes operations on file systems and their components, such as paths, regular files, and directories.
A file system is a collection of files and their attributes.
A file is an object within a file system that holds user or system data.
Files can be written to, or read from, or both.
A file has certain attributes, including type.
File types include regular files and directories.
Other types of files, such as symbolic links, may be supported by the implementation.
A directory is a file within a file system that acts as a container of directory entries that contain information about other files, possibly including other directory files.
The parent directory of a directory is the directory that both contains a directory entry for the given directory and is represented by the filename dot-dot in the given directory.
The parent directory of other types of files is a directory containing a directory entry for the file under discussion.
A link is an object that associates a filename with a file.
Several links can associate names with the same file.
A hard link is a link to an existing file.
Some file systems support multiple hard links to a file.
If the last hard link to a file is removed, the file itself is removed.
[Note
:
A hard link can be thought of as a shared-ownership smart pointer to a file.
end note
]
A symbolic link is a type of file with the property that when the file is encountered during pathname resolution ([fs.class.path]), a string stored by the file is used to modify the pathname resolution.
[Note
:
Symbolic links are often called symlinks.
A symbolic link can be thought of as a raw pointer to a file.
If the file pointed to does not exist, the symbolic link is said to be a “dangling” symbolic link.
end note
]

30.10.2 Conformance [fs.conformance]

Conformance is specified in terms of behavior.
Ideal behavior is not always implementable, so the conformance subclauses take that into account.

30.10.2.1 POSIX conformance [fs.conform.9945]

Some behavior is specified by reference to POSIX ([fs.norm.ref]).
How such behavior is actually implemented is unspecified.
[Note
:
This constitutes an “as if” rule allowing implementations to call native operating system or other APIs.
end note
]
Implementations should provide such behavior as it is defined by POSIX.
Implementations shall document any behavior that differs from the behavior defined by POSIX.
Implementations that do not support exact POSIX behavior should provide behavior as close to POSIX behavior as is reasonable given the limitations of actual operating systems and file systems.
If an implementation cannot provide any reasonable behavior, the implementation shall report an error as specified in [fs.err.report].
[Note
:
This allows users to rely on an exception being thrown or an error code being set when an implementation cannot provide any reasonable behavior.
end note
]
Implementations are not required to provide behavior that is not supported by a particular file system.
[Example
:
The FAT file system used by some memory cards, camera memory, and floppy disks does not support hard links, symlinks, and many other features of more capable file systems, so implementations are not required to support those features on the FAT file system but instead are required to report an error as described above.
end example
]

30.10.2.2 Operating system dependent behavior conformance [fs.conform.os]

Behavior that is specified as being operating system dependent is dependent upon the behavior and characteristics of an operating system.
The operating system an implementation is dependent upon is implementation-defined.
It is permissible for an implementation to be dependent upon an operating system emulator rather than the actual underlying operating system.

30.10.2.3 File system race behavior [fs.race.behavior]

A file system race is the condition that occurs when multiple threads, processes, or computers interleave access and modification of the same object within a file system.
Behavior is undefined if calls to functions provided by this subclause introduce a file system race.
If the possibility of a file system race would make it unreliable for a program to test for a precondition before calling a function described herein, Requires: is not specified for the function.
[Note
:
As a design practice, preconditions are not specified when it is unreasonable for a program to detect them prior to calling the function.
end note
]

30.10.3 Normative references [fs.norm.ref]

This subclause mentions commercially available operating systems for purposes of exposition.329
POSIX® is a registered trademark of The IEEE.
Windows® is a registered trademark of Microsoft Corporation.
This information is given for the convenience of users of this document and does not constitute an endorsement by ISO or IEC of these products.

30.10.4 Requirements [fs.req]

Throughout this subclause, char, wchar_­t, char16_­t, and char32_­t are collectively called encoded character types.
Functions with template parameters named EcharT shall not participate in overload resolution unless EcharT is one of the encoded character types.
Template parameters named InputIterator shall meet the input iterator requirements and shall have a value type that is one of the encoded character types.
[Note
:
Use of an encoded character type implies an associated character set and encoding.
Since signed char and unsigned char have no implied character set and encoding, they are not included as permitted types.
end note
]
Template parameters named Allocator shall meet the Allocator requirements.

30.10.4.1 Namespaces and headers [fs.req.namespace]

Unless otherwise specified, references to entities described in this subclause are assumed to be qualified with ​::​std​::​filesystem​::​.

30.10.5 Header <filesystem> synopsis [fs.filesystem.syn]

namespace std::filesystem {
  // [fs.class.path], paths
  class path;

  // [fs.path.nonmember], path non-member functions
  void swap(path& lhs, path& rhs) noexcept;
  size_t hash_value(const path& p) noexcept;

  bool operator==(const path& lhs, const path& rhs) noexcept;
  bool operator!=(const path& lhs, const path& rhs) noexcept;
  bool operator< (const path& lhs, const path& rhs) noexcept;
  bool operator<=(const path& lhs, const path& rhs) noexcept;
  bool operator> (const path& lhs, const path& rhs) noexcept;
  bool operator>=(const path& lhs, const path& rhs) noexcept;

  path operator/ (const path& lhs, const path& rhs);

  // [fs.path.io], path inserter and extractor
  template <class charT, class traits>
    basic_ostream<charT, traits>&
      operator<<(basic_ostream<charT, traits>& os, const path& p);
  template <class charT, class traits>
    basic_istream<charT, traits>&
      operator>>(basic_istream<charT, traits>& is, path& p);

  // [fs.path.factory], path factory functions
  template <class Source>
    path u8path(const Source& source);
  template <class InputIterator>
    path u8path(InputIterator first, InputIterator last);

  // [fs.class.filesystem_error], filesystem errors
  class filesystem_error;

  // [fs.class.directory_entry], directory entries
  class directory_entry;

  // [fs.class.directory_iterator], directory iterators
  class directory_iterator;

  // [fs.dir.itr.nonmembers], range access for directory iterators
  directory_iterator begin(directory_iterator iter) noexcept;
  directory_iterator end(const directory_iterator&) noexcept;

  // [fs.class.rec.dir.itr], recursive directory iterators
  class recursive_directory_iterator;

  // [fs.rec.dir.itr.nonmembers], range access for recursive directory iterators
  recursive_directory_iterator begin(recursive_directory_iterator iter) noexcept;
  recursive_directory_iterator end(const recursive_directory_iterator&) noexcept;

  // [fs.class.file_status], file status
  class file_status;

  struct space_info {
    uintmax_t capacity;
    uintmax_t free;
    uintmax_t available;
  };

  // [fs.enum], enumerations
  enum class file_type;
  enum class perms;
  enum class perm_options;
  enum class copy_options;
  enum class directory_options;

  using file_time_type = chrono::time_point<trivial-clock>;

  // [fs.op.funcs], filesystem operations
  path absolute(const path& p);
  path absolute(const path& p, error_code& ec);

  path canonical(const path& p);
  path canonical(const path& p, error_code& ec);

  void copy(const path& from, const path& to);
  void copy(const path& from, const path& to, error_code& ec) noexcept;
  void copy(const path& from, const path& to, copy_options options);
  void copy(const path& from, const path& to, copy_options options,
            error_code& ec) noexcept;

  bool copy_file(const path& from, const path& to);
  bool copy_file(const path& from, const path& to, error_code& ec) noexcept;
  bool copy_file(const path& from, const path& to, copy_options option);
  bool copy_file(const path& from, const path& to, copy_options option,
                 error_code& ec) noexcept;

  void copy_symlink(const path& existing_symlink, const path& new_symlink);
  void copy_symlink(const path& existing_symlink, const path& new_symlink,
                    error_code& ec) noexcept;

  bool create_directories(const path& p);
  bool create_directories(const path& p, error_code& ec) noexcept;

  bool create_directory(const path& p);
  bool create_directory(const path& p, error_code& ec) noexcept;

  bool create_directory(const path& p, const path& attributes);
  bool create_directory(const path& p, const path& attributes,
                        error_code& ec) noexcept;

  void create_directory_symlink(const path& to, const path& new_symlink);
  void create_directory_symlink(const path& to, const path& new_symlink,
                                error_code& ec) noexcept;

  void create_hard_link(const path& to, const path& new_hard_link);
  void create_hard_link(const path& to, const path& new_hard_link,
                        error_code& ec) noexcept;

  void create_symlink(const path& to, const path& new_symlink);
  void create_symlink(const path& to, const path& new_symlink,
                      error_code& ec) noexcept;

  path current_path();
  path current_path(error_code& ec);
  void current_path(const path& p);
  void current_path(const path& p, error_code& ec) noexcept;

  bool equivalent(const path& p1, const path& p2);
  bool equivalent(const path& p1, const path& p2, error_code& ec) noexcept;

  bool exists(file_status s) noexcept;
  bool exists(const path& p);
  bool exists(const path& p, error_code& ec) noexcept;

  uintmax_t file_size(const path& p);
  uintmax_t file_size(const path& p, error_code& ec) noexcept;

  uintmax_t hard_link_count(const path& p);
  uintmax_t hard_link_count(const path& p, error_code& ec) noexcept;

  bool is_block_file(file_status s) noexcept;
  bool is_block_file(const path& p);
  bool is_block_file(const path& p, error_code& ec) noexcept;

  bool is_character_file(file_status s) noexcept;
  bool is_character_file(const path& p);
  bool is_character_file(const path& p, error_code& ec) noexcept;

  bool is_directory(file_status s) noexcept;
  bool is_directory(const path& p);
  bool is_directory(const path& p, error_code& ec) noexcept;

  bool is_empty(const path& p);
  bool is_empty(const path& p, error_code& ec) noexcept;

  bool is_fifo(file_status s) noexcept;
  bool is_fifo(const path& p);
  bool is_fifo(const path& p, error_code& ec) noexcept;

  bool is_other(file_status s) noexcept;
  bool is_other(const path& p);
  bool is_other(const path& p, error_code& ec) noexcept;

  bool is_regular_file(file_status s) noexcept;
  bool is_regular_file(const path& p);
  bool is_regular_file(const path& p, error_code& ec) noexcept;

  bool is_socket(file_status s) noexcept;
  bool is_socket(const path& p);
  bool is_socket(const path& p, error_code& ec) noexcept;

  bool is_symlink(file_status s) noexcept;
  bool is_symlink(const path& p);
  bool is_symlink(const path& p, error_code& ec) noexcept;

  file_time_type last_write_time(const path& p);
  file_time_type last_write_time(const path& p, error_code& ec) noexcept;
  void last_write_time(const path& p, file_time_type new_time);
  void last_write_time(const path& p, file_time_type new_time,
                       error_code& ec) noexcept;

  void permissions(const path& p, perms prms, perm_options opts=perm_options::replace);
  void permissions(const path& p, perms prms, error_code& ec) noexcept;
  void permissions(const path& p, perms prms, perm_options opts, error_code& ec);

  path proximate(const path& p, error_code& ec);
  path proximate(const path& p, const path& base = current_path());
  path proximate(const path& p, const path& base, error_code& ec);

  path read_symlink(const path& p);
  path read_symlink(const path& p, error_code& ec);

  path relative(const path& p, error_code& ec);
  path relative(const path& p, const path& base = current_path());
  path relative(const path& p, const path& base, error_code& ec);

  bool remove(const path& p);
  bool remove(const path& p, error_code& ec) noexcept;

  uintmax_t remove_all(const path& p);
  uintmax_t remove_all(const path& p, error_code& ec) noexcept;

  void rename(const path& from, const path& to);
  void rename(const path& from, const path& to, error_code& ec) noexcept;

  void resize_file(const path& p, uintmax_t size);
  void resize_file(const path& p, uintmax_t size, error_code& ec) noexcept;

  space_info space(const path& p);
  space_info space(const path& p, error_code& ec) noexcept;

  file_status status(const path& p);
  file_status status(const path& p, error_code& ec) noexcept;

  bool status_known(file_status s) noexcept;

  file_status symlink_status(const path& p);
  file_status symlink_status(const path& p, error_code& ec) noexcept;

  path temp_directory_path();
  path temp_directory_path(error_code& ec);

  path weakly_canonical(const path& p);
  path weakly_canonical(const path& p, error_code& ec);
}
trivial-clock is an implementation-defined type that satisfies the TrivialClock requirements and that is capable of representing and measuring file time values.
Implementations should ensure that the resolution and range of file_­time_­type reflect the operating system dependent resolution and range of file time values.

30.10.6 Error reporting [fs.err.report]

Filesystem library functions often provide two overloads, one that throws an exception to report file system errors, and another that sets an error_­code.
[Note
:
This supports two common use cases:
  • Uses where file system errors are truly exceptional and indicate a serious failure.
    Throwing an exception is an appropriate response.
  • Uses where file system errors are routine and do not necessarily represent failure.
    Returning an error code is the most appropriate response.
    This allows application specific error handling, including simply ignoring the error.
end note
]
Functions not having an argument of type error_­code& handle errors as follows, unless otherwise specified:
  • When a call by the implementation to an operating system or other underlying API results in an error that prevents the function from meeting its specifications, an exception of type filesystem_­error shall be thrown.
    For functions with a single path argument, that argument shall be passed to the filesystem_­error constructor with a single path argument.
    For functions with two path arguments, the first of these arguments shall be passed to the filesystem_­error constructor as the path1 argument, and the second shall be passed as the path2 argument.
    The filesystem_­error constructor's error_­code argument is set as appropriate for the specific operating system dependent error.
  • Failure to allocate storage is reported by throwing an exception as described in [res.on.exception.handling].
  • Destructors throw nothing.
Functions having an argument of type error_­code& handle errors as follows, unless otherwise specified:
  • If a call by the implementation to an operating system or other underlying API results in an error that prevents the function from meeting its specifications, the error_­code& argument is set as appropriate for the specific operating system dependent error.
    Otherwise, clear() is called on the error_­code& argument.

30.10.7 Class path [fs.class.path]

An object of class path represents a path and contains a pathname.
Such an object is concerned only with the lexical and syntactic aspects of a path.
The path does not necessarily exist in external storage, and the pathname is not necessarily valid for the current operating system or for a particular file system.
[Note
:
Class path is used to support the differences between the string types used by different operating systems to represent pathnames, and to perform conversions between encodings when necessary.
end note
]
A path is a sequence of elements that identify the location of a file within a filesystem.
The elements are the root-name, root-directory, and an optional sequence of filenames.
The maximum number of elements in the sequence is operating system dependent ([fs.conform.os]).
An absolute path is a path that unambiguously identifies the location of a file without reference to an additional starting location.
The elements of a path that determine if it is absolute are operating system dependent.
A relative path is a path that is not absolute, and as such, only unambiguously identifies the location of a file when resolved relative to an implied starting location.
The elements of a path that determine if it is relative are operating system dependent.
[Note
:
Pathnames “.
” and “.
” are relative paths.
end note
]
A pathname is a character string that represents the name of a path.
Pathnames are formatted according to the generic pathname format grammar ([fs.path.generic]) or according to an operating system dependent native pathname format accepted by the host operating system.
Pathname resolution is the operating system dependent mechanism for resolving a pathname to a particular file in a file hierarchy.
There may be multiple pathnames that resolve to the same file.
[Example
:
POSIX specifies the mechanism in section 4.11, Pathname resolution.
end example
]
namespace std::filesystem {
  class path {
  public:
    using value_type  = see below;
    using string_type = basic_string<value_type>;
    static constexpr value_type preferred_separator = see below;

    // [fs.enum.path.format], enumeration format
    enum format;

    // [fs.path.construct], constructors and destructor
    path() noexcept;
    path(const path& p);
    path(path&& p) noexcept;
    path(string_type&& source, format fmt = auto_format);
    template <class Source>
      path(const Source& source, format fmt = auto_format);
    template <class InputIterator>
      path(InputIterator first, InputIterator last, format fmt = auto_format);
    template <class Source>
      path(const Source& source, const locale& loc, format fmt = auto_format);
    template <class InputIterator>
      path(InputIterator first, InputIterator last, const locale& loc, format fmt = auto_format);
    ~path();

    // [fs.path.assign], assignments
    path& operator=(const path& p);
    path& operator=(path&& p) noexcept;
    path& operator=(string_type&& source);
    path& assign(string_type&& source);
    template <class Source>
      path& operator=(const Source& source);
    template <class Source>
      path& assign(const Source& source);
    template <class InputIterator>
      path& assign(InputIterator first, InputIterator last);

    // [fs.path.append], appends
    path& operator/=(const path& p);
    template <class Source>
      path& operator/=(const Source& source);
    template <class Source>
      path& append(const Source& source);
    template <class InputIterator>
      path& append(InputIterator first, InputIterator last);

    // [fs.path.concat], concatenation
    path& operator+=(const path& x);
    path& operator+=(const string_type& x);
    path& operator+=(basic_string_view<value_type> x);
    path& operator+=(const value_type* x);
    path& operator+=(value_type x);
    template <class Source>
      path& operator+=(const Source& x);
    template <class EcharT>
      path& operator+=(EcharT x);
    template <class Source>
      path& concat(const Source& x);
    template <class InputIterator>
      path& concat(InputIterator first, InputIterator last);

    // [fs.path.modifiers], modifiers
    void  clear() noexcept;
    path& make_preferred();
    path& remove_filename();
    path& replace_filename(const path& replacement);
    path& replace_extension(const path& replacement = path());
    void  swap(path& rhs) noexcept;

    // [fs.path.native.obs], native format observers
    const string_type& native() const noexcept;
    const value_type*  c_str() const noexcept;
    operator string_type() const;

    template <class EcharT, class traits = char_traits<EcharT>,
              class Allocator = allocator<EcharT>>
      basic_string<EcharT, traits, Allocator>
        string(const Allocator& a = Allocator()) const;
    std::string    string() const;
    std::wstring   wstring() const;
    std::string    u8string() const;
    std::u16string u16string() const;
    std::u32string u32string() const;

    // [fs.path.generic.obs], generic format observers
    template <class EcharT, class traits = char_traits<EcharT>,
              class Allocator = allocator<EcharT>>
      basic_string<EcharT, traits, Allocator>
        generic_string(const Allocator& a = Allocator()) const;
    std::string    generic_string() const;
    std::wstring   generic_wstring() const;
    std::string    generic_u8string() const;
    std::u16string generic_u16string() const;
    std::u32string generic_u32string() const;

    // [fs.path.compare], compare
    int compare(const path& p) const noexcept;
    int compare(const string_type& s) const;
    int compare(basic_string_view<value_type> s) const;
    int compare(const value_type* s) const;

    // [fs.path.decompose], decomposition
    path root_name() const;
    path root_directory() const;
    path root_path() const;
    path relative_path() const;
    path parent_path() const;
    path filename() const;
    path stem() const;
    path extension() const;

    // [fs.path.query], query
    bool empty() const noexcept;
    bool has_root_name() const;
    bool has_root_directory() const;
    bool has_root_path() const;
    bool has_relative_path() const;
    bool has_parent_path() const;
    bool has_filename() const;
    bool has_stem() const;
    bool has_extension() const;
    bool is_absolute() const;
    bool is_relative() const;

    // [fs.path.gen], generation
    path lexically_normal() const;
    path lexically_relative(const path& base) const;
    path lexically_proximate(const path& base) const;

    // [fs.path.itr], iterators
    class iterator;
    using const_iterator = iterator;

    iterator begin() const;
    iterator end() const;
  };
}
value_­type is a typedef for the operating system dependent encoded character type used to represent pathnames.
The value of the preferred_­separator member is the operating system dependent preferred-separator character ([fs.path.generic]).
[Example
:
For POSIX-based operating systems, value_­type is char and preferred_­separator is the slash character ('/').
For Windows-based operating systems, value_­type is wchar_­t and preferred_­separator is the backslash character (L'\\').
end example
]

30.10.7.1 Generic pathname format [fs.path.generic]

pathname:
	root-name root-directory relative-path
root-name:
	operating system dependent sequences of characters
	implementation-defined sequences of characters
root-directory:
	directory-separator
relative-path:
	filename
	filename directory-separator relative-path
	an empty path
filename:
	non-empty sequence of characters other than directory-separator characters
directory-separator:
	preferred-separator directory-separator
	fallback-separator directory-separator
preferred-separator:
	operating system dependent directory separator character
fallback-separator:
	/, if preferred-separator is not /
A filename is the name of a file.
Filenames dot and dot-dot, consisting solely of one and two period characters respectively, have special meaning.
The following characteristics of filenames are operating system dependent:
  • The permitted characters.
    [Example
    :
    Some operating systems prohibit the ASCII control characters (0x00 – 0x1F) in filenames.
    end example
    ]
    [Note
    :
    For wide portability, users may wish to limit filename characters to the POSIX Portable Filename Character Set:
    A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
    a b c d e f g h i j k l m n o p q r s t u v w x y z
    0 1 2 3 4 5 6 7 8 9 . _­ -
    end note
    ]
  • The maximum permitted length.
  • Filenames that are not permitted.
  • Filenames that have special meaning.
  • Case awareness and sensitivity during path resolution.
  • Special rules that may apply to file types other than regular files, such as directories.
Except in a root-name, multiple successive directory-separator characters are considered to be the same as one directory-separator character.
The filename dot is treated as a reference to the current directory.
The filename dot-dot is treated as a reference to the parent directory.
What the filename dot-dot refers to relative to root-directory is implementation-defined.
Specific filenames may have special meanings for a particular operating system.
A root-name identifies the starting location for pathname resolution ([fs.class.path]).
If there are no operating system dependent root-names, at least one implementation-defined root-name is required.
[Note
:
Many operating systems define a name beginning with two directory-separator characters as a root-name that identifies network or other resource locations.
Some operating systems define a single letter followed by a colon as a drive specifier – a root-name identifying a specific device such as a disk drive.
end note
]
If a root-name is otherwise ambiguous, the possibility with the longest sequence of characters is chosen.
[Note
:
On a POSIX-like operating system, it is impossible to have a root-name and a relative-path without an intervening root-directory element.
end note
]
Normalization of a generic format pathname means:
  1. 1.
    If the path is empty, stop.
  2. 2.
    Replace each slash character in the root-name with a preferred-separator.
  3. 3.
    [Note
    :
    The generic pathname grammar ([fs.path.generic]) defines directory-separator as one or more slashes and preferred-separators.
    end note
    ]
  4. 4.
    Remove each dot filename and any immediately following directory-separator.
  5. 5.
    As long as any appear, remove a non-dot-dot filename immediately followed by a directory-separator and a dot-dot filename, along with any immediately following directory-separator.
  6. 6.
    If there is a root-directory, remove all dot-dot filenames and any directory-separators immediately following them.
    [Note
    :
    These dot-dot filenames attempt to refer to nonexistent parent directories.
    end note
    ]
  7. 7.
    If the last filename is dot-dot, remove any trailing directory-separator.
  8. 8.
    If the path is empty, add a dot.
The result of normalization is a path in normal form, which is said to be normalized.

30.10.7.2 path conversions [fs.path.cvt]

30.10.7.2.1 path argument format conversions [fs.path.fmt.cvt]

[Note
:
The format conversions described in this section are not applied on POSIX-based operating systems because on these systems:
  • The generic format is acceptable as a native path.
  • There is no need to distinguish between native format and generic format in function arguments.
  • Paths for regular files and paths for directories share the same syntax.
end note
]
Several functions are defined to accept detected-format arguments, which are character sequences.
A detected-format argument represents a path using either a pathname in the generic format ([fs.path.generic]) or a pathname in the native format ([fs.class.path]).
Such an argument is taken to be in the generic format if and only if it matches the generic format and is not acceptable to the operating system as a native path.
[Note
:
Some operating systems may have no unambiguous way to distinguish between native format and generic format arguments.
This is by design as it simplifies use for operating systems that do not require disambiguation.
An implementation for an operating system where disambiguation is required is permitted to distinguish between the formats.
end note
]
Pathnames are converted as needed between the generic and native formats in an operating-system-dependent manner.
Let G(n) and N(g) in a mathematical sense be the implementation's functions that convert native-to-generic and generic-to-native formats respectively.
If g=G(n) for some n, then G(N(g))=g; if n=N(g) for some g, then N(G(n))=n.
[Note
:
Neither G nor N need be invertible.
end note
]
If the native format requires paths for regular files to be formatted differently from paths for directories, the path shall be treated as a directory path if its last element is a directory-separator, otherwise it shall be treated as a path to a regular file.
[Note
:
A path stores a native format pathname ([fs.path.native.obs]) and acts as if it also stores a generic format pathname, related as given below.
The implementation may generate the generic format pathname based on the native format pathname (and possibly other information) when requested.
end note
]
When a path is constructed from or is assigned a single representation separate from any path, the other representation is selected by the appropriate conversion function (G or N).
When the (new) value p of one representation of a path is derived from the representation of that or another path, a value q is chosen for the other representation.
The value q converts to p (by G or N as appropriate) if any such value does so; q is otherwise unspecified.
[Note
:
If q is the result of converting any path at all, it is the result of converting p.
end note
]

30.10.7.2.2 path type and encoding conversions [fs.path.type.cvt]

The native encoding of a narrow character string is the operating system dependent current encoding for pathnames ([fs.class.path]).
The native encoding for wide character strings is the implementation-defined execution wide-character set encoding ([lex.charset]).
For member function arguments that take character sequences representing paths and for member functions returning strings, value type and encoding conversion is performed if the value type of the argument or return value differs from path​::​value_­type.
For the argument or return value, the method of conversion and the encoding to be converted to is determined by its value type:
  • char: The encoding is the native narrow encoding.
    The method of conversion, if any, is operating system dependent.
    [Note
    :
    For POSIX-based operating systems path​::​value_­type is char so no conversion from char value type arguments or to char value type return values is performed.
    For Windows-based operating systems, the native narrow encoding is determined by calling a Windows API function.
    end note
    ]
    [Note
    :
    This results in behavior identical to other C and C++ standard library functions that perform file operations using narrow character strings to identify paths.
    Changing this behavior would be surprising and error prone.
    end note
    ]
  • wchar_­t: The encoding is the native wide encoding.
    The method of conversion is unspecified.
    [Note
    :
    For Windows-based operating systems path​::​value_­type is wchar_­t so no conversion from wchar_­t value type arguments or to wchar_­t value type return values is performed.
    end note
    ]
  • char16_­t: The encoding is UTF-16.
    The method of conversion is unspecified.
  • char32_­t: The encoding is UTF-32.
    The method of conversion is unspecified.
If the encoding being converted to has no representation for source characters, the resulting converted characters, if any, are unspecified.
Implementations should not modify member function arguments if already of type path​::​value_­type.

30.10.7.3 path requirements [fs.path.req]

In addition to the requirements ([fs.req]), function template parameters named Source shall be one of:
  • basic_­string<EcharT, traits, Allocator>.
    A function argument const Source& source shall have an effective range [source.begin(), source.end()).
  • basic_­string_­view<EcharT, traits>.
    A function argument const Source& source shall have an effective range [source.begin(), source.end()).
  • A type meeting the input iterator requirements that iterates over a NTCTS.
    The value type shall be an encoded character type.
    A function argument const Source& source shall have an effective range [source, end) where end is the first iterator value with an element value equal to iterator_­traits<Source>​::​value_­type().
  • A character array that after array-to-pointer decay results in a pointer to the start of a NTCTS.
    The value type shall be an encoded character type.
    A function argument const Source& source shall have an effective range [source, end) where end is the first iterator value with an element value equal to iterator_­traits<decay_­t<Source>>​::​value_­type().
Functions taking template parameters named Source shall not participate in overload resolution unless either
  • Source is a specialization of basic_­string or basic_­string_­view, or
  • the qualified-id iterator_­traits<decay_­t<Source>>​::​value_­type is valid and denotes a possibly const encoded character type ([temp.deduct]).
[Note
:
See path conversions for how the value types above and their encodings convert to path​::​value_­type and its encoding.
end note
]
Arguments of type Source shall not be null pointers.

30.10.7.4 path members [fs.path.member]

30.10.7.4.1 path constructors [fs.path.construct]

path() noexcept;
Effects: Constructs an object of class path.
Postconditions: empty() == true.
path(const path& p); path(path&& p) noexcept;
Effects: Constructs an object of class path having the same pathname in the native and generic formats, respectively, as the original value of p.
In the second form, p is left in a valid but unspecified state.
path(string_type&& source, format fmt = auto_format);
Effects: Constructs an object of class path for which the pathname in the detected-format of source has the original value of source ([fs.path.fmt.cvt]), converting format if required ([fs.path.fmt.cvt]).
source is left in a valid but unspecified state.
template <class Source> path(const Source& source, format fmt = auto_format); template <class InputIterator> path(InputIterator first, InputIterator last, format fmt = auto_format);
Effects: Let s be the effective range of source ([fs.path.req]) or the range [first, last), with the encoding converted if required ([fs.path.cvt]).
Finds the detected-format of s ([fs.path.fmt.cvt]) and constructs an object of class path for which the pathname in that format is s.
template <class Source> path(const Source& source, const locale& loc, format fmt = auto_format); template <class InputIterator> path(InputIterator first, InputIterator last, const locale& loc, format fmt = auto_format);
Requires: The value type of Source and InputIterator is char.
Effects: Let s be the effective range of source or the range [first, last), after converting the encoding as follows:
  • If value_­type is wchar_­t, converts to the native wide encoding ([fs.path.type.cvt]) using the codecvt<​wchar_­t, char, mbstate_­t> facet of loc.
  • Otherwise a conversion is performed using the codecvt<wchar_­t, char, mbstate_­t> facet of loc, and then a second conversion to the current narrow encoding.
Finds the detected-format of s ([fs.path.fmt.cvt]) and constructs an object of class path for which the pathname in that format is s.
[Example
:
A string is to be read from a database that is encoded in ISO/IEC 8859-1, and used to create a directory:
namespace fs = std::filesystem;
std::string latin1_string = read_latin1_data();
codecvt_8859_1<wchar_t> latin1_facet;
std::locale latin1_locale(std::locale(), latin1_facet);
fs::create_directory(fs::path(latin1_string, latin1_locale));
For POSIX-based operating systems, the path is constructed by first using latin1_­facet to convert ISO/IEC 8859-1 encoded latin1_­string to a wide character string in the native wide encoding ([fs.path.type.cvt]).
The resulting wide string is then converted to a narrow character pathname string in the current native narrow encoding.
If the native wide encoding is UTF-16 or UTF-32, and the current native narrow encoding is UTF-8, all of the characters in the ISO/IEC 8859-1 character set will be converted to their Unicode representation, but for other native narrow encodings some characters may have no representation.
For Windows-based operating systems, the path is constructed by using latin1_­facet to convert ISO/IEC 8859-1 encoded latin1_­string to a UTF-16 encoded wide character pathname string.
All of the characters in the ISO/IEC 8859-1 character set will be converted to their Unicode representation.
end example
]

30.10.7.4.2 path assignments [fs.path.assign]

path& operator=(const path& p);
Effects: If *this and p are the same object, has no effect.
Otherwise, sets both respective pathnames of *this to the respective pathnames of p.
Returns: *this.
path& operator=(path&& p) noexcept;
Effects: If *this and p are the same object, has no effect.
Otherwise, sets both respective pathnames of *this to the respective pathnames of p.
p is left in a valid but unspecified state.
[Note
:
A valid implementation is swap(p).
end note
]
Returns: *this.
path& operator=(string_type&& source); path& assign(string_type&& source);
Effects: Sets the pathname in the detected-format of source to the original value of source.
source is left in a valid but unspecified state.
Returns: *this.
template <class Source> path& operator=(const Source& source); template <class Source> path& assign(const Source& source); template <class InputIterator> path& assign(InputIterator first, InputIterator last);
Effects: Let s be the effective range of source ([fs.path.req]) or the range [first, last), with the encoding converted if required ([fs.path.cvt]).
Finds the detected-format of s ([fs.path.fmt.cvt]) and sets the pathname in that format to s.
Returns: *this.

30.10.7.4.3 path appends [fs.path.append]

The append operations use operator/= to denote their semantic effect of appending preferred-separator when needed.
path& operator/=(const path& p);
Effects: If p.is_­absolute() || (p.has_­root_­name() && p.root_­name() != root_­name()), then operator=(p).
Otherwise, modifies *this as if by these steps:
  • If p.has_­root_­directory(), then removes any root directory and relative path from the generic format pathname.
    Otherwise, if !has_­root_­directory() && is_­absolute() is true or if has_­filename() is true, then appends path​::​preferred_­separator to the generic format pathname.
  • Then appends the native format pathname of p, omitting any root-name from its generic format pathname, to the native format pathname.
[Example
:
Even if //host is interpreted as a root-name, both of the paths path("//host")/"foo" and path("//host/")/"foo" equal "//host/foo".
Expression examples:
// On POSIX,
path("foo") / "";     // yields "foo/"
path("foo") / "/bar"; // yields "/bar"
// On Windows, backslashes replace slashes in the above yields

// On Windows,
path("foo") / "c:/bar";  // yields "c:/bar"
path("foo") / "c:";      // yields "c:"
path("c:") / "";         // yields "c:"
path("c:foo") / "/bar";  // yields "c:/bar"
path("c:foo") / "c:bar"; // yields "c:foo/bar"
end example
]
Returns: *this.
template <class Source> path& operator/=(const Source& source); template <class Source> path& append(const Source& source);
Effects: Equivalent to: return operator/=(path(source));
template <class InputIterator> path& append(InputIterator first, InputIterator last);
Effects: Equivalent to: return operator/=(path(first, last));

30.10.7.4.4 path concatenation [fs.path.concat]

path& operator+=(const path& x); path& operator+=(const string_type& x); path& operator+=(basic_string_view<value_type> x); path& operator+=(const value_type* x); path& operator+=(value_type x); template <class Source> path& operator+=(const Source& x); template <class EcharT> path& operator+=(EcharT x); template <class Source> path& concat(const Source& x);
Effects: Appends path(x).native() to the pathname in the native format.
[Note
:
This directly manipulates the value of native() and may not be portable between operating systems.
end note
]
Returns: *this.
template <class InputIterator> path& concat(InputIterator first, InputIterator last);
Effects: Equivalent to return *this += path(first, last).

30.10.7.4.5 path modifiers [fs.path.modifiers]

void clear() noexcept;
Postconditions: empty() == true.
path& make_preferred();
Effects: Each directory-separator of the pathname in the generic format is converted to preferred-separator.
Returns: *this.
[Example
:
path p("foo/bar");
std::cout << p << '\n';
p.make_preferred();
std::cout << p << '\n';
On an operating system where preferred-separator is a slash, the output is:
"foo/bar"
"foo/bar"
On an operating system where preferred-separator is a backslash, the output is:
"foo/bar"
"foo\bar"
end example
]
path& remove_filename();
Postconditions: !has_­filename().
Effects: Remove the generic format pathname of filename() from the generic format pathname.
Returns: *this.
[Example
:
path("foo/bar").remove_filename(); // yields "foo/"
path("foo/").remove_filename();    // yields "foo/"
path("/foo").remove_filename();    // yields "/"
path("/").remove_filename();       // yields "/"
end example
]
path& replace_filename(const path& replacement);
Effects: Equivalent to:
remove_filename();
operator/=(replacement);
Returns: *this.
[Example
:
path("/foo").replace_filename("bar");  // yields "/bar" on POSIX
path("/").replace_filename("bar");     // yields "/bar" on POSIX
end example
]
path& replace_extension(const path& replacement = path());
Effects:
  • Any existing extension()([fs.path.decompose]) is removed from the pathname in the generic format, then
  • If replacement is not empty and does not begin with a dot character, a dot character is appended to the pathname in the generic format, then
  • operator+=(replacement);.
Returns: *this.
void swap(path& rhs) noexcept;
Effects: Swaps the contents (in all formats) of the two paths.
Complexity: Constant time.

30.10.7.4.6 path native format observers [fs.path.native.obs]

The string returned by all native format observers is in the native pathname format ([fs.class.path]).
const string_type& native() const noexcept;
Returns: The pathname in the native format.
const value_type* c_str() const noexcept;
Returns: Equivalent to native().c_­str().
operator string_type() const;
Returns: native().
[Note
:
Conversion to string_­type is provided so that an object of class path can be given as an argument to existing standard library file stream constructors and open functions.
end note
]
template <class EcharT, class traits = char_traits<EcharT>, class Allocator = allocator<EcharT>> basic_string<EcharT, traits, Allocator> string(const Allocator& a = Allocator()) const;
Returns: native().
Remarks: All memory allocation, including for the return value, shall be performed by a.
Conversion, if any, is specified by [fs.path.cvt].
std::string string() const; std::wstring wstring() const; std::string u8string() const; std::u16string u16string() const; std::u32string u32string() const;
Returns: native().
Remarks: Conversion, if any, is performed as specified by [fs.path.cvt].
The encoding of the string returned by u8string() is always UTF-8.

30.10.7.4.7 path generic format observers [fs.path.generic.obs]

Generic format observer functions return strings formatted according to the generic pathname format.
A single slash ('/') character is used as the directory-separator.
[Example
:
On an operating system that uses backslash as its preferred-separator,
path("foo\\bar").generic_string()
returns "foo/bar".
end example
]
template <class EcharT, class traits = char_traits<EcharT>, class Allocator = allocator<EcharT>> basic_string<EcharT, traits, Allocator> generic_string(const Allocator& a = Allocator()) const;
Returns: The pathname in the generic format.
Remarks: All memory allocation, including for the return value, shall be performed by a.
Conversion, if any, is specified by [fs.path.cvt].
std::string generic_string() const; std::wstring generic_wstring() const; std::string generic_u8string() const; std::u16string generic_u16string() const; std::u32string generic_u32string() const;
Returns: The pathname in the generic format.
Remarks: Conversion, if any, is specified by [fs.path.cvt].
The encoding of the string returned by generic_­u8string() is always UTF-8.

30.10.7.4.8 path compare [fs.path.compare]

int compare(const path& p) const noexcept;
Returns:
  • A value less than 0, if native() for the elements of *this are lexicographically less than native() for the elements of p; otherwise,
  • a value greater than 0, if native() for the elements of *this are lexicographically greater than native() for the elements of p; otherwise,
  • 0.
Remarks: The elements are determined as if by iteration over the half-open range [begin(), end()) for *this and p.
int compare(const string_type& s) const int compare(basic_string_view<value_type> s) const;
Returns: compare(path(s)).
int compare(const value_type* s) const
Returns: compare(path(s)).

30.10.7.4.9 path decomposition [fs.path.decompose]

path root_name() const;
Returns: root-name, if the pathname in the generic format includes root-name, otherwise path().
path root_directory() const;
Returns: root-directory, if the pathname in the generic format includes root-directory, otherwise path().
path root_path() const;
Returns: root_­name() / root_­directory().
path relative_path() const;
Returns: A path composed from the pathname in the generic format, if empty() is false, beginning with the first filename after root-path.
Otherwise, path().
path parent_path() const;
Returns: *this if has_­relative_­path() is false, otherwise a path whose generic format pathname is the longest prefix of the generic format pathname of *this that produces one fewer element in its iteration.
path filename() const;
Returns: relative_­path().empty() ? path() : *--end().
[Example
:
path("/foo/bar.txt").filename();   // yields "bar.txt"
path("/foo/bar").filename();       // yields "bar"
path("/foo/bar/").filename();      // yields ""
path("/").filename();              // yields ""
path("//host").filename();         // yields ""
path(".").filename();              // yields "."
path("..").filename();             // yields ".."
end example
]
path stem() const;
Returns: Let f be the generic format pathname of filename().
Returns a path whose pathname in the generic format is
  • f, if it contains no periods other than a leading period or consists solely of one or two periods;
  • otherwise, the prefix of f ending before its last period.
[Example
:
std::cout << path("/foo/bar.txt").stem(); // outputs "bar"
path p = "foo.bar.baz.tar";
for (; !p.extension().empty(); p = p.stem())
  std::cout << p.extension() << '\n';
  // outputs: .tar
  //          .baz
  //          .bar
end example
]
path extension() const;
Returns: A path whose pathname in the generic format is the suffix of filename() not included in stem().
[Example
:
path("/foo/bar.txt").extension();  // yields ".txt" and stem() is "bar"
path("/foo/bar").extension();      // yields "" and stem() is "bar"
path("/foo/.profile").extension(); // yields "" and stem() is ".profile"
path(".bar").extension();          // yields "" and stem() is ".bar"
path("..bar").extension();         // yields ".bar" and stem() is "."
end example
]
[Note
:
The period is included in the return value so that it is possible to distinguish between no extension and an empty extension.
end note
]
[Note
:
On non-POSIX operating systems, for a path p, it may not be the case that p.stem() + p.extension() == p.filename(), even though the generic format pathnames are the same.
end note
]

30.10.7.4.10 path query [fs.path.query]

bool empty() const noexcept;
Returns: true if the pathname in the generic format is empty, else false.
bool has_root_path() const;
Returns: !root_­path().empty().
bool has_root_name() const;
Returns: !root_­name().empty().
bool has_root_directory() const;
Returns: !root_­directory().empty().
bool has_relative_path() const;
Returns: !relative_­path().empty().
bool has_parent_path() const;
Returns: !parent_­path().empty().
bool has_filename() const;
Returns: !filename().empty().
bool has_stem() const;
Returns: !stem().empty().
bool has_extension() const;
Returns: !extension().empty().
bool is_absolute() const;
Returns: true if the pathname in the native format contains an absolute path ([fs.class.path]), else false.
[Example
:
path("/").is_­absolute() is true for POSIX-based operating systems, and false for Windows-based operating systems.
end example
]
bool is_relative() const;
Returns: !is_­absolute().

30.10.7.4.11 path generation [fs.path.gen]

path lexically_normal() const;
Returns: A path whose pathname in the generic format is the normal form ([fs.path.generic]) of the pathname in the generic format of *this.
[Example
:
assert(path("foo/./bar/..").lexically_normal() == "foo/");
assert(path("foo/.///bar/../").lexically_normal() == "foo/");
The above assertions will succeed.
On Windows, the returned path's directory-separator characters will be backslashes rather than slashes, but that does not affect path equality.
end example
]
path lexically_relative(const path& base) const;
Returns: *this made relative to base.
Does not resolve ([fs.class.path]) symlinks.
Does not first normalize ([fs.path.generic]) *this or base.
Effects: If root_­name() != base.root_­name() is true or is_­absolute() != base.is_­absolute() is true or !has_­root_­directory() && base.has_­root_­directory() is true, returns path().
Determines the first mismatched element of *this and base as if by:
auto [a, b] = mismatch(begin(), end(), base.begin(), base.end());
Then,
  • if a == end() and b == base.end(), returns path("."); otherwise
  • let n be the number of filename elements in [b, base.end()) that are not dot or dot-dot minus the number that are dot-dot.
    If n<0, returns path(); otherwise
  • returns an object of class path that is default-constructed, followed by
    • application of operator/=(path("..")) n times, and then
    • application of operator/= for each element in [a, end()).
[Example
:
assert(path("/a/d").lexically_relative("/a/b/c") == "../../d");
assert(path("/a/b/c").lexically_relative("/a/d") == "../b/c");
assert(path("a/b/c").lexically_relative("a") == "b/c");
assert(path("a/b/c").lexically_relative("a/b/c/x/y") == "../..");
assert(path("a/b/c").lexically_relative("a/b/c") == ".");
assert(path("a/b").lexically_relative("c/d") == "../../a/b");
The above assertions will succeed.
On Windows, the returned path's directory-separator characters will be backslashes rather than slashes, but that does not affect path equality.
end example
]
[Note
:
If symlink following semantics are desired, use the operational function relative().
end note
]
[Note
:
If normalization ([fs.path.generic]) is needed to ensure consistent matching of elements, apply lexically_­normal() to *this, base, or both.
end note
]
path lexically_proximate(const path& base) const;
Returns: If the value of lexically_­relative(base) is not an empty path, return it.
Otherwise return *this.
[Note
:
If symlink following semantics are desired, use the operational function proximate().
end note
]
[Note
:
If normalization ([fs.path.generic]) is needed to ensure consistent matching of elements, apply lexically_­normal() to *this, base, or both.
end note
]

30.10.7.5 path iterators [fs.path.itr]

Path iterators iterate over the elements of the pathname in the generic format.
A path​::​iterator is a constant iterator satisfying all the requirements of a bidirectional iterator except that, for dereferenceable iterators a and b of type path​::​iterator with a == b, there is no requirement that *a and *b are bound to the same object.
Its value_­type is path.
Calling any non-const member function of a path object invalidates all iterators referring to elements of that object.
For the elements of the pathname in the generic format, the forward traversal order is as follows:
The backward traversal order is the reverse of forward traversal.
iterator begin() const;
Returns: An iterator for the first present element in the traversal list above.
If no elements are present, the end iterator.
iterator end() const;
Returns: The end iterator.

30.10.7.6 path non-member functions [fs.path.nonmember]

void swap(path& lhs, path& rhs) noexcept;
Effects: Equivalent to: lhs.swap(rhs);
size_t hash_value (const path& p) noexcept;
Returns: A hash value for the path p.
If for two paths, p1 == p2 then hash_­value(p1) == hash_­value(p2).
bool operator< (const path& lhs, const path& rhs) noexcept;
Returns: lhs.compare(rhs) < 0.
bool operator<=(const path& lhs, const path& rhs) noexcept;
Returns: !(rhs < lhs).
bool operator> (const path& lhs, const path& rhs) noexcept;
Returns: rhs < lh