27 Input/output library [input.output]

27.1 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 [tab:iostreams.lib.summary].

Table 121 — Input/output library summary
Subclause Header(s)
[iostreams.requirements] Requirements
[iostream.forward] Forward declarations <iosfwd>
[iostream.objects] Standard iostream objects <iostream>
[iostreams.base] Iostreams base classes <ios>
[stream.buffers] Stream buffers <streambuf>
[iostream.format] Formatting and manipulators <istream>
<ostream>
<iomanip>
[string.streams] String streams <sstream>
[file.streams] File streams <fstream>
<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]

27.2 Iostreams requirements [iostreams.requirements]

27.2.1 Imbue limitations [iostream.limits.imbue]

No function described in Clause [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 Clause [input.output] or as an overriding virtual function of any class declared in Clause [input.output] calls imbue, the behavior is undefined.

27.2.2 Positioning type limitations [iostreams.limits.pos]

The classes of Clause [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 Clause [input.output], a template formal 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.

27.2.3 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.

27.3 Forward declarations [iostream.forward]

Header <iosfwd> synopsis

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;

  typedef basic_ios<char>       ios;
  typedef basic_ios<wchar_t>    wios;

  typedef basic_streambuf<char> streambuf;
  typedef basic_istream<char>   istream;
  typedef basic_ostream<char>   ostream;
  typedef basic_iostream<char>  iostream;

  typedef basic_stringbuf<char>     stringbuf;
  typedef basic_istringstream<char> istringstream;
  typedef basic_ostringstream<char> ostringstream;
  typedef basic_stringstream<char>  stringstream;

  typedef basic_filebuf<char>  filebuf;
  typedef basic_ifstream<char> ifstream;
  typedef basic_ofstream<char> ofstream;
  typedef basic_fstream<char>  fstream;

  typedef basic_streambuf<wchar_t> wstreambuf;
  typedef basic_istream<wchar_t>   wistream;
  typedef basic_ostream<wchar_t>   wostream;
  typedef basic_iostream<wchar_t>  wiostream;

  typedef basic_stringbuf<wchar_t>     wstringbuf;
  typedef basic_istringstream<wchar_t> wistringstream;
  typedef basic_ostringstream<wchar_t> wostringstream;
  typedef basic_stringstream<wchar_t>  wstringstream;

  typedef basic_filebuf<wchar_t>  wfilebuf;
  typedef basic_ifstream<wchar_t> wifstream;
  typedef basic_ofstream<wchar_t> wofstream;
  typedef basic_fstream<wchar_t>  wfstream;

  template <class state> class fpos;
  typedef fpos<char_traits<char>::state_type>    streampos;
  typedef fpos<char_traits<wchar_t>::state_type> wstreampos;
}

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.294

Note: 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 typedefs 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.

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
typedef ... _STATE;             // implementation private declaration of stateT

typedef fpos<_STATE> streampos;

template<> struct char_traits<char> {
  typedef streampos
  pos_type;
}

 — end note ]

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.

27.4 Standard iostream objects [iostream.objects]

27.4.1 Overview [iostream.objects.overview]

Header <iostream> synopsis

#include <ios>
#include <streambuf>
#include <istream>
#include <ostream>

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;
}

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.295 The objects are not destroyed during program execution.296 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. Similarly, the entire program shall behave as if there were at least one 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 Amendment 1 of the ISO C standard.

Concurrent access to a synchronized ([ios.members.static]) standard iostream object's formatted and unformatted input ([istream]) and output ([ostream]) functions or a standard C stream by multiple threads shall not result in a data race ([intro.multithread]). [ Note: Users must still synchronize concurrent use of these objects and streams by multiple threads if they wish to avoid interleaved characters.  — end note ]

If it is possible for them to do so, implementations are encouraged to 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.

27.4.2 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 ([basic.ios.cons]).

ostream cout;

The object cout controls output to a stream buffer associated with the object stdout, declared in <cstdio> ([c.files]).

ostream cerr;

The object cerr controls output to a stream buffer associated with the object stderr, declared in <cstdio> ([c.files]).

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 ([basic.ios.cons]).

ostream clog;

The object clog controls output to a stream buffer associated with the object stderr, declared in <cstdio> ([c.files]).

27.4.3 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 ([basic.ios.cons]).

wostream wcout;

The object wcout controls output to a stream buffer associated with the object stdout, declared in <cstdio> ([c.files]).

wostream wcerr;

The object wcerr controls output to a stream buffer associated with the object stderr, declared in <cstdio> ([c.files]).

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 ([basic.ios.cons]).

wostream wclog;

The object wclog controls output to a stream buffer associated with the object stderr, declared in <cstdio> ([c.files]).

27.5 Iostreams base classes [iostreams.base]

27.5.1 Overview [iostreams.base.overview]

Header <ios> synopsis

#include <iosfwd>

namespace std {
  typedef implementation-defined streamoff;
  typedef implementation-defined streamsize;
  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);
  error_condition make_error_condition(io_errc e);
  const error_category& iostream_category();
}

27.5.2 Types [stream.types]

typedef implementation-defined streamoff;

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.297

typedef implementation-defined streamsize;

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.298

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).

27.5.3 Class ios_base [ios.base]

namespace std {
  class ios_base {
  public:
    class failure;

    // [ios::fmtflags] fmtflags
    typedef T1 fmtflags;
    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
    typedef T2 iostate;
    static constexpr iostate badbit = unspecified;
    static constexpr iostate eofbit = unspecified;
    static constexpr iostate failbit = unspecified;
    static constexpr iostate goodbit = see below;

    // [ios::openmode] openmode
    typedef T3 openmode;
    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
    typedef T4 seekdir;
    static constexpr fmtflags beg = unspecified;
    static constexpr fmtflags cur = unspecified;
    static constexpr fmtflags 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 };
    typedef void (*event_callback)(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 class failure 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 ]

27.5.3.1 Types [ios.types]

27.5.3.1.1 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);
  };
}

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.

27.5.3.1.2 Type ios_base::fmtflags [ios::fmtflags]

typedef T1 fmtflags;

The type fmtflags is a bitmask type ([bitmask.types]). Setting its elements has the effects indicated in Table [tab:iostreams.fmtflags.effects].

Table 122fmtflags effects
ElementEffect(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 [tab:iostreams.fmtflags.constants].

Table 123fmtflags constants
ConstantAllowable values
adjustfield left | right | internal
basefield dec | oct | hex
floatfield scientific | fixed

27.5.3.1.3 Type ios_base::iostate [ios::iostate]

typedef T2 iostate;

The type iostate is a bitmask type ([bitmask.types]) that contains the elements indicated in Table [tab:iostreams.iostate.effects].

Table 124iostate effects
ElementEffect(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:

  • goodbit, the value zero.

27.5.3.1.4 Type ios_base::openmode [ios::openmode]

typedef T3 openmode;

The type openmode is a bitmask type ([bitmask.types]). It contains the elements indicated in Table [tab:iostreams.openmode.effects].

Table 125openmode effects
ElementEffect(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

27.5.3.1.5 Type ios_base::seekdir [ios::seekdir]

typedef T4 seekdir;

The type seekdir is an enumerated type ([enumerated.types]) that contains the elements indicated in Table [tab:iostreams.seekdir.effects].

Table 126seekdir effects
ElementMeaning
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

27.5.3.1.6 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> ([c.files]).

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().

27.5.3.2 ios_base state functions [fmtflags.state]

fmtflags flags() const;

Returns: The format control information for both input and output.

fmtflags flags(fmtflags fmtfl);

Postcondition: 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);

Postcondition: 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);

Postcondition: wide == width().

Returns: The previous value of width().

27.5.3.3 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().

Postcondition: 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.

27.5.3.4 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 ([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(c);

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.299

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.

27.5.3.5 ios_base storage functions [ios.base.storage]

static int xalloc();

Returns: index ++.

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.300 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 fails301 and *this is a base 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 fails302 and *this is a base 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.

27.5.3.6 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.

27.5.3.7 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.

27.5.4 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
  };
}

27.5.4.1 fpos members [fpos.members]

void state(stateT s);

Effects: Assign s to st.

stateT state() const;

Returns: Current value of st.

27.5.4.2 fpos requirements [fpos.operations]

Operations specified in Table [tab:iostreams.position.requirements] 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 127 — Position type requirements
ExpressionReturn typeOperationalAssertion/note
semanticspre-/post-condition
P(i) p == P(i)
note: a destructor is assumed.
P p(i);
P p = i;
post: 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.

27.5.5 Class template basic_ios [ios]

27.5.5.1 Overview [ios.overview]

namespace std {
  template <class charT, class traits = char_traits<charT> >
  class basic_ios : public ios_base {
  public:

    // types:
    typedef charT                     char_type;
    typedef typename traits::int_type int_type;
    typedef typename traits::pos_type pos_type;
    typedef typename traits::off_type off_type;
    typedef traits                    traits_type;

    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);

  };
}

27.5.5.2 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 [tab:iostreams.basicios.init.effects].

Table 128basic_ios::init() effects
ElementValue
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

27.5.5.3 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 must not be reachable by traversing the linked list of tied stream objects starting from tiestr->tie().

Postcondition: 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);

Postcondition: sb == rdbuf().

Effects: Calls clear().

Returns: The previous value of rdbuf().

locale imbue(const locale& loc);

Effects: Calls ios_base::imbue(loc) ([ios.base.locales]) and if rdbuf()!=0 then rdbuf()->pubimbue(loc) ([streambuf.locales]).

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);

Postcondition: 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. calls each registered callback pair (fn, index) as (*fn)(erase_event, *this, index);

  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;303 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. calls each callback pair that was copied from rhs as (*fn)(copyfmt_event, *this, index);

  4. calls exceptions(rhs.except()).

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.

Postconditions: The postconditions of this function are indicated in Table [tab:iostreams.copyfmt.effects].

Table 129basic_ios::copyfmt() effects
ElementValue
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 non-zero.

27.5.5.4 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);

Postcondition: 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 fail of class basic_ios::failure ([ios::failure]), constructed with implementation-defined argument values.

void setstate(iostate state);

Effects: Calls clear(rdstate() | state) (which may throw basic_ios::failure ([ios::failure])).

bool good() const;

Returns: rdstate() == 0

bool eof() const;

Returns: true if eofbit is set in rdstate().

bool fail() const;

Returns: true if failbit or badbit is set in rdstate().304

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);

Postcondition: except == exceptions().

Effects: Calls clear(rdstate()).

Checking badbit also for fail() is historical practice.

27.5.6 ios_base manipulators [std.ios.manip]

27.5.6.1 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.

27.5.6.2 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.

27.5.6.3 basefield manipulators [basefield.manip]

ios_base& dec(ios_base& str);

Effects: Calls str.setf(ios_base::dec, ios_base::basefield).

Returns: str305.

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.

27.5.6.4 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.

27.5.6.5 Error reporting [error.reporting]

error_code make_error_code(io_errc e);

Returns: error_code(static_cast<int>(e), iostream_category()).

error_condition make_error_condition(io_errc e);

Returns: error_condition(static_cast<int>(e), iostream_category()).

const error_category& iostream_category();

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".

27.6 Stream buffers [stream.buffers]

27.6.1 Overview [stream.buffers.overview]

Header <streambuf> synopsis

namespace std {
  template <class charT, class traits = char_traits<charT> >
    class basic_streambuf;
  typedef basic_streambuf<char>     streambuf;
  typedef basic_streambuf<wchar_t> wstreambuf;
}

The header <streambuf> defines types that control input from and output to character sequences.

27.6.2 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).

27.6.3 Class template basic_streambuf<charT,traits> [streambuf]

namespace std {
  template <class charT, class traits = char_traits<charT> >
  class basic_streambuf {
  public:

    // types:
    typedef charT                     char_type;
    typedef typename traits::int_type int_type;
    typedef typename traits::pos_type pos_type;
    typedef typename traits::off_type off_type;
    typedef traits                    traits_type;

    virtual ~basic_streambuf();

    // [streambuf.locales] locales:
    locale   pubimbue(const locale& loc);
    locale   getloc() const;

    // [streambuf.buffer] buffer and positioning:
    basic_streambuf<char_type,traits>*
       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:
    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:
    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<char_type,traits>*
         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<charT,traits> serves as an abstract base class for deriving various stream buffers whose objects each control two character sequences:

27.6.3.1 basic_streambuf constructors [streambuf.cons]

basic_streambuf();

Effects: Constructs an object of class basic_streambuf<charT,traits> and initializes:306

  • 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.

27.6.3.2 basic_streambuf public member functions [streambuf.members]

27.6.3.2.1 Locales [streambuf.locales]

locale pubimbue(const locale& loc);

Postcondition: 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.

27.6.3.2.2 Buffer management and positioning [streambuf.buffer]

basic_streambuf<char_type,traits>* 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().

27.6.3.2.3 Get area [streambuf.pub.get]

streamsize in_avail();

Returns: If a read position is available, returns egptr() - gptr(). Otherwise returns showmanyc() ([streambuf.virt.get]).

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).

27.6.3.2.4 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()).

27.6.3.2.5 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).

27.6.3.3 basic_streambuf protected member functions [streambuf.protected]

27.6.3.3.1 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.

27.6.3.3.2 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().

27.6.3.3.3 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().

27.6.3.4 basic_streambuf virtual functions [streambuf.virtuals]

27.6.3.4.1 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.

27.6.3.4.2 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.

27.6.3.4.3 Get area [streambuf.virt.get]

streamsize showmanyc();307

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.308

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.309

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:

  1. If gptr() is non- NULL, then the egptr() - gptr() characters starting at gptr(), otherwise the empty sequence.

  2. Some sequence (possibly empty) of characters read from the input sequence.

The result character is

  1. If the pending sequence is non-empty, the first character of the sequence.

  2. If the pending sequence is empty then the next character that would be read from the input sequence.

The backup sequence is defined as the concatenation of:

  1. If eback() is null then empty,

  2. Otherwise the gptr() - eback() characters beginning at eback().

Effects: The function sets up the gptr() and egptr() satisfying one of:

  1. If the pending sequence is non-empty, egptr() is non-null and egptr() - gptr() characters starting at gptr() are the characters in the pending sequence

  2. If the pending sequence is empty, either gptr() is null or gptr() and egptr() are set to the same non-NULL pointer.

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:

  1. If the backup sequence contains at least gptr() - eback() characters, then the gptr() - eback() characters starting at eback() agree with the last gptr() - eback() characters of the backup sequence.

  2. Or the n characters starting at gptr() - n 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.

27.6.3.4.4 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()) return false, then c is prepended. Whether the input sequence is backed up or modified in any other way is unspecified.

Postcondition: 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().

27.6.3.4.5 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(). Is 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

  1. if pbase() is NULL then the empty sequence otherwise, pptr() - pbase() characters beginning at pbase().

  2. if traits::eq_int_type(c,traits::eof()) returns true, then the empty sequence 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. The effect of consuming a character on the associated output sequence is specified310

  2. Let r be the number of characters in the pending sequence not consumed. If r is non-zero 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 NULL, or pbase() and pptr() are both set to the same NULL non-value.

  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.311

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).

27.7 Formatting and manipulators [iostream.format]

27.7.1 Overview [iostream.format.overview]

Header <istream> synopsis

namespace std {
  template <class charT, class traits = char_traits<charT> >
    class basic_istream;
  typedef basic_istream<char>     istream;
  typedef basic_istream<wchar_t> wistream;

  template <class charT, class traits = char_traits<charT> >
    class basic_iostream;
  typedef basic_iostream<char>    iostream;
  typedef basic_iostream<wchar_t> wiostream;

  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);
}

Header <ostream> synopsis

namespace std {
  template <class charT, class traits = char_traits<charT> >
    class basic_ostream;
  typedef basic_ostream<char>     ostream;
  typedef basic_ostream<wchar_t> wostream;

  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);
}

Header <iomanip> synopsis

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<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);
}

27.7.2 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.

27.7.2.1 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 ([ios])):
    typedef charT                     char_type;
    typedef typename traits::int_type int_type;
    typedef typename traits::pos_type pos_type;
    typedef typename traits::off_type off_type;
    typedef traits                    traits_type;

    // [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:
    basic_istream(const basic_istream& rhs) = delete;
    basic_istream(basic_istream&& rhs);

    // [istream.assign] Assign/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 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.

27.7.2.1.1 basic_istream constructors [istream.cons]

explicit basic_istream(basic_streambuf<charT,traits>* sb);

Effects: Constructs an object of class basic_istream, assigning initial values to the base class by calling basic_ios::init(sb) ([basic.ios.cons]).

Postcondition: 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().

27.7.2.1.2 Class basic_istream assign and swap [istream.assign]

basic_istream& operator=(basic_istream&& rhs);

Effects: 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().

27.7.2.1.3 Class basic_istream::sentry [istream::sentry]

namespace std {
  template <class charT,class traits = char_traits<charT> >
  class basic_istream<charT,traits>::sentry {
    typedef traits traits_type;
    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.312 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]))313

~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.

27.7.2.2 Formatted input functions [istream.formatted]

27.7.2.2.1 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 on314 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.

27.7.2.2.2 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<> ([locale.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:

typedef num_get< charT,istreambuf_iterator<charT,traits> > numget;
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):

typedef num_get<charT,istreambuf_iterator<charT,traits> > numget;
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):

typedef num_get<charT,istreambuf_iterator<charT,traits> > numget;
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);

27.7.2.2.3 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).315

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).316 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;

  • ct.is(ct.space,c) is true for the next available input character c, where ct is use_facet<ctype<charT> >(in.getloc()).

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 (as described in [istream.unformatted], paragraph 1). 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&) ([istream.manip]).

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

27.7.2.3 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 non-zero 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 on317 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 in [istream.unformatted], paragraph 1).

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 in [istream.unformatted], paragraph 1). 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 in [istream.unformatted], paragraph 1). After constructing a sentry object, extracts a character, if one is available, and assigns it to c.318 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 in [istream.unformatted], paragraph 1). After constructing a sentry object, extracts characters and stores them into successive locations of an array whose first element is designated by s.319 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 in [istream.unformatted], paragraph 1). 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 in [istream.unformatted], paragraph 1). After constructing a sentry object, extracts characters and stores them into successive locations of an array whose first element is designated by s.320 Characters are extracted and stored until one of the following occurs:

  1. end-of-file occurs on the input sequence (in which case the function calls setstate(eofbit));

  2. traits::eq(c, delim) for the next available input character c (in which case the input character is extracted but not stored);321

  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.322

If the function extracts no characters, it calls setstate(failbit) (which may throw ios_base::failure ([iostate.flags])).323

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 in [istream.unformatted], paragraph 1). After constructing a sentry object, extracts characters and discards them. Characters are extracted until any of the following occurs:

  • if n != numeric_limits<streamsize>::max() ([limits]), n characters are extracted

  • 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 in [istream.unformatted], paragraph 1). 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 in [istream.unformatted], paragraph 1). 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.324 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 in [istream.unformatted], paragraph 1). 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 in [istream.unformatted], paragraph 1), 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 in [istream.unformatted], paragraph 1), 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 in [istream.unformatted], paragraph 1), 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 in [istream.unformatted], paragraph 1), 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 in [istream.unformatted], paragraph 1), 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 in [istream.unformatted], paragraph 1), 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 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.

27.7.2.4 Standard basic_istream manipulators [istream.manip]

namespace std { template <class charT, class traits> basic_istream<charT,traits>& ws(basic_istream<charT,traits>& is); }

Effects: Behaves as an unformatted input function (as described in [istream.unformatted], paragraph 1), 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 ([istream::sentry]). If ws stops extracting characters because there are no more available it sets eofbit, but not failbit.

Returns: is.

27.7.2.5 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:
    // types:
    typedef charT                     char_type;
    typedef typename traits::int_type int_type;
    typedef typename traits::pos_type pos_type;
    typedef typename traits::off_type off_type;
    typedef traits                    traits_type;

    // constructor/destructor
    explicit basic_iostream(basic_streambuf<charT,traits>* sb);
    virtual ~basic_iostream();

  protected:
    basic_iostream(const basic_iostream& rhs) = delete;
    basic_iostream(basic_iostream&& rhs);

    // assign/swap
    basic_iostream& operator=(const basic_iostream& rhs) = delete;
    basic_iostream& operator=(basic_iostream&& rhs);
    void swap(basic_iostream& rhs);
  };
}

The class basic_iostream inherits a number of functions that allow reading input and writing output to sequences controlled by a stream buffer.

27.7.2.5.1 basic_iostream constructors [iostream.cons]

explicit basic_iostream(basic_streambuf<charT,traits>* sb);

Effects: Constructs an object of class basic_iostream, assigning initial values to the base classes by calling basic_istream<charT,traits>(sb) ([istream]) and basic_ostream<charT,traits>(sb) ([ostream])

Postcondition: 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).

27.7.2.5.2 basic_iostream destructor [iostream.dest]

virtual ~basic_iostream();

Effects: Destroys an object of class basic_iostream.

Remarks: Does not perform any operations on rdbuf().

27.7.2.5.3 basic_iostream assign and swap [iostream.assign]

basic_iostream& operator=(basic_iostream&& rhs);

Effects: swap(rhs).

void swap(basic_iostream& rhs);

Effects: Calls basic_istream<charT, traits>::swap(rhs).

27.7.2.6 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: is >> x

Returns: is

27.7.3 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.

27.7.3.1 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 ([ios])):
    typedef charT                     char_type;
    typedef typename traits::int_type int_type;
    typedef typename traits::pos_type pos_type;
    typedef typename traits::off_type off_type;
    typedef traits                    traits_type;

    // [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<<(
      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:
    basic_ostream(const basic_ostream& rhs) = delete;
    basic_ostream(basic_ostream&& rhs);

    // [ostream.assign] Assign/swap
    basic_ostream& operator=(basic_ostream& rhs) = delete;
    basic_ostream& operator=(const 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);

  // signed and unsigned
  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*);

  // signed and unsigned
  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 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.

27.7.3.2 basic_ostream constructors [ostream.cons]

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

Effects: Constructs an object of class basic_ostream, assigning initial values to the base class by calling basic_ios<charT,traits>::init(sb) ([basic.ios.cons]).

Postcondition: rdbuf() == sb.

virtual ~basic_ostream();

Effects: Destroys an object of class basic_ostream.

Remarks: Does not perform any operations on rdbuf().

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.

27.7.3.3 Class basic_ostream assign and swap [ostream.assign]

basic_ostream& operator=(basic_ostream&& rhs);

Effects: swap(rhs).

Returns: *this.

void swap(basic_ostream& rhs);

Effects: Calls basic_ios<charT, traits>::swap(rhs).

27.7.3.4 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().325

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]))326

~sentry();

If ((os.flags() & ios_base::unitbuf) && !uncaught_exception() && 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.

27.7.3.5 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).

Returns: *this.

27.7.3.6 Formatted output functions [ostream.formatted]

27.7.3.6.1 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 on327 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 operations describe how they perform output and do not mention the sentry object.

without causing an ios::failure to be thrown.

27.7.3.6.2 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 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.

27.7.3.6.3 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).328

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.329

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 (as described in [ostream.unformatted], paragraph 1). 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.

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

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

27.7.3.6.4 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 like a formatted inserter (as described in [ostream.formatted.reqmts]) of out. After a sentry object is constructed it inserts characters. In case c has type char and the character type of the stream is not char, then the character to be inserted is out.widen(c); otherwise the character is c. Padding is determined as described in [facet.num.put.virtuals]. width(0) is called. The insertion character and any required padding are inserted into out.

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. After a sentry object is constructed it inserts n characters starting at s, 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*,

  • std::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.

Padding is determined as described in [facet.num.put.virtuals]. The n characters starting at s are widened using out.widen ([basic.ios.members]). The widened characters and any required padding are inserted into out. Calls width(0).

Returns: out.

27.7.3.7 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 on330 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 in [ostream.unformatted], paragraph 1). After constructing a sentry object, inserts the character c, if possible.331

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 in [ostream.unformatted], paragraph 1). After constructing a sentry object, obtains characters to insert from successive locations of an array whose first element is designated by s.332 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 in [ostream.formatted.reqmts], paragraph 1). 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.

27.7.3.8 Standard basic_ostream manipulators [ostream.manip]

namespace std { template <class charT, class traits> basic_ostream<charT,traits>& endl(basic_ostream<charT,traits>& os); }

Effects: Calls os.put(os.widen('\n')), then os.flush().

Returns: os.

namespace std { 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.

namespace std { template <class charT, class traits> basic_ostream<charT,traits>& flush(basic_ostream<charT,traits>& os); }

Effects: Calls os.flush().

Returns: os.

27.7.3.9 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: os << x

Returns: os

27.7.4 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:333

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).

27.7.5 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 (Clause [strings]).

Effects: The expression in >> get_money(mon, intl) described below behaves as a formatted input function ([istream.formatted.reqmts]).

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) {
  typedef istreambuf_iterator<charT, traits> Iter;
  typedef money_get<charT, Iter> MoneyGet;

  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 (Clause [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 input 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) {
  typedef ostreambuf_iterator<charT, traits> Iter;
  typedef money_put<charT, Iter> MoneyPut;

  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, 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 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) {
  typedef istreambuf_iterator<charT, traits> Iter;
  typedef time_get<charT, Iter> TimeGet;

  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) {
  typedef ostreambuf_iterator<charT, traits> Iter;
  typedef time_put<charT, Iter> TimePut;

  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_istream<charT, traits>& and value out.

27.8 String-based streams [string.streams]

27.8.1 Overview [string.streams.overview]

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].

Header <sstream> synopsis

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

  typedef basic_stringbuf<char>     stringbuf;
  typedef basic_stringbuf<wchar_t> wstringbuf;

  template <class charT, class traits = char_traits<charT>,
        class Allocator = allocator<charT> >
    class basic_istringstream;

  typedef basic_istringstream<char>     istringstream;
  typedef basic_istringstream<wchar_t> wistringstream;

  template <class charT, class traits = char_traits<charT>,
        class Allocator = allocator<charT> >
    class basic_ostringstream;
  typedef basic_ostringstream<char>     ostringstream;
  typedef basic_ostringstream<wchar_t> wostringstream;

  template <class charT, class traits = char_traits<charT>,
        class Allocator = allocator<charT> >
    class basic_stringstream;
  typedef basic_stringstream<char>     stringstream;
  typedef basic_stringstream<wchar_t> wstringstream;
}

27.8.2 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:
    typedef charT                     char_type;
    typedef typename traits::int_type int_type;
    typedef typename traits::pos_type pos_type;
    typedef typename traits::off_type off_type;
    typedef traits                    traits_type;
    typedef Allocator                 allocator_type;

    // [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:
    virtual int_type   underflow();
    virtual int_type   pbackfail(int_type c = traits::eof());
    virtual int_type   overflow (int_type c = traits::eof());
    virtual  basic_streambuf<charT,traits>* setbuf(charT*, streamsize);


    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);

  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.

27.8.2.1 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.

Postcondition: 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()

27.8.2.2 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: x.swap(y).

27.8.2.3 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 true 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 true, pbase() points to the first underlying character and epptr() >= pbase() + s.size() holds; in addition, if mode & ios_base::ate is true, pptr() == pbase() + s.size() holds, otherwise pptr() == pbase() is true. If mode & ios_base::in is true, eback() points to the first underlying character, and both gptr() == eback() and egptr() == eback() + s.size() hold.

27.8.2.4 Overridden virtual functions [stringbuf.virtuals]

int_type underflow();

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());

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: 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());

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: 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);

Effects: Alters the stream position within one of the controlled sequences, if possible, as indicated in Table [tab:iostreams.seekoff.positioning].

Table 130seekoff positioning
ConditionsResult
(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, if its next pointer (either gptr() or pptr()) is a null pointer and the new offset newoff is nonzero, the positioning operation fails. Otherwise, the function determines newoff as indicated in Table [tab:iostreams.newoff.values].

Table 131newoff values
Conditionnewoff 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 (as defined in [stringbuf.members] paragraph 1), 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);

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.

27.8.3 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:
    typedef charT                     char_type;
    typedef typename traits::int_type int_type;
    typedef typename traits::pos_type pos_type;
    typedef typename traits::off_type off_type;
    typedef traits                    traits_type;
    typedef Allocator                 allocator_type;

    // [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.

27.8.3.1 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(&sb) 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(&sb) 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.

27.8.3.2 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: x.swap(y).

27.8.3.3 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).

27.8.4 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:

    // types:
    typedef charT                     char_type;
    typedef typename traits::int_type int_type;
    typedef typename traits::pos_type pos_type;
    typedef typename traits::off_type off_type;
    typedef traits                    traits_type;
    typedef Allocator                 allocator_type;

    // [ostringstream.cons] Constructors/destructor:
    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/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.

27.8.4.1 basic_ostringstream constructors [ostringstream.cons]

explicit basic_ostringstream(ios_base::openmode which = ios_base::out);

Effects: Constructs an object of class basic_ostringstream, initializing the base class with basic_ostream(&sb) 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(&sb) 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.

27.8.4.2 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: x.swap(y).

27.8.4.3 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).

27.8.5 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:

    // types:
    typedef charT                     char_type;
    typedef typename traits::int_type int_type;
    typedef typename traits::pos_type pos_type;
    typedef typename traits::off_type off_type;
    typedef traits                    traits_type;
    typedef Allocator                 allocator_type;

    // constructors/destructor
    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/swap:
    basic_stringstream& operator=(const basic_stringstream& rhs) = delete;
    basic_stringstream& operator=(basic_stringstream&& rhs);
    void swap(basic_stringstream& rhs);

    // 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.

27.8.6 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(&sb) 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(&sb) 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.

27.8.6.1 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: x.swap(y).

27.8.7 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).

27.9 File-based streams [file.streams]

27.9.1 File streams [fstreams]

The header <fstream> defines four class templates and eight types that associate stream buffers with files and assist reading and writing files.

Header <fstream> synopsis

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

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

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

  template <class charT, class traits = char_traits<charT> >
    class basic_fstream;
  typedef basic_fstream<char>     fstream;
  typedef basic_fstream<wchar_t> wfstream;
}

In this subclause, the type name FILE refers to the type FILE declared in <cstdio> ([c.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 ]

27.9.1.1 Class template basic_filebuf [filebuf]

namespace std {
  template <class charT, class traits = char_traits<charT> >
  class basic_filebuf : public basic_streambuf<charT,traits> {
  public:
    typedef charT                     char_type;
    typedef typename traits::int_type int_type;
    typedef typename traits::pos_type pos_type;
    typedef typename traits::off_type off_type;
    typedef traits                    traits_type;

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

    // [filebuf.assign] Assign/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<charT,traits>* open(const char* s,
        ios_base::openmode mode);
    basic_filebuf<charT,traits>* open(const string& s,
        ios_base::openmode mode);
    basic_filebuf<charT,traits>* close();

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

    virtual basic_streambuf<charT,traits>*
        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();
    virtual void     imbue(const locale& loc);
  };

  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 Standard C 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());

27.9.1.2 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]).

Postcondition: 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]).

27.9.1.3 Assign and swap [filebuf.assign]

basic_filebuf& operator=(basic_filebuf&& rhs);

Effects: Calls this->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: x.swap(y).

27.9.1.4 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<charT,traits>* open(const char* s, ios_base::openmode mode);

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 std::fopen(s,modstr)). The ntbs modstr is determined from mode & ~ios_base::ate as indicated in Table [tab:iostreams.file.open.modes]. If mode is not some combination of flags shown in the table then the open fails.

Table 132 — 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 std::fseek(file,0,SEEK_END)).334

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<charT,traits>* open(const string& s, ios_base::openmode mode);

Returns: open(s.c_str(), mode);

basic_filebuf<charT,traits>* 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 std::fclose(file)).335 If any of the calls made by the function, including std::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.

Postcondition: 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> ([c.files]).

The function signature fclose(FILE*) is declared in <cstdio> ([c.files]).

27.9.1.5 Overridden virtual functions [filebuf.virtuals]

streamsize showmanyc();

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();

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();

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());

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: 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());

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);

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);

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 std::fseek(file, width * off, whence), otherwise call std::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 [tab:iostreams.seekoff.effects].

Table 133seekoff effects
way Valuestdio 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);

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. if (om & ios_base::out) != 0, then update the output sequence and write any unshift sequence;

  2. set the file position to sp;

  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();

Effects: If a put area exists, calls filebuf::overflow to write the characters to the file. If a get area exists, the effect is implementation-defined.

void imbue(const locale& loc);

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.

Remark: 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.

27.9.1.6 Class template basic_ifstream [ifstream]

namespace std {
  template <class charT, class traits = char_traits<charT> >
  class basic_ifstream : public basic_istream<charT,traits> {
  public:
    typedef charT                     char_type;
    typedef typename traits::int_type int_type;
    typedef typename traits::pos_type pos_type;
    typedef typename traits::off_type off_type;
    typedef traits                    traits_type;

    // [ifstream.cons] Constructors:
    basic_ifstream();
    explicit basic_ifstream(const char* s,
        ios_base::openmode mode = ios_base::in);
    explicit basic_ifstream(const string& s,
        ios_base::openmode mode = ios_base::in);
    basic_ifstream(const basic_ifstream& rhs) = delete;
    basic_ifstream(basic_ifstream&& rhs);

    // [ifstream.assign] Assign/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 string& 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:

  • sb, the filebuf object.

27.9.1.7 basic_ifstream constructors [ifstream.cons]

basic_ifstream();

Effects: Constructs an object of class basic_ifstream<charT,traits>, initializing the base class with basic_istream(&sb) and initializing sb with basic_filebuf<charT,traits>()) ([istream.cons], [filebuf.cons]).

explicit basic_ifstream(const char* s, ios_base::openmode mode = ios_base::in);

Effects: Constructs an object of class basic_ifstream, initializing the base class with basic_istream(&sb) and initializing sb with basic_filebuf<charT, traits>()) ([istream.cons], [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);

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.

27.9.1.8 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: x.swap(y).

27.9.1.9 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);

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);

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])).

27.9.1.10 Class template basic_ofstream [ofstream]

namespace std {
  template <class charT, class traits = char_traits<charT> >
  class basic_ofstream : public basic_ostream<charT,traits> {
  public:
    typedef charT                     char_type;
    typedef typename traits::int_type int_type;
    typedef typename traits::pos_type pos_type;
    typedef typename traits::off_type off_type;
    typedef traits                    traits_type;

    // [ofstream.cons] Constructors:
    basic_ofstream();
    explicit basic_ofstream(const char* s,
        ios_base::openmode mode = ios_base::out);
    explicit basic_ofstream(const string& s,
        ios_base::openmode mode = ios_base::out);
    basic_ofstream(const basic_ofstream& rhs) = delete;
    basic_ofstream(basic_ofstream&& rhs);

    // [ofstream.assign] Assign/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 string& 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:

  • sb, the filebuf object.

27.9.1.11 basic_ofstream constructors [ofstream.cons]

basic_ofstream();

Effects: Constructs an object of class basic_ofstream<charT,traits>, initializing the base class with basic_ostream(&sb) and initializing sb with basic_filebuf<charT,traits>()) ([ostream.cons], [filebuf.cons]).

explicit basic_ofstream(const char* s, ios_base::openmode mode = ios_base::out);

Effects: Constructs an object of class basic_ofstream<charT,traits>, initializing the base class with basic_ostream(&sb) and initializing sb with basic_filebuf<charT,traits>()) ([ostream.cons], [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);

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.

27.9.1.12 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: x.swap(y).

27.9.1.13 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);

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);

Effects: calls open(s.c_str(), mode);

27.9.1.14 Class template basic_fstream [fstream]

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

  public:
    typedef charT                     char_type;
    typedef typename traits::int_type int_type;
    typedef typename traits::pos_type pos_type;
    typedef typename traits::off_type off_type;
    typedef traits                    traits_type;

    // constructors/destructor
    basic_fstream();
    explicit basic_fstream(const char* s,
        ios_base::openmode mode = ios_base::in|ios_base::out);
    explicit basic_fstream(const string& 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/swap:
    basic_fstream& operator=(const basic_fstream& rhs) = delete;
    basic_fstream& operator=(basic_fstream&& rhs);
    void swap(basic_fstream& rhs);

    // 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 string& 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.

27.9.1.15 basic_fstream constructors [fstream.cons]

basic_fstream();

Effects: Constructs an object of class basic_fstream<charT,traits>, initializing the base class with basic_iostream(&sb) and initializing sb with basic_filebuf<charT,traits>().

explicit basic_fstream(const char* s, ios_base::openmode mode = ios_base::in|ios_base::out);

Effects: Constructs an object of class basic_fstream<charT, traits>, initializing the base class with basic_iostream(&sb) 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);

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.

27.9.1.16 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: x.swap(y).

27.9.1.17 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);

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);

Effects: calls open(s.c_str(), mode);

void close();

Effects: Calls rdbuf()->close() and, if that function returns returns a null pointer, calls setstate(failbit) ([iostate.flags]) (which may throw ios_base::failure).

27.9.2 C library files [c.files]

Table [tab:iostreams.hdr.cstdio] describes header <cstdio>.

Table 134 — Header <cstdio> synopsis
TypeName(s)
Macros:
BUFSIZ FOPEN_MAX SEEK_CUR TMP_MAX _IONBF stdout
EOF L_tmpnam SEEK_END _IOFBF stderr
FILENAME_MAX NULL <cstdio> SEEK_SET _IOLBF stdin
Types: FILE fpos_t size_t <cstdio>
Functions:
clearerr fopen fsetpos putc setbuf vprintf
fclose fprintf ftell putchar setvbuf vscanf
feof fputc fwrite puts snprintf vsnprintf
ferror fputs getc rename sprintf vsprintf
fflush fread getchar remove tmpfile vsscanf
fgetc freopen gets rewind tmpnam
fgetpos fscanf perror scanf ungetc
fgets fseek printf sscanf vfprintf

Calls to the function tmpnam with an argument of NULL may introduce a data race ([res.on.data.races]) with other calls to tmpnam with an argument of NULL.

See also: ISO C 7.9, Amendment 1 4.6.2.

Table [tab:iostreams.hdr.cinttypes] describes header <cinttypes>. [ Note: The macros defined by <cinttypes> are provided unconditionally. In particular, the symbol __STDC_FORMAT_MACROS, mentioned in footnote 182 of the C standard, plays no role in C++.  — end note ]

Table 135 — Header <cinttypes> synopsis
TypeName(s)
Macros:
PRI{d i o u x X}[FAST LEAST]{8 16 32 64}
PRI{d i o u x X}{MAX PTR}
SCN{d i o u x X}[FAST LEAST]{8 16 32 64}
SCN{d i o u x X}{MAX PTR}
Types: imaxdiv_t
Functions:
abs imaxabs strtoimax wcstoimax
div imaxdiv strtoumax wcstoumax

The contents of header <cinttypes> are the same as the Standard C Library header <inttypes.h>, with the following changes:

  • the header <cinttypes> includes the header <cstdint> instead of <stdint.h>, and

  • if and only if the type intmax_t designates an extended integer type ([basic.fundamental]), the following function signatures are added:

    intmax_t abs(intmax_t);
    imaxdiv_t div(intmax_t, intmax_t);
    

    which shall have the same semantics as the function signatures intmax_t imaxabs(intmax_t) and imaxdiv_t imaxdiv(intmax_t, intmax_t), respectively.