istream

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// Input streams -*- C++ -*-
 
// Copyright (C) 1997-2025 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.
 
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
 
// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.
 
// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.
 
//
// ISO C++ 14882: 27.6.1  Input streams
//
 
/** @file include/istream
 *  This is a Standard C++ Library header.
 */
 
#ifndef _GLIBCXX_ISTREAM
#define _GLIBCXX_ISTREAM 1
 
#ifdef _GLIBCXX_SYSHDR
#pragma GCC system_header
#endif
 
#include <bits/requires_hosted.h> // iostreams
 
#include <ios>
#include <ostream>
 
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
 
  /**
   *  @brief  Template class basic_istream.
   *  @ingroup io
   *
   *  @tparam _CharT  Type of character stream.
   *  @tparam _Traits  Traits for character type, defaults to
   *                   char_traits<_CharT>.
   *
   *  This is the base class for all input streams.  It provides text
   *  formatting of all builtin types, and communicates with any class
   *  derived from basic_streambuf to do the actual input.
  */
  template<typename _CharT, typename _Traits>
    class basic_istream : virtual public basic_ios<_CharT, _Traits>
    {
    public:
      // Types (inherited from basic_ios (27.4.4)):
      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;
 
      // Non-standard Types:
      typedef basic_streambuf<_CharT, _Traits>          __streambuf_type;
      typedef basic_ios<_CharT, _Traits>                __ios_type;
      typedef basic_istream<_CharT, _Traits>            __istream_type;
      typedef num_get<_CharT, istreambuf_iterator<_CharT, _Traits> >
                                                        __num_get_type;
      typedef ctype<_CharT>                             __ctype_type;
 
    protected:
      // Data Members:
      /**
       *  The number of characters extracted in the previous unformatted
       *  function; see gcount().
      */
      streamsize                _M_gcount;
 
    public:
      /**
       *  @brief  Base constructor.
       *
       *  This ctor is almost never called by the user directly, rather from
       *  derived classes' initialization lists, which pass a pointer to
       *  their own stream buffer.
      */
      explicit
      basic_istream(__streambuf_type* __sb)
      : _M_gcount(streamsize(0))
      { this->init(__sb); }
 
      /**
       *  @brief  Base destructor.
       *
       *  This does very little apart from providing a virtual base dtor.
      */
      virtual
      ~basic_istream()
      { _M_gcount = streamsize(0); }
 
      /// Safe prefix/suffix operations.
      class sentry;
      friend class sentry;
 
      ///@{
      /**
       *  @brief  Interface for manipulators.
       *
       *  Manipulators such as @c std::ws and @c std::dec use these
       *  functions in constructs like
       *  <code>std::cin >> std::ws</code>.
       *  For more information, see the iomanip header.
      */
      __istream_type&
      operator>>(__istream_type& (*__pf)(__istream_type&))
      { return __pf(*this); }
 
      __istream_type&
      operator>>(__ios_type& (*__pf)(__ios_type&))
      {
        __pf(*this);
        return *this;
      }
 
      __istream_type&
      operator>>(ios_base& (*__pf)(ios_base&))
      {
        __pf(*this);
        return *this;
      }
      ///@}
 
      ///@{
      /**
       *  @name Extractors
       *
       *  All the @c operator>> functions (aka <em>formatted input
       *  functions</em>) have some common behavior.  Each starts by
       *  constructing a temporary object of type std::basic_istream::sentry
       *  with the second argument (noskipws) set to false.  This has several
       *  effects, concluding with the setting of a status flag; see the
       *  sentry documentation for more.
       *
       *  If the sentry status is good, the function tries to extract
       *  whatever data is appropriate for the type of the argument.
       *
       *  If an exception is thrown during extraction, ios_base::badbit
       *  will be turned on in the stream's error state (without causing an
       *  ios_base::failure to be thrown) and the original exception will
       *  be rethrown if badbit is set in the exceptions mask.
      */
 
      ///@{
      /**
       *  @brief  Integer arithmetic extractors
       *  @param  __n A variable of builtin integral type.
       *  @return  @c *this if successful
       *
       *  These functions use the stream's current locale (specifically, the
       *  @c num_get facet) to parse the input data.
      */
      __istream_type&
      operator>>(bool& __n)
      { return _M_extract(__n); }
 
      __istream_type&
      operator>>(short& __n);
 
      __istream_type&
      operator>>(unsigned short& __n)
      { return _M_extract(__n); }
 
      __istream_type&
      operator>>(int& __n);
 
      __istream_type&
      operator>>(unsigned int& __n)
      { return _M_extract(__n); }
 
      __istream_type&
      operator>>(long& __n)
      { return _M_extract(__n); }
 
      __istream_type&
      operator>>(unsigned long& __n)
      { return _M_extract(__n); }
 
#ifdef _GLIBCXX_USE_LONG_LONG
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wlong-long"
      __istream_type&
      operator>>(long long& __n)
      { return _M_extract(__n); }
 
      __istream_type&
      operator>>(unsigned long long& __n)
      { return _M_extract(__n); }
#pragma GCC diagnostic pop
#endif
      ///@}
 
      ///@{
      /**
       *  @brief  Floating point arithmetic extractors
       *  @param  __f A variable of builtin floating point type.
       *  @return  @c *this if successful
       *
       *  These functions use the stream's current locale (specifically, the
       *  @c num_get facet) to parse the input data.
      */
      __istream_type&
      operator>>(float& __f)
      { return _M_extract(__f); }
 
      __istream_type&
      operator>>(double& __f)
      { return _M_extract(__f); }
 
      __istream_type&
      operator>>(long double& __f)
      { return _M_extract(__f); }
      ///@}
 
#if defined(__STDCPP_FLOAT16_T__) && defined(_GLIBCXX_FLOAT_IS_IEEE_BINARY32)
      __attribute__((__always_inline__))
      __istream_type&
      operator>>(_Float16& __f)
      {
        float __flt;
        __istream_type& __ret = _M_extract(__flt);
        ios_base::iostate __err = ios_base::goodbit;
        if (__flt < -__FLT16_MAX__)
          {
            __f = -__FLT16_MAX__;
            __err = ios_base::failbit;
          }
        else if (__flt > __FLT16_MAX__)
          {
            __f = __FLT16_MAX__;
            __err = ios_base::failbit;
          }
        else
          __f = static_cast<_Float16>(__flt);
        if (__err)
          this->setstate(__err);
        return __ret;
      }
#endif
 
#if defined(__STDCPP_FLOAT32_T__) && defined(_GLIBCXX_FLOAT_IS_IEEE_BINARY32)
      __attribute__((__always_inline__))
      __istream_type&
      operator>>(_Float32& __f)
      {
        float __flt;
        __istream_type& __ret = _M_extract(__flt);
        __f = static_cast<_Float32> (__flt);
        return __ret;
      }
#endif
 
#if defined(__STDCPP_FLOAT64_T__) && defined(_GLIBCXX_DOUBLE_IS_IEEE_BINARY64)
      __attribute__((__always_inline__))
      __istream_type&
      operator>>(_Float64& __f)
      {
        double __dbl;
        __istream_type& __ret = _M_extract(__dbl);
        __f = static_cast<_Float64> (__dbl);
        return __ret;
      }
#endif
 
#if defined(__STDCPP_FLOAT128_T__) && defined(_GLIBCXX_LDOUBLE_IS_IEEE_BINARY128)
      __attribute__((__always_inline__))
      __istream_type&
      operator>>(_Float128& __f)
      {
        long double __ldbl;
        __istream_type& __ret = _M_extract(__ldbl);
        __f = static_cast<_Float128> (__ldbl);
        return __ret;
      }
#endif
 
#if defined(__STDCPP_BFLOAT16_T__) && defined(_GLIBCXX_FLOAT_IS_IEEE_BINARY32)
      __attribute__((__always_inline__))
      __istream_type&
      operator>>(__gnu_cxx::__bfloat16_t & __f)
      {
        float __flt;
        __istream_type& __ret = _M_extract(__flt);
        ios_base::iostate __err = ios_base::goodbit;
        if (__flt < -__BFLT16_MAX__)
          {
            __f = -__BFLT16_MAX__;
            __err = ios_base::failbit;
          }
        else if (__flt > __BFLT16_MAX__)
          {
            __f = __BFLT16_MAX__;
            __err = ios_base::failbit;
          }
        else
          __f = static_cast<__gnu_cxx::__bfloat16_t>(__flt);
        if (__err)
          this->setstate(__err);
        return __ret;
      }
#endif
 
      /**
       *  @brief  Basic arithmetic extractors
       *  @param  __p A variable of pointer type.
       *  @return  @c *this if successful
       *
       *  These functions use the stream's current locale (specifically, the
       *  @c num_get facet) to parse the input data.
      */
      __istream_type&
      operator>>(void*& __p)
      { return _M_extract(__p); }
 
      /**
       *  @brief  Extracting into another streambuf.
       *  @param  __sb  A pointer to a streambuf
       *
       *  This function behaves like one of the basic arithmetic extractors,
       *  in that it also constructs a sentry object and has the same error
       *  handling behavior.
       *
       *  If @p __sb is NULL, the stream will set failbit in its error state.
       *
       *  Characters are extracted from this stream and inserted into the
       *  @p __sb streambuf until one of the following occurs:
       *
       *  - the input stream reaches end-of-file,
       *  - insertion into the output buffer fails (in this case, the
       *    character that would have been inserted is not extracted), or
       *  - an exception occurs (and in this case is caught)
       *
       *  If the function inserts no characters, failbit is set.
      */
      __istream_type&
      operator>>(__streambuf_type* __sb);
      ///@}
 
      // [27.6.1.3] unformatted input
      /**
       *  @brief  Character counting
       *  @return  The number of characters extracted by the previous
       *           unformatted input function dispatched for this stream.
      */
      streamsize
      gcount() const
      { return _M_gcount; }
 
      ///@{
      /**
       *  @name Unformatted Input Functions
       *
       *  All the unformatted input functions have some common behavior.
       *  Each starts by constructing a temporary object of type
       *  std::basic_istream::sentry with the second argument (noskipws)
       *  set to true.  This has several effects, concluding with the
       *  setting of a status flag; see the sentry documentation for more.
       *
       *  If the sentry status is good, the function tries to extract
       *  whatever data is appropriate for the type of the argument.
       *
       *  The number of characters extracted is stored for later retrieval
       *  by gcount().
       *
       *  If an exception is thrown during extraction, ios_base::badbit
       *  will be turned on in the stream's error state (without causing an
       *  ios_base::failure to be thrown) and the original exception will
       *  be rethrown if badbit is set in the exceptions mask.
      */
 
      /**
       *  @brief  Simple extraction.
       *  @return  A character, or eof().
       *
       *  Tries to extract a character.  If none are available, sets failbit
       *  and returns traits::eof().
      */
      int_type
      get();
 
      /**
       *  @brief  Simple extraction.
       *  @param  __c  The character in which to store data.
       *  @return  *this
       *
       *  Tries to extract a character and store it in @a __c.  If none are
       *  available, sets failbit and returns traits::eof().
       *
       *  @note  This function is not overloaded on signed char and
       *         unsigned char.
      */
      __istream_type&
      get(char_type& __c);
 
      /**
       *  @brief  Simple multiple-character extraction.
       *  @param  __s  Pointer to an array.
       *  @param  __n  Maximum number of characters to store in @a __s.
       *  @param  __delim  A "stop" character.
       *  @return  *this
       *
       *  Characters are extracted and stored into @a __s until one of the
       *  following happens:
       *
       *  - @c __n-1 characters are stored
       *  - the input sequence reaches EOF
       *  - the next character equals @a __delim, in which case the character
       *    is not extracted
       *
       * If no characters are stored, failbit is set in the stream's error
       * state.
       *
       * In any case, a null character is stored into the next location in
       * the array.
       *
       *  @note  This function is not overloaded on signed char and
       *         unsigned char.
      */
      __istream_type&
      get(char_type* __s, streamsize __n, char_type __delim);
 
      /**
       *  @brief  Simple multiple-character extraction.
       *  @param  __s  Pointer to an array.
       *  @param  __n  Maximum number of characters to store in @a s.
       *  @return  *this
       *
       *  Returns @c get(__s,__n,widen(&apos;\\n&apos;)).
      */
      __istream_type&
      get(char_type* __s, streamsize __n)
      { return this->get(__s, __n, this->widen('\n')); }
 
      /**
       *  @brief  Extraction into another streambuf.
       *  @param  __sb  A streambuf in which to store data.
       *  @param  __delim  A "stop" character.
       *  @return  *this
       *
       *  Characters are extracted and inserted into @a __sb until one of the
       *  following happens:
       *
       *  - the input sequence reaches EOF
       *  - insertion into the output buffer fails (in this case, the
       *    character that would have been inserted is not extracted)
       *  - the next character equals @a __delim (in this case, the character
       *    is not extracted)
       *  - an exception occurs (and in this case is caught)
       *
       * If no characters are stored, failbit is set in the stream's error
       * state.
      */
      __istream_type&
      get(__streambuf_type& __sb, char_type __delim);
 
      /**
       *  @brief  Extraction into another streambuf.
       *  @param  __sb  A streambuf in which to store data.
       *  @return  *this
       *
       *  Returns @c get(__sb,widen(&apos;\\n&apos;)).
      */
      __istream_type&
      get(__streambuf_type& __sb)
      { return this->get(__sb, this->widen('\n')); }
 
      /**
       *  @brief  String extraction.
       *  @param  __s  A character array in which to store the data.
       *  @param  __n  Maximum number of characters to extract.
       *  @param  __delim  A "stop" character.
       *  @return  *this
       *
       *  Extracts and stores characters into @a __s until one of the
       *  following happens.  Note that these criteria are required to be
       *  tested in the order listed here, to allow an input line to exactly
       *  fill the @a __s array without setting failbit.
       *
       *  -# the input sequence reaches end-of-file, in which case eofbit
       *     is set in the stream error state
       *  -# the next character equals @c __delim, in which case the character
       *     is extracted (and therefore counted in @c gcount()) but not stored
       *  -# @c __n-1 characters are stored, in which case failbit is set
       *     in the stream error state
       *
       *  If no characters are extracted, failbit is set.  (An empty line of
       *  input should therefore not cause failbit to be set.)
       *
       *  In any case, a null character is stored in the next location in
       *  the array.
      */
      __istream_type&
      getline(char_type* __s, streamsize __n, char_type __delim);
 
      /**
       *  @brief  String extraction.
       *  @param  __s  A character array in which to store the data.
       *  @param  __n  Maximum number of characters to extract.
       *  @return  *this
       *
       *  Returns @c getline(__s,__n,widen(&apos;\\n&apos;)).
      */
      __istream_type&
      getline(char_type* __s, streamsize __n)
      { return this->getline(__s, __n, this->widen('\n')); }
 
      /**
       *  @brief  Discarding characters
       *  @param  __n  Number of characters to discard.
       *  @param  __delim  A "stop" character.
       *  @return  *this
       *
       *  Extracts characters and throws them away until one of the
       *  following happens:
       *  - if @a __n @c != @c std::numeric_limits<int>::max(), @a __n
       *    characters are extracted
       *  - the input sequence reaches end-of-file
       *  - the next character equals @a __delim (in this case, the character
       *    is extracted); note that this condition will never occur if
       *    @a __delim equals @c traits::eof().
       *
       *  NB: Provide three overloads, instead of the single function
       *  (with defaults) mandated by the Standard: this leads to a
       *  better performing implementation, while still conforming to
       *  the Standard.
      */
      __istream_type&
      ignore(streamsize __n, int_type __delim);
 
      __istream_type&
      ignore(streamsize __n);
 
      __istream_type&
      ignore();
 
      /**
       *  @brief  Looking ahead in the stream
       *  @return  The next character, or eof().
       *
       *  If, after constructing the sentry object, @c good() is false,
       *  returns @c traits::eof().  Otherwise reads but does not extract
       *  the next input character.
      */
      int_type
      peek();
 
      /**
       *  @brief  Extraction without delimiters.
       *  @param  __s  A character array.
       *  @param  __n  Maximum number of characters to store.
       *  @return  *this
       *
       *  If the stream state is @c good(), extracts characters and stores
       *  them into @a __s until one of the following happens:
       *  - @a __n characters are stored
       *  - the input sequence reaches end-of-file, in which case the error
       *    state is set to @c failbit|eofbit.
       *
       *  @note  This function is not overloaded on signed char and
       *         unsigned char.
      */
      __istream_type&
      read(char_type* __s, streamsize __n);
 
      /**
       *  @brief  Extraction until the buffer is exhausted, but no more.
       *  @param  __s  A character array.
       *  @param  __n  Maximum number of characters to store.
       *  @return  The number of characters extracted.
       *
       *  Extracts characters and stores them into @a __s depending on the
       *  number of characters remaining in the streambuf's buffer,
       *  @c rdbuf()->in_avail(), called @c A here:
       *  - if @c A @c == @c -1, sets eofbit and extracts no characters
       *  - if @c A @c == @c 0, extracts no characters
       *  - if @c A @c > @c 0, extracts @c min(A,n)
       *
       *  The goal is to empty the current buffer, and to not request any
       *  more from the external input sequence controlled by the streambuf.
      */
      streamsize
      readsome(char_type* __s, streamsize __n);
 
      /**
       *  @brief  Unextracting a single character.
       *  @param  __c  The character to push back into the input stream.
       *  @return  *this
       *
       *  If @c rdbuf() is not null, calls @c rdbuf()->sputbackc(c).
       *
       *  If @c rdbuf() is null or if @c sputbackc() fails, sets badbit in
       *  the error state.
       *
       *  @note  This function first clears eofbit.  Since no characters
       *         are extracted, the next call to @c gcount() will return 0,
       *         as required by DR 60.
      */
      __istream_type&
      putback(char_type __c);
 
      /**
       *  @brief  Unextracting the previous character.
       *  @return  *this
       *
       *  If @c rdbuf() is not null, calls @c rdbuf()->sungetc(c).
       *
       *  If @c rdbuf() is null or if @c sungetc() fails, sets badbit in
       *  the error state.
       *
       *  @note  This function first clears eofbit.  Since no characters
       *         are extracted, the next call to @c gcount() will return 0,
       *         as required by DR 60.
      */
      __istream_type&
      unget();
 
      /**
       *  @brief  Synchronizing the stream buffer.
       *  @return  0 on success, -1 on failure
       *
       *  If @c rdbuf() is a null pointer, returns -1.
       *
       *  Otherwise, calls @c rdbuf()->pubsync(), and if that returns -1,
       *  sets badbit and returns -1.
       *
       *  Otherwise, returns 0.
       *
       *  @note  This function does not count the number of characters
       *         extracted, if any, and therefore does not affect the next
       *         call to @c gcount().
      */
      int
      sync();
 
      /**
       *  @brief  Getting the current read position.
       *  @return  A file position object.
       *
       *  If @c fail() is not false, returns @c pos_type(-1) to indicate
       *  failure.  Otherwise returns @c rdbuf()->pubseekoff(0,cur,in).
       *
       *  @note  This function does not count the number of characters
       *         extracted, if any, and therefore does not affect the next
       *         call to @c gcount().  At variance with putback, unget and
       *         seekg, eofbit is not cleared first.
      */
      pos_type
      tellg();
 
      /**
       *  @brief  Changing the current read position.
       *  @param  __pos  A file position object.
       *  @return  *this
       *
       *  If @c fail() is not true, calls @c rdbuf()->pubseekpos(__pos).  If
       *  that function fails, sets failbit.
       *
       *  @note  This function first clears eofbit.  It does not count the
       *         number of characters extracted, if any, and therefore does
       *         not affect the next call to @c gcount().
      */
      __istream_type&
      seekg(pos_type);
 
      /**
       *  @brief  Changing the current read position.
       *  @param  __off  A file offset object.
       *  @param  __dir  The direction in which to seek.
       *  @return  *this
       *
       *  If @c fail() is not true, calls @c rdbuf()->pubseekoff(__off,__dir).
       *  If that function fails, sets failbit.
       *
       *  @note  This function first clears eofbit.  It does not count the
       *         number of characters extracted, if any, and therefore does
       *         not affect the next call to @c gcount().
      */
      __istream_type&
      seekg(off_type, ios_base::seekdir);
      ///@}
 
    protected:
      basic_istream()
      : _M_gcount(streamsize(0))
      { this->init(0); }
 
#if __cplusplus >= 201103L
      basic_istream(const basic_istream&) = delete;
 
      basic_istream(basic_istream&& __rhs)
      : __ios_type(), _M_gcount(__rhs._M_gcount)
      {
        __ios_type::move(__rhs);
        __rhs._M_gcount = 0;
      }
 
      // 27.7.3.3 Assign/swap
 
      basic_istream& operator=(const basic_istream&) = delete;
 
      basic_istream&
      operator=(basic_istream&& __rhs)
      {
        swap(__rhs);
        return *this;
      }
 
      void
      swap(basic_istream& __rhs)
      {
        __ios_type::swap(__rhs);
        std::swap(_M_gcount, __rhs._M_gcount);
      }
#endif
 
      template<typename _ValueT>
        __istream_type&
        _M_extract(_ValueT& __v);
    };
 
  /// Explicit specialization declarations, defined in src/istream.cc.
  template<>
    basic_istream<char>&
    basic_istream<char>::
    getline(char_type* __s, streamsize __n, char_type __delim);
 
  template<>
    basic_istream<char>&
    basic_istream<char>::
    ignore(streamsize __n);
 
  template<>
    basic_istream<char>&
    basic_istream<char>::
    ignore(streamsize __n, int_type __delim);
 
#ifdef _GLIBCXX_USE_WCHAR_T
  template<>
    basic_istream<wchar_t>&
    basic_istream<wchar_t>::
    getline(char_type* __s, streamsize __n, char_type __delim);
 
  template<>
    basic_istream<wchar_t>&
    basic_istream<wchar_t>::
    ignore(streamsize __n);
 
  template<>
    basic_istream<wchar_t>&
    basic_istream<wchar_t>::
    ignore(streamsize __n, int_type __delim);
#endif
 
  /**
   *  @brief  Performs setup work for input streams.
   *
   *  Objects of this class are created before all of the standard
   *  extractors are run.  It is responsible for <em>exception-safe
   *  prefix and suffix operations,</em> although only prefix actions
   *  are currently required by the standard.
  */
  template<typename _CharT, typename _Traits>
    class basic_istream<_CharT, _Traits>::sentry
    {
      // Data Members.
      bool _M_ok;
 
    public:
      /// Easy access to dependent types.
      typedef _Traits                                   traits_type;
      typedef basic_streambuf<_CharT, _Traits>          __streambuf_type;
      typedef basic_istream<_CharT, _Traits>            __istream_type;
      typedef typename __istream_type::__ctype_type     __ctype_type;
      typedef typename _Traits::int_type                __int_type;
 
      /**
       *  @brief  The constructor performs all the work.
       *  @param  __is  The input stream to guard.
       *  @param  __noskipws  Whether to consume whitespace or not.
       *
       *  If the stream state is good (@a __is.good() is true), then the
       *  following actions are performed, otherwise the sentry state
       *  is false (<em>not okay</em>) and failbit is set in the
       *  stream state.
       *
       *  The sentry's preparatory actions are:
       *
       *  -# if the stream is tied to an output stream, @c is.tie()->flush()
       *     is called to synchronize the output sequence
       *  -# if @a __noskipws is false, and @c ios_base::skipws is set in
       *     @c is.flags(), the sentry extracts and discards whitespace
       *     characters from the stream.  The currently imbued locale is
       *     used to determine whether each character is whitespace.
       *
       *  If the stream state is still good, then the sentry state becomes
       *  true (@a okay).
      */
      explicit
      sentry(basic_istream<_CharT, _Traits>& __is, bool __noskipws = false);
 
      /**
       *  @brief  Quick status checking.
       *  @return  The sentry state.
       *
       *  For ease of use, sentries may be converted to booleans.  The
       *  return value is that of the sentry state (true == okay).
      */
#if __cplusplus >= 201103L
      explicit
#endif
      operator bool() const
      { return _M_ok; }
    };
 
  ///@{
  /**
   *  @brief  Character extractors
   *  @param  __in  An input stream.
   *  @param  __c  A character reference.
   *  @return  in
   *
   *  Behaves like one of the formatted arithmetic extractors described in
   *  std::basic_istream.  After constructing a sentry object with good
   *  status, this function extracts a character (if one is available) and
   *  stores it in @a __c.  Otherwise, sets failbit in the input stream.
  */
  template<typename _CharT, typename _Traits>
    basic_istream<_CharT, _Traits>&
    operator>>(basic_istream<_CharT, _Traits>& __in, _CharT& __c);
 
  template<class _Traits>
    inline basic_istream<char, _Traits>&
    operator>>(basic_istream<char, _Traits>& __in, unsigned char& __c)
    { return (__in >> reinterpret_cast<char&>(__c)); }
 
  template<class _Traits>
    inline basic_istream<char, _Traits>&
    operator>>(basic_istream<char, _Traits>& __in, signed char& __c)
    { return (__in >> reinterpret_cast<char&>(__c)); }
  ///@}
 
 
  template<typename _CharT, typename _Traits>
    void
    __istream_extract(basic_istream<_CharT, _Traits>&, _CharT*, streamsize);
 
  void __istream_extract(istream&, char*, streamsize);
 
  ///@{
  /**
   *  @brief  Character string extractors
   *  @param  __in  An input stream.
   *  @param  __s  A character array (or a pointer to an array before C++20).
   *  @return  __in
   *
   *  Behaves like one of the formatted arithmetic extractors described in
   *  `std::basic_istream`.  After constructing a sentry object with good
   *  status, this function extracts up to `n` characters and stores them
   *  into the array `__s`.  `n` is defined as:
   *
   *  - if `width()` is greater than zero, `n` is `min(width(), n)`
   *  - otherwise `n` is the number of elements of the array
   *  - (before C++20 the pointer is assumed to point to an array of
   *    the largest possible size for an array of `char_type`).
   *
   *  Characters are extracted and stored until one of the following happens:
   *  - `n - 1` characters are stored
   *  - EOF is reached
   *  - the next character is whitespace according to the current locale
   *
   *  `width(0)` is then called for the input stream.
   *
   *  If no characters are extracted, sets failbit.
  */
 
#if __cplusplus <= 201703L
  template<typename _CharT, typename _Traits>
    __attribute__((__nonnull__(2), __access__(__write_only__, 2)))
    inline basic_istream<_CharT, _Traits>&
    operator>>(basic_istream<_CharT, _Traits>& __in, _CharT* __s)
    {
#ifdef __OPTIMIZE__
      // Function inlining might make the buffer size known, allowing us to
      // prevent overflow.
      size_t __n = __builtin_object_size(__s, 0);
      if (__n < sizeof(_CharT))
        {
          // There is not even space for the required null terminator.
          __glibcxx_assert(__n >= sizeof(_CharT));
          // No point calling __istream_extract, but still need to reset width.
          __in.width(0);
          __in.setstate(ios_base::failbit);
        }
      else if (__n != (size_t)-1)
        {
          __n /= sizeof(_CharT);
          streamsize __w = __in.width();
          std::__istream_extract(__in, __s, __n);
          if (__in.good() && (__w <= 0 || __n < (size_t)__w))
            {
              // Stopped extracting early to avoid overflowing the buffer,
              // but might have stopped anyway (and set eofbit) if at EOF.
              const typename _Traits::int_type __c = __in.rdbuf()->sgetc();
              const bool __eof = _Traits::eq_int_type(__c, _Traits::eof());
              if (__builtin_expect(__eof, true)) // Assume EOF, not overflow.
                __in.setstate(ios_base::eofbit);
            }
        }
      else
#endif // __OPTIMIZE
        {
          // Buffer size is unknown, have to assume it's huge.
          streamsize __n = __gnu_cxx::__numeric_traits<streamsize>::__max;
          __n /= sizeof(_CharT);
          std::__istream_extract(__in, __s, __n);
        }
      return __in;
    }
 
  template<class _Traits>
    __attribute__((__nonnull__(2), __access__(__write_only__, 2)))
    inline basic_istream<char, _Traits>&
    operator>>(basic_istream<char, _Traits>& __in, unsigned char* __s)
    { return __in >> reinterpret_cast<char*>(__s); }
 
  template<class _Traits>
    __attribute__((__nonnull__(2), __access__(__write_only__, 2)))
    inline basic_istream<char, _Traits>&
    operator>>(basic_istream<char, _Traits>& __in, signed char* __s)
    { return __in >> reinterpret_cast<char*>(__s); }
#else
  // _GLIBCXX_RESOLVE_LIB_DEFECTS
  // 2499. operator>>(istream&, char*) makes it hard to avoid buffer overflows
  template<typename _CharT, typename _Traits, size_t _Num>
    inline basic_istream<_CharT, _Traits>&
    operator>>(basic_istream<_CharT, _Traits>& __in, _CharT (&__s)[_Num])
    {
      static_assert(_Num <= __gnu_cxx::__numeric_traits<streamsize>::__max);
      std::__istream_extract(__in, __s, _Num);
      return __in;
    }
 
  template<class _Traits, size_t _Num>
    inline basic_istream<char, _Traits>&
    operator>>(basic_istream<char, _Traits>& __in, unsigned char (&__s)[_Num])
    { return __in >> reinterpret_cast<char(&)[_Num]>(__s); }
 
  template<class _Traits, size_t _Num>
    inline basic_istream<char, _Traits>&
    operator>>(basic_istream<char, _Traits>& __in, signed char (&__s)[_Num])
    { return __in >> reinterpret_cast<char(&)[_Num]>(__s); }
#endif
  ///@}
 
  /**
   *  @brief  Template class basic_iostream
   *  @ingroup io
   *
   *  @tparam _CharT  Type of character stream.
   *  @tparam _Traits  Traits for character type, defaults to
   *                   char_traits<_CharT>.
   *
   *  This class multiply inherits from the input and output stream classes
   *  simply to provide a single interface.
  */
  template<typename _CharT, typename _Traits>
    class basic_iostream
    : public basic_istream<_CharT, _Traits>,
      public basic_ostream<_CharT, _Traits>
    {
    public:
      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 271. basic_iostream missing typedefs
      // Types (inherited):
      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;
 
      // Non-standard Types:
      typedef basic_istream<_CharT, _Traits>            __istream_type;
      typedef basic_ostream<_CharT, _Traits>            __ostream_type;
 
      /**
       *  @brief  Constructor does nothing.
       *
       *  Both of the parent classes are initialized with the same
       *  streambuf pointer passed to this constructor.
      */
      explicit
      basic_iostream(basic_streambuf<_CharT, _Traits>* __sb)
      : __istream_type(__sb), __ostream_type(__sb) { }
 
      /**
       *  @brief  Destructor does nothing.
      */
      virtual
      ~basic_iostream() { }
 
    protected:
      basic_iostream()
      : __istream_type(), __ostream_type() { }
 
#if __cplusplus >= 201103L
      basic_iostream(const basic_iostream&) = delete;
 
      basic_iostream(basic_iostream&& __rhs)
      : __istream_type(std::move(__rhs)), __ostream_type(*this)
      { }
 
      // 27.7.3.3 Assign/swap
 
      basic_iostream& operator=(const basic_iostream&) = delete;
 
      basic_iostream&
      operator=(basic_iostream&& __rhs)
      {
        swap(__rhs);
        return *this;
      }
 
      void
      swap(basic_iostream& __rhs)
      { __istream_type::swap(__rhs); }
#endif
    };
 
  /**
   *  @brief  Quick and easy way to eat whitespace
   *
   *  This manipulator extracts whitespace characters, stopping when the
   *  next character is non-whitespace, or when the input sequence is empty.
   *  If the sequence is empty, @c eofbit is set in the stream, but not
   *  @c failbit.
   *
   *  The current locale is used to distinguish whitespace characters.
   *
   *  Example:
   *  @code
   *     MyClass   mc;
   *
   *     std::cin >> std::ws >> mc;
   *  @endcode
   *  will skip leading whitespace before calling operator>> on cin and your
   *  object.  Note that the same effect can be achieved by creating a
   *  std::basic_istream::sentry inside your definition of operator>>.
  */
  template<typename _CharT, typename _Traits>
    basic_istream<_CharT, _Traits>&
    ws(basic_istream<_CharT, _Traits>& __is);
 
#if __cplusplus >= 201103L
  // C++11 27.7.2.6 Rvalue stream extraction [istream.rvalue]
  // _GLIBCXX_RESOLVE_LIB_DEFECTS
  // 2328. Rvalue stream extraction should use perfect forwarding
  // 1203. More useful rvalue stream insertion
 
#if __cpp_concepts >= 201907L && __glibcxx_type_trait_variable_templates
  template<typename _Is, typename _Tp>
    requires __derived_from_ios_base<_Is>
      && requires (_Is& __is, _Tp&& __t) { __is >> std::forward<_Tp>(__t); }
    using __rvalue_stream_extraction_t = _Is&&;
#else
  template<typename _Is, typename _Tp,
           typename = _Require_derived_from_ios_base<_Is>,
           typename = decltype(std::declval<_Is&>() >> std::declval<_Tp>())>
    using __rvalue_stream_extraction_t = _Is&&;
#endif
 
  /**
   *  @brief  Generic extractor for rvalue stream
   *  @param  __is  An input stream.
   *  @param  __x  A reference to the extraction target.
   *  @return  __is
   *
   *  This is just a forwarding function to allow extraction from
   *  rvalue streams since they won't bind to the extractor functions
   *  that take an lvalue reference.
  */
  template<typename _Istream, typename _Tp>
    inline __rvalue_stream_extraction_t<_Istream, _Tp>
    operator>>(_Istream&& __is, _Tp&& __x)
    {
      __is >> std::forward<_Tp>(__x);
      return std::move(__is);
    }
#endif // C++11
 
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace
 
#include <bits/istream.tcc>
 
#endif  /* _GLIBCXX_ISTREAM */