libstdc++-v3/include/std/streambuf

   1 // Stream buffer classes -*- C++ -*-
   2
   3 // Copyright (C) 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005,
   4 // 2006, 2007, 2008 Free Software Foundation, Inc.
   5 //
   6 // This file is part of the GNU ISO C++ Library.  This library is free
   7 // software; you can redistribute it and/or modify it under the
   8 // terms of the GNU General Public License as published by the
   9 // Free Software Foundation; either version 2, or (at your option)
  10 // any later version.
  11
  12 // This library is distributed in the hope that it will be useful,
  13 // but WITHOUT ANY WARRANTY; without even the implied warranty of
  14 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15 // GNU General Public License for more details.
  16
  17 // You should have received a copy of the GNU General Public License
  18 // along with this library; see the file COPYING.  If not, write to
  19 // the Free Software Foundation, 51 Franklin Street, Fifth Floor,
  20 // Boston, MA 02110-1301, USA.
  21
  22 // As a special exception, you may use this file as part of a free software
  23 // library without restriction.  Specifically, if other files instantiate
  24 // templates or use macros or inline functions from this file, or you compile
  25 // this file and link it with other files to produce an executable, this
  26 // file does not by itself cause the resulting executable to be covered by
  27 // the GNU General Public License.  This exception does not however
  28 // invalidate any other reasons why the executable file might be covered by
  29 // the GNU General Public License.
  30
  31 /** @file streambuf
  32  *  This is a Standard C++ Library header.
  33  */
  34
  35 //
  36 // ISO C++ 14882: 27.5  Stream buffers
  37 //
  38
  39 #ifndef _GLIBXX_STREAMBUF
  40 #define _GLIBXX_STREAMBUF 1
  41
  42 #pragma GCC system_header
  43
  44 #include <bits/c++config.h>
  45 #include <iosfwd>
  46 #include <bits/localefwd.h>
  47 #include <bits/ios_base.h>
  48 #include <bits/cpp_type_traits.h>
  49 #include <ext/type_traits.h>
  50
  51 _GLIBCXX_BEGIN_NAMESPACE(std)
  52
  53   template<typename _CharT, typename _Traits>
  54     streamsize
  55     __copy_streambufs_eof(basic_streambuf<_CharT, _Traits>*,
  56                           basic_streambuf<_CharT, _Traits>*, bool&);
  57
  58   /**
  59    *  @brief  The actual work of input and output (interface).
  60    *
  61    *  This is a base class.  Derived stream buffers each control a
  62    *  pair of character sequences:  one for input, and one for output.
  63    *
  64    *  Section [27.5.1] of the standard describes the requirements and
  65    *  behavior of stream buffer classes.  That section (three paragraphs)
  66    *  is reproduced here, for simplicity and accuracy.
  67    *
  68    *  -# Stream buffers can impose various constraints on the sequences
  69    *     they control.  Some constraints are:
  70    *     - The controlled input sequence can be not readable.
  71    *     - The controlled output sequence can be not writable.
  72    *     - The controlled sequences can be associated with the contents of
  73    *       other representations for character sequences, such as external
  74    *       files.
  75    *     - The controlled sequences can support operations @e directly to or
  76    *       from associated sequences.
  77    *     - The controlled sequences can impose limitations on how the
  78    *       program can read characters from a sequence, write characters to
  79    *       a sequence, put characters back into an input sequence, or alter
  80    *       the stream position.
  81    *     .
  82    *  -# Each sequence is characterized by three pointers which, if non-null,
  83    *     all point into the same @c charT array object.  The array object
  84    *     represents, at any moment, a (sub)sequence of characters from the
  85    *     sequence.  Operations performed on a sequence alter the values
  86    *     stored in these pointers, perform reads and writes directly to or
  87    *     from associated sequences, and alter "the stream position" and
  88    *     conversion state as needed to maintain this subsequence relationship.
  89    *     The three pointers are:
  90    *     - the <em>beginning pointer</em>, or lowest element address in the
  91    *       array (called @e xbeg here);
  92    *     - the <em>next pointer</em>, or next element address that is a
  93    *       current candidate for reading or writing (called @e xnext here);
  94    *     - the <em>end pointer</em>, or first element address beyond the
  95    *       end of the array (called @e xend here).
  96    *     .
  97    *  -# The following semantic constraints shall always apply for any set
  98    *     of three pointers for a sequence, using the pointer names given
  99    *     immediately above:
 100    *     - If @e xnext is not a null pointer, then @e xbeg and @e xend shall
 101    *       also be non-null pointers into the same @c charT array, as
 102    *       described above; otherwise, @e xbeg and @e xend shall also be null.
 103    *     - If @e xnext is not a null pointer and @e xnext < @e xend for an
 104    *       output sequence, then a <em>write position</em> is available.
 105    *       In this case, @e *xnext shall be assignable as the next element
 106    *       to write (to put, or to store a character value, into the sequence).
 107    *     - If @e xnext is not a null pointer and @e xbeg < @e xnext for an
 108    *       input sequence, then a <em>putback position</em> is available.
 109    *       In this case, @e xnext[-1] shall have a defined value and is the
 110    *       next (preceding) element to store a character that is put back
 111    *       into the input sequence.
 112    *     - If @e xnext is not a null pointer and @e xnext< @e xend for an
 113    *       input sequence, then a <em>read position</em> is available.
 114    *       In this case, @e *xnext shall have a defined value and is the
 115    *       next element to read (to get, or to obtain a character value,
 116    *       from the sequence).
 117   */
 118   template<typename _CharT, typename _Traits>
 119     class basic_streambuf
 120     {
 121     public:
 122       //@{
 123       /**
 124        *  These are standard types.  They permit a standardized way of
 125        *  referring to names of (or names dependant on) the template
 126        *  parameters, which are specific to the implementation.
 127       */
 128       typedef _CharT                                    char_type;
 129       typedef _Traits                                   traits_type;
 130       typedef typename traits_type::int_type            int_type;
 131       typedef typename traits_type::pos_type            pos_type;
 132       typedef typename traits_type::off_type            off_type;
 133       //@}
 134
 135       //@{
 136       /// This is a non-standard type.
 137       typedef basic_streambuf<char_type, traits_type>   __streambuf_type;
 138       //@}
 139
 140       friend class basic_ios<char_type, traits_type>;
 141       friend class basic_istream<char_type, traits_type>;
 142       friend class basic_ostream<char_type, traits_type>;
 143       friend class istreambuf_iterator<char_type, traits_type>;
 144       friend class ostreambuf_iterator<char_type, traits_type>;
 145
 146       friend streamsize
 147       __copy_streambufs_eof<>(__streambuf_type*, __streambuf_type*, bool&);
 148
 149       template<bool _IsMove, typename _CharT2>
 150         friend typename __gnu_cxx::__enable_if<__is_char<_CharT2>::__value,
 151                                                _CharT2*>::__type
 152         __copy_move_a2(istreambuf_iterator<_CharT2>,
 153                        istreambuf_iterator<_CharT2>, _CharT2*);
 154
 155       template<typename _CharT2>
 156         friend typename __gnu_cxx::__enable_if<__is_char<_CharT2>::__value,
 157                                   istreambuf_iterator<_CharT2> >::__type
 158         find(istreambuf_iterator<_CharT2>, istreambuf_iterator<_CharT2>,
 159              const _CharT2&);
 160
 161       template<typename _CharT2, typename _Traits2>
 162         friend basic_istream<_CharT2, _Traits2>&
 163         operator>>(basic_istream<_CharT2, _Traits2>&, _CharT2*);
 164
 165       template<typename _CharT2, typename _Traits2, typename _Alloc>
 166         friend basic_istream<_CharT2, _Traits2>&
 167         operator>>(basic_istream<_CharT2, _Traits2>&,
 168                    basic_string<_CharT2, _Traits2, _Alloc>&);
 169
 170       template<typename _CharT2, typename _Traits2, typename _Alloc>
 171         friend basic_istream<_CharT2, _Traits2>&
 172         getline(basic_istream<_CharT2, _Traits2>&,
 173                 basic_string<_CharT2, _Traits2, _Alloc>&, _CharT2);
 174
 175     protected:
 176       //@{
 177       /**
 178        *  This is based on _IO_FILE, just reordered to be more consistent,
 179        *  and is intended to be the most minimal abstraction for an
 180        *  internal buffer.
 181        *  -  get == input == read
 182        *  -  put == output == write
 183       */
 184       char_type*                _M_in_beg;     // Start of get area.
 185       char_type*                _M_in_cur;     // Current read area.
 186       char_type*                _M_in_end;     // End of get area.
 187       char_type*                _M_out_beg;    // Start of put area.
 188       char_type*                _M_out_cur;    // Current put area.
 189       char_type*                _M_out_end;    // End of put area.
 190
 191       /// Current locale setting.
 192       locale                    _M_buf_locale;
 193
 194   public:
 195       /// Destructor deallocates no buffer space.
 196       virtual
 197       ~basic_streambuf()
 198       { }
 199
 200       // [27.5.2.2.1] locales
 201       /**
 202        *  @brief  Entry point for imbue().
 203        *  @param  loc  The new locale.
 204        *  @return  The previous locale.
 205        *
 206        *  Calls the derived imbue(loc).
 207       */
 208       locale
 209       pubimbue(const locale &__loc)
 210       {
 211         locale __tmp(this->getloc());
 212         this->imbue(__loc);
 213         _M_buf_locale = __loc;
 214         return __tmp;
 215       }
 216
 217       /**
 218        *  @brief  Locale access.
 219        *  @return  The current locale in effect.
 220        *
 221        *  If pubimbue(loc) has been called, then the most recent @c loc
 222        *  is returned.  Otherwise the global locale in effect at the time
 223        *  of construction is returned.
 224       */
 225       locale
 226       getloc() const
 227       { return _M_buf_locale; }
 228
 229       // [27.5.2.2.2] buffer management and positioning
 230       //@{
 231       /**
 232        *  @brief  Entry points for derived buffer functions.
 233        *
 234        *  The public versions of @c pubfoo dispatch to the protected
 235        *  derived @c foo member functions, passing the arguments (if any)
 236        *  and returning the result unchanged.
 237       */
 238       __streambuf_type*
 239       pubsetbuf(char_type* __s, streamsize __n)
 240       { return this->setbuf(__s, __n); }
 241
 242       pos_type
 243       pubseekoff(off_type __off, ios_base::seekdir __way,
 244                  ios_base::openmode __mode = ios_base::in | ios_base::out)
 245       { return this->seekoff(__off, __way, __mode); }
 246
 247       pos_type
 248       pubseekpos(pos_type __sp,
 249                  ios_base::openmode __mode = ios_base::in | ios_base::out)
 250       { return this->seekpos(__sp, __mode); }
 251
 252       int
 253       pubsync() { return this->sync(); }
 254       //@}
 255
 256       // [27.5.2.2.3] get area
 257       /**
 258        *  @brief  Looking ahead into the stream.
 259        *  @return  The number of characters available.
 260        *
 261        *  If a read position is available, returns the number of characters
 262        *  available for reading before the buffer must be refilled.
 263        *  Otherwise returns the derived @c showmanyc().
 264       */
 265       streamsize
 266       in_avail()
 267       {
 268         const streamsize __ret = this->egptr() - this->gptr();
 269         return __ret ? __ret : this->showmanyc();
 270       }
 271
 272       /**
 273        *  @brief  Getting the next character.
 274        *  @return  The next character, or eof.
 275        *
 276        *  Calls @c sbumpc(), and if that function returns
 277        *  @c traits::eof(), so does this function.  Otherwise, @c sgetc().
 278       */
 279       int_type
 280       snextc()
 281       {
 282         int_type __ret = traits_type::eof();
 283         if (__builtin_expect(!traits_type::eq_int_type(this->sbumpc(),
 284                                                        __ret), true))
 285           __ret = this->sgetc();
 286         return __ret;
 287       }
 288
 289       /**
 290        *  @brief  Getting the next character.
 291        *  @return  The next character, or eof.
 292        *
 293        *  If the input read position is available, returns that character
 294        *  and increments the read pointer, otherwise calls and returns
 295        *  @c uflow().
 296       */
 297       int_type
 298       sbumpc()
 299       {
 300         int_type __ret;
 301         if (__builtin_expect(this->gptr() < this->egptr(), true))
 302           {
 303             __ret = traits_type::to_int_type(*this->gptr());
 304             this->gbump(1);
 305           }
 306         else
 307           __ret = this->uflow();
 308         return __ret;
 309       }
 310
 311       /**
 312        *  @brief  Getting the next character.
 313        *  @return  The next character, or eof.
 314        *
 315        *  If the input read position is available, returns that character,
 316        *  otherwise calls and returns @c underflow().  Does not move the
 317        *  read position after fetching the character.
 318       */
 319       int_type
 320       sgetc()
 321       {
 322         int_type __ret;
 323         if (__builtin_expect(this->gptr() < this->egptr(), true))
 324           __ret = traits_type::to_int_type(*this->gptr());
 325         else
 326           __ret = this->underflow();
 327         return __ret;
 328       }
 329
 330       /**
 331        *  @brief  Entry point for xsgetn.
 332        *  @param  s  A buffer area.
 333        *  @param  n  A count.
 334        *
 335        *  Returns xsgetn(s,n).  The effect is to fill @a s[0] through
 336        *  @a s[n-1] with characters from the input sequence, if possible.
 337       */
 338       streamsize
 339       sgetn(char_type* __s, streamsize __n)
 340       { return this->xsgetn(__s, __n); }
 341
 342       // [27.5.2.2.4] putback
 343       /**
 344        *  @brief  Pushing characters back into the input stream.
 345        *  @param  c  The character to push back.
 346        *  @return  The previous character, if possible.
 347        *
 348        *  Similar to sungetc(), but @a c is pushed onto the stream instead
 349        *  of "the previous character".  If successful, the next character
 350        *  fetched from the input stream will be @a c.
 351       */
 352       int_type
 353       sputbackc(char_type __c)
 354       {
 355         int_type __ret;
 356         const bool __testpos = this->eback() < this->gptr();
 357         if (__builtin_expect(!__testpos ||
 358                              !traits_type::eq(__c, this->gptr()[-1]), false))
 359           __ret = this->pbackfail(traits_type::to_int_type(__c));
 360         else
 361           {
 362             this->gbump(-1);
 363             __ret = traits_type::to_int_type(*this->gptr());
 364           }
 365         return __ret;
 366       }
 367
 368       /**
 369        *  @brief  Moving backwards in the input stream.
 370        *  @return  The previous character, if possible.
 371        *
 372        *  If a putback position is available, this function decrements the
 373        *  input pointer and returns that character.  Otherwise, calls and
 374        *  returns pbackfail().  The effect is to "unget" the last character
 375        *  "gotten".
 376       */
 377       int_type
 378       sungetc()
 379       {
 380         int_type __ret;
 381         if (__builtin_expect(this->eback() < this->gptr(), true))
 382           {
 383             this->gbump(-1);
 384             __ret = traits_type::to_int_type(*this->gptr());
 385           }
 386         else
 387           __ret = this->pbackfail();
 388         return __ret;
 389       }
 390
 391       // [27.5.2.2.5] put area
 392       /**
 393        *  @brief  Entry point for all single-character output functions.
 394        *  @param  c  A character to output.
 395        *  @return  @a c, if possible.
 396        *
 397        *  One of two public output functions.
 398        *
 399        *  If a write position is available for the output sequence (i.e.,
 400        *  the buffer is not full), stores @a c in that position, increments
 401        *  the position, and returns @c traits::to_int_type(c).  If a write
 402        *  position is not available, returns @c overflow(c).
 403       */
 404       int_type
 405       sputc(char_type __c)
 406       {
 407         int_type __ret;
 408         if (__builtin_expect(this->pptr() < this->epptr(), true))
 409           {
 410             *this->pptr() = __c;
 411             this->pbump(1);
 412             __ret = traits_type::to_int_type(__c);
 413           }
 414         else
 415           __ret = this->overflow(traits_type::to_int_type(__c));
 416         return __ret;
 417       }
 418
 419       /**
 420        *  @brief  Entry point for all single-character output functions.
 421        *  @param  s  A buffer read area.
 422        *  @param  n  A count.
 423        *
 424        *  One of two public output functions.
 425        *
 426        *
 427        *  Returns xsputn(s,n).  The effect is to write @a s[0] through
 428        *  @a s[n-1] to the output sequence, if possible.
 429       */
 430       streamsize
 431       sputn(const char_type* __s, streamsize __n)
 432       { return this->xsputn(__s, __n); }
 433
 434     protected:
 435       /**
 436        *  @brief  Base constructor.
 437        *
 438        *  Only called from derived constructors, and sets up all the
 439        *  buffer data to zero, including the pointers described in the
 440        *  basic_streambuf class description.  Note that, as a result,
 441        *  - the class starts with no read nor write positions available,
 442        *  - this is not an error
 443       */
 444       basic_streambuf()
 445       : _M_in_beg(0), _M_in_cur(0), _M_in_end(0),
 446       _M_out_beg(0), _M_out_cur(0), _M_out_end(0),
 447       _M_buf_locale(locale())
 448       { }
 449
 450       // [27.5.2.3.1] get area access
 451       //@{
 452       /**
 453        *  @brief  Access to the get area.
 454        *
 455        *  These functions are only available to other protected functions,
 456        *  including derived classes.
 457        *
 458        *  - eback() returns the beginning pointer for the input sequence
 459        *  - gptr() returns the next pointer for the input sequence
 460        *  - egptr() returns the end pointer for the input sequence
 461       */
 462       char_type*
 463       eback() const { return _M_in_beg; }
 464
 465       char_type*
 466       gptr()  const { return _M_in_cur;  }
 467
 468       char_type*
 469       egptr() const { return _M_in_end; }
 470       //@}
 471
 472       /**
 473        *  @brief  Moving the read position.
 474        *  @param  n  The delta by which to move.
 475        *
 476        *  This just advances the read position without returning any data.
 477       */
 478       void
 479       gbump(int __n) { _M_in_cur += __n; }
 480
 481       /**
 482        *  @brief  Setting the three read area pointers.
 483        *  @param  gbeg  A pointer.
 484        *  @param  gnext  A pointer.
 485        *  @param  gend  A pointer.
 486        *  @post  @a gbeg == @c eback(), @a gnext == @c gptr(), and
 487        *         @a gend == @c egptr()
 488       */
 489       void
 490       setg(char_type* __gbeg, char_type* __gnext, char_type* __gend)
 491       {
 492         _M_in_beg = __gbeg;
 493         _M_in_cur = __gnext;
 494         _M_in_end = __gend;
 495       }
 496
 497       // [27.5.2.3.2] put area access
 498       //@{
 499       /**
 500        *  @brief  Access to the put area.
 501        *
 502        *  These functions are only available to other protected functions,
 503        *  including derived classes.
 504        *
 505        *  - pbase() returns the beginning pointer for the output sequence
 506        *  - pptr() returns the next pointer for the output sequence
 507        *  - epptr() returns the end pointer for the output sequence
 508       */
 509       char_type*
 510       pbase() const { return _M_out_beg; }
 511
 512       char_type*
 513       pptr() const { return _M_out_cur; }
 514
 515       char_type*
 516       epptr() const { return _M_out_end; }
 517       //@}
 518
 519       /**
 520        *  @brief  Moving the write position.
 521        *  @param  n  The delta by which to move.
 522        *
 523        *  This just advances the write position without returning any data.
 524       */
 525       void
 526       pbump(int __n) { _M_out_cur += __n; }
 527
 528       /**
 529        *  @brief  Setting the three write area pointers.
 530        *  @param  pbeg  A pointer.
 531        *  @param  pend  A pointer.
 532        *  @post  @a pbeg == @c pbase(), @a pbeg == @c pptr(), and
 533        *         @a pend == @c epptr()
 534       */
 535       void
 536       setp(char_type* __pbeg, char_type* __pend)
 537       {
 538         _M_out_beg = _M_out_cur = __pbeg;
 539         _M_out_end = __pend;
 540       }
 541
 542       // [27.5.2.4] virtual functions
 543       // [27.5.2.4.1] locales
 544       /**
 545        *  @brief  Changes translations.
 546        *  @param  loc  A new locale.
 547        *
 548        *  Translations done during I/O which depend on the current locale
 549        *  are changed by this call.  The standard adds, "Between invocations
 550        *  of this function a class derived from streambuf can safely cache
 551        *  results of calls to locale functions and to members of facets
 552        *  so obtained."
 553        *
 554        *  @note  Base class version does nothing.
 555       */
 556       virtual void
 557       imbue(const locale&)
 558       { }
 559
 560       // [27.5.2.4.2] buffer management and positioning
 561       /**
 562        *  @brief  Manipulates the buffer.
 563        *
 564        *  Each derived class provides its own appropriate behavior.  See
 565        *  the next-to-last paragraph of
 566        *  http://gcc.gnu.org/onlinedocs/libstdc++/27_io/howto.html#2 for
 567        *  more on this function.
 568        *
 569        *  @note  Base class version does nothing, returns @c this.
 570       */
 571       virtual basic_streambuf<char_type,_Traits>*
 572       setbuf(char_type*, streamsize)
 573       { return this; }
 574
 575       /**
 576        *  @brief  Alters the stream positions.
 577        *
 578        *  Each derived class provides its own appropriate behavior.
 579        *  @note  Base class version does nothing, returns a @c pos_type
 580        *         that represents an invalid stream position.
 581       */
 582       virtual pos_type
 583       seekoff(off_type, ios_base::seekdir,
 584               ios_base::openmode /*__mode*/ = ios_base::in | ios_base::out)
 585       { return pos_type(off_type(-1)); }
 586
 587       /**
 588        *  @brief  Alters the stream positions.
 589        *
 590        *  Each derived class provides its own appropriate behavior.
 591        *  @note  Base class version does nothing, returns a @c pos_type
 592        *         that represents an invalid stream position.
 593       */
 594       virtual pos_type
 595       seekpos(pos_type,
 596               ios_base::openmode /*__mode*/ = ios_base::in | ios_base::out)
 597       { return pos_type(off_type(-1)); }
 598
 599       /**
 600        *  @brief  Synchronizes the buffer arrays with the controlled sequences.
 601        *  @return  -1 on failure.
 602        *
 603        *  Each derived class provides its own appropriate behavior,
 604        *  including the definition of "failure".
 605        *  @note  Base class version does nothing, returns zero.
 606       */
 607       virtual int
 608       sync() { return 0; }
 609
 610       // [27.5.2.4.3] get area
 611       /**
 612        *  @brief  Investigating the data available.
 613        *  @return  An estimate of the number of characters available in the
 614        *           input sequence, or -1.
 615        *
 616        *  "If it returns a positive value, then successive calls to
 617        *  @c underflow() will not return @c traits::eof() until at least that
 618        *  number of characters have been supplied.  If @c showmanyc()
 619        *  returns -1, then calls to @c underflow() or @c uflow() will fail."
 620        *  [27.5.2.4.3]/1
 621        *
 622        *  @note  Base class version does nothing, returns zero.
 623        *  @note  The standard adds that "the intention is not only that the
 624        *         calls [to underflow or uflow] will not return @c eof() but
 625        *         that they will return "immediately".
 626        *  @note  The standard adds that "the morphemes of @c showmanyc are
 627        *         "es-how-many-see", not "show-manic".
 628       */
 629       virtual streamsize
 630       showmanyc() { return 0; }
 631
 632       /**
 633        *  @brief  Multiple character extraction.
 634        *  @param  s  A buffer area.
 635        *  @param  n  Maximum number of characters to assign.
 636        *  @return  The number of characters assigned.
 637        *
 638        *  Fills @a s[0] through @a s[n-1] with characters from the input
 639        *  sequence, as if by @c sbumpc().  Stops when either @a n characters
 640        *  have been copied, or when @c traits::eof() would be copied.
 641        *
 642        *  It is expected that derived classes provide a more efficient
 643        *  implementation by overriding this definition.
 644       */
 645       virtual streamsize
 646       xsgetn(char_type* __s, streamsize __n);
 647
 648       /**
 649        *  @brief  Fetches more data from the controlled sequence.
 650        *  @return  The first character from the <em>pending sequence</em>.
 651        *
 652        *  Informally, this function is called when the input buffer is
 653        *  exhausted (or does not exist, as buffering need not actually be
 654        *  done).  If a buffer exists, it is "refilled".  In either case, the
 655        *  next available character is returned, or @c traits::eof() to
 656        *  indicate a null pending sequence.
 657        *
 658        *  For a formal definition of the pending sequence, see a good text
 659        *  such as Langer & Kreft, or [27.5.2.4.3]/7-14.
 660        *
 661        *  A functioning input streambuf can be created by overriding only
 662        *  this function (no buffer area will be used).  For an example, see
 663        *  http://gcc.gnu.org/onlinedocs/libstdc++/27_io/howto.html#6
 664        *
 665        *  @note  Base class version does nothing, returns eof().
 666       */
 667       virtual int_type
 668       underflow()
 669       { return traits_type::eof(); }
 670
 671       /**
 672        *  @brief  Fetches more data from the controlled sequence.
 673        *  @return  The first character from the <em>pending sequence</em>.
 674        *
 675        *  Informally, this function does the same thing as @c underflow(),
 676        *  and in fact is required to call that function.  It also returns
 677        *  the new character, like @c underflow() does.  However, this
 678        *  function also moves the read position forward by one.
 679       */
 680       virtual int_type
 681       uflow()
 682       {
 683         int_type __ret = traits_type::eof();
 684         const bool __testeof = traits_type::eq_int_type(this->underflow(),
 685                                                         __ret);
 686         if (!__testeof)
 687           {
 688             __ret = traits_type::to_int_type(*this->gptr());
 689             this->gbump(1);
 690           }
 691         return __ret;
 692       }
 693
 694       // [27.5.2.4.4] putback
 695       /**
 696        *  @brief  Tries to back up the input sequence.
 697        *  @param  c  The character to be inserted back into the sequence.
 698        *  @return  eof() on failure, "some other value" on success
 699        *  @post  The constraints of @c gptr(), @c eback(), and @c pptr()
 700        *         are the same as for @c underflow().
 701        *
 702        *  @note  Base class version does nothing, returns eof().
 703       */
 704       virtual int_type
 705       pbackfail(int_type /* __c */  = traits_type::eof())
 706       { return traits_type::eof(); }
 707
 708       // Put area:
 709       /**
 710        *  @brief  Multiple character insertion.
 711        *  @param  s  A buffer area.
 712        *  @param  n  Maximum number of characters to write.
 713        *  @return  The number of characters written.
 714        *
 715        *  Writes @a s[0] through @a s[n-1] to the output sequence, as if
 716        *  by @c sputc().  Stops when either @a n characters have been
 717        *  copied, or when @c sputc() would return @c traits::eof().
 718        *
 719        *  It is expected that derived classes provide a more efficient
 720        *  implementation by overriding this definition.
 721       */
 722       virtual streamsize
 723       xsputn(const char_type* __s, streamsize __n);
 724
 725       /**
 726        *  @brief  Consumes data from the buffer; writes to the
 727        *          controlled sequence.
 728        *  @param  c  An additional character to consume.
 729        *  @return  eof() to indicate failure, something else (usually
 730        *           @a c, or not_eof())
 731        *
 732        *  Informally, this function is called when the output buffer is full
 733        *  (or does not exist, as buffering need not actually be done).  If a
 734        *  buffer exists, it is "consumed", with "some effect" on the
 735        *  controlled sequence.  (Typically, the buffer is written out to the
 736        *  sequence verbatim.)  In either case, the character @a c is also
 737        *  written out, if @a c is not @c eof().
 738        *
 739        *  For a formal definition of this function, see a good text
 740        *  such as Langer & Kreft, or [27.5.2.4.5]/3-7.
 741        *
 742        *  A functioning output streambuf can be created by overriding only
 743        *  this function (no buffer area will be used).
 744        *
 745        *  @note  Base class version does nothing, returns eof().
 746       */
 747       virtual int_type
 748       overflow(int_type /* __c */ = traits_type::eof())
 749       { return traits_type::eof(); }
 750
 751 #if _GLIBCXX_DEPRECATED
 752     // Annex D.6
 753     public:
 754       /**
 755        *  @brief  Tosses a character.
 756        *
 757        *  Advances the read pointer, ignoring the character that would have
 758        *  been read.
 759        *
 760        *  See http://gcc.gnu.org/ml/libstdc++/2002-05/msg00168.html
 761        */
 762       void
 763       stossc()
 764       {
 765         if (this->gptr() < this->egptr())
 766           this->gbump(1);
 767         else
 768           this->uflow();
 769       }
 770 #endif
 771
 772     private:
 773       // _GLIBCXX_RESOLVE_LIB_DEFECTS
 774       // Side effect of DR 50.
 775       basic_streambuf(const __streambuf_type& __sb)
 776       : _M_in_beg(__sb._M_in_beg), _M_in_cur(__sb._M_in_cur),
 777       _M_in_end(__sb._M_in_end), _M_out_beg(__sb._M_out_beg),
 778       _M_out_cur(__sb._M_out_cur), _M_out_end(__sb._M_out_cur),
 779       _M_buf_locale(__sb._M_buf_locale)
 780       { }
 781
 782       __streambuf_type&
 783       operator=(const __streambuf_type&) { return *this; };
 784     };
 785
 786   // Explicit specialization declarations, defined in src/streambuf.cc.
 787   template<>
 788     streamsize
 789     __copy_streambufs_eof(basic_streambuf<char>* __sbin,
 790                           basic_streambuf<char>* __sbout, bool& __ineof);
 791 #ifdef _GLIBCXX_USE_WCHAR_T
 792   template<>
 793     streamsize
 794     __copy_streambufs_eof(basic_streambuf<wchar_t>* __sbin,
 795                           basic_streambuf<wchar_t>* __sbout, bool& __ineof);
 796 #endif
 797
 798 _GLIBCXX_END_NAMESPACE
 799
 800 #ifndef _GLIBCXX_EXPORT_TEMPLATE
 801 # include <bits/streambuf.tcc>
 802 #endif
 803
 804 #endif /* _GLIBCXX_STREAMBUF */