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54 <p>So you want to copy a file quickly and easily, and most important,
55 completely portably. And since this is C++, you have an open
56 ifstream (call it IN) and an open ofstream (call it OUT):
57 </p>
58 <pre>
61 std::ifstream IN ("input_file");
63 <p>Here's the easiest way to get it completely wrong:
64 </p>
65 <pre>
67 <p>For those of you who don't already know why this doesn't work
68 (probably from having done it before), I invite you to quickly
70 the sentence
71 </p>
72 <pre>
73 The quick brown fox jumped over the lazy dog.</pre>
74 <p>surrounded by blank lines. Code it up and try it. The contents
76 </p>
77 <p>Seriously, go do it. Get surprised, then come back. It's worth it.
78 </p>
81 handle formatting, nothing else. In particular, they break up on
82 whitespace. The actual reading, writing, and storing of data is
85 a pointer-to-streambuf, in order to help with just this kind of
87 </p>
89 the [io]streams hold pointers (or references, depending on the
90 implementation) to their buffers, not the actual
91 buffers. This allows polymorphic behavior on the part of the buffers
92 as well as the streams themselves. The pointer is easily retrieved
94 way to copy the file is:
95 </p>
96 <pre>
99 behavior, since that particular << isn't defined by the Standard.
100 I have seen instances where it is implemented, but the character
101 extraction process removes all the whitespace, leaving you with no
103 libraries that do not define that operator, IN (or one of IN's
104 member pointers) sometimes gets converted to a void*, and the output
105 file then contains a perfect text representation of a hexidecimal
106 address (quite a big surprise). Others don't compile at all.
107 </p>
109 The operators shown above are all defined in the parent
110 basic_ostream class and are therefore available with all possible
111 descendents.
112 </p>
115 </p>
117 <hr />
119 <!--
120 This is not written very well. I need to redo this section.
121 -->
122 <p>First, are you sure that you understand buffering? Particularly
123 the fact that C++ may not, in fact, have anything to do with it?
124 </p>
125 <p>The rules for buffering can be a little odd, but they aren't any
126 different from those of C. (Maybe that's why they can be a bit
127 odd.) Many people think that writing a newline to an output
128 stream automatically flushes the output buffer. This is true only
129 when the output stream is, in fact, a terminal and not a file
131 since C++ says nothing about files nor terminals. All of that is
132 system-dependent. (The "newline-buffer-flushing only occurring
133 on terminals" thing is mostly true on Unix systems, though.)
134 </p>
136 output stream only writes a newline. This is incorrect; after a
137 newline is written, the buffer is also flushed. Perhaps this
138 is the effect you want when writing to a screen -- get the text
139 out as soon as possible, etc -- but the buffering is largely
140 wasted when doing this to a file:
141 </p>
142 <pre>
146 <p>The proper thing to do in this case to just write the data out
147 and let the libraries and the system worry about the buffering.
148 If you need a newline, just write a newline:
149 </p>
150 <pre>
154 <p>I have also joined the output statements into a single statement.
155 You could make the code prettier by moving the single newline to
156 the start of the quoted text on the last line, for example.
157 </p>
158 <p>If you do need to flush the buffer above, you can send an
160 yourself:
161 </p>
162 <pre>
164 output.flush(); // or call a member fn </pre>
165 <p>On the other hand, there are times when writing to a file should
166 be like writing to standard error; no buffering should be done
167 because the data needs to appear quickly (a prime example is a
168 log file for security-related information). The way to do this is
169 just to turn off the buffering <em>before any I/O operations at
170 all</em> have been done (note that opening counts as an I/O operation):
171 </p>
172 <pre>
173 std::ofstream os;
174 std::ifstream is;
175 int i;
182 ...
185 <p>Since all aspects of buffering are handled by a streambuf-derived
188 arguments are the same as those for the Standard C I/O Library
189 function (a buffer area followed by its size).
190 </p>
191 <p>A great deal of this is implementation-dependent. For example,
203 which you must allocate and deallocate.)
204 </p>
205 <p>A last reminder: there are usually more buffers involved than
206 just those at the language/library level. Kernel buffers, disk
207 buffers, and the like will also have an effect. Inspecting and
208 changing those are system-dependent.
209 </p>
212 </p>
214 <hr />
216 <p>The first and most important thing to remember about binary I/O is
220 operators of the normal fstreams to do binary I/O.
221 </p>
222 <p>Sorry. Them's the breaks.
223 </p>
224 <p>This isn't going to try and be a complete tutorial on reading and
227 up a couple of misconceptions and common errors.
228 </p>
230 and no less. Normal text mode has to be concerned with the newline
231 characters, and the runtime system will translate between (for
232 example) '\n' and the appropriate end-of-line sequence (LF on Unix,
233 CRLF on DOS, CR on Macintosh, etc). (There are other things that
234 normal mode does, but that's the most obvious.) Opening a file in
235 binary mode disables this conversion, so reading a CRLF sequence
236 under Windows won't accidentally get mapped to a '\n' character, etc.
237 Binary mode is not supposed to suddenly give you a bitstream, and
238 if it is doing so in your program then you've discovered a bug in
239 your vendor's compiler (or some other part of the C++ implementation,
240 possibly the runtime system).
241 </p>
243 read isn't going to work with the standard file stream classes, even
246 not reading and writing. Their job is to interpret the data into
247 text characters, and that's exactly what you don't want to happen
248 during binary I/O.
249 </p>
251 functions still aren't guaranteed to help you. These are
253 (This may or may not be what you want, see below.)
254 </p>
255 <p>Notice how all the problems here are due to the inappropriate use
258 seemingly infinite number of solutions, and a few are listed here:
259 </p>
260 <ul>
263 types you're using." This is a Bad Thing, because while
264 the compiler would probably be just fine with it, other humans
265 are going to be confused. The overloaded bitshift operators
266 have a well-defined meaning (formatting), and this breaks it.
267 </li>
269 the file and copy the structure." Well, this is easy to
270 make work, and easy to break, and is pretty equivalent to
272 makes no use of the iostream library at all...
273 </li>
275 While not trivial for the beginner, this is the best of all
276 solutions. The streambuf/filebuf layer is the layer that is
277 responsible for actual I/O. If you want to use the C++
278 library for binary I/O, this is where you start.
279 </li>
280 </ul>
281 <p>How to go about using streambufs is a bit beyond the scope of this
282 document (at least for now), but while streambufs go a long way,
283 they still leave a couple of things up to you, the programmer.
284 As an example, byte ordering is completely between you and the
285 operating system, and you have to handle it yourself.
286 </p>
287 <p>Deriving a streambuf or filebuf
288 class from the standard ones, one that is specific to your data
289 types (or an abstraction thereof) is probably a good idea, and
290 lots of examples exist in journals and on Usenet. Using the
291 standard filebufs directly (either by declaring your own or by
293 is certainly feasible as well.
294 </p>
295 <p>One area that causes problems is trying to do bit-by-bit operations
296 with filebufs. C++ is no different from C in this respect: I/O
297 must be done at the byte level. If you're trying to read or write
298 a few bits at a time, you're going about it the wrong way. You
299 must read/write an integral number of bytes and then process the
300 bytes. (For example, the streambuf functions take and return
302 </p>
303 <p>Another area of problems is opening text files in binary mode.
304 Generally, binary mode is intended for binary files, and opening
305 text files in binary mode means that you now have to deal with all of
306 those end-of-line and end-of-file problems that we mentioned before.
307 An instructive thread from comp.lang.c++.moderated delved off into
308 this topic starting more or less at
310 article and continuing to the end of the thread. (You'll have to
311 sort through some flames every couple of paragraphs, but the points
312 made are good ones.)
313 </p>
315 <hr />
318 are in this author's opinion one of the coolest things since
319 sliced time. An example of their use is in the Received Wisdom
320 section for Chapter 21 (Strings),
322 format strings</a>.
323 </p>
324 <p>The quick definition is: they are siblings of ifstream and ofstream,
330 ostringstream. You've read a string as input and need to parse it?
331 Initialize an istringstream with that string, and then pull pieces
334 member function.
335 </p>
336 <p>This only works if you've written your
338 and correctly means that they take istreams and ostreams as
340 take the latter, then your I/O operators will work fine with
341 file streams, but with nothing else -- including stringstreams.
342 </p>
343 <p>If you are a user of the strstream classes, you need to update
345 terminate the C-style character array, you don't have to mess with
347 memory yourself. The strstreams have been officially deprecated,
348 which means that 1) future revisions of the C++ Standard won't
349 support them, and 2) if you use them, people will laugh at you.
350 </p>
352 <hr />
354 <p>Creating your own stream buffers for I/O can be remarkably easy.
355 If you are interested in doing so, we highly recommend two very
356 excellent books:
361 Addison-Wesley, who isn't paying us a cent for saying that, honest.
362 </p>
363 <p>Here is a simple example, io/outbuf1, from the Josuttis text. It
364 transforms everything sent through it to uppercase. This version
365 assumes many things about the nature of the character type being
366 used (for more information, read the books or the newsgroups):
367 </p>
368 <pre>
374 class outbuf : public std::streambuf
375 {
376 protected:
377 /* central output function
378 * - print characters in uppercase mode
379 */
380 virtual int_type overflow (int_type c) {
381 if (c != EOF) {
382 // convert lowercase to uppercase
385 // and write the character to the standard output
386 if (putchar(c) == EOF) {
387 return EOF;
388 }
389 }
390 return c;
391 }
392 };
394 int main()
395 {
396 // create special output buffer
397 outbuf ob;
398 // initialize output stream with that output buffer
399 std::ostream out(&ob);
403 return 0;
404 }
405 </pre>
410 </p>
412 <hr />
416 same time. One notable place was Usenet, where James Kanze and
417 Dietmar Kühl separately posted articles on why attempting
418 generic binary I/O was not a good idea. (Here are copies of
421 </p>
422 <p>Briefly, the problems of byte ordering and type sizes mean that
425 between arbitrary programs, or across a network, or from one
426 invocation of a program to another invocation of the same program
427 on a different platform, etc.
428 </p>
429 <p>The entire Usenet thread is instructive, and took place under the
431 and comp.lang.c++.moderated in parallel. Also in that thread,
432 Dietmar Kühl mentioned that he had written a pair of stream
433 classes that would read and write XDR, which is a good step towards
434 a portable binary format.
435 </p>
437 <hr />
439 <p>It sounds like a flame on C, but it isn't. Really. Calm down.
440 I'm just saying it to get your attention.
441 </p>
442 <p>Because the C++ library includes the C library, both C-style and
443 C++-style I/O have to work at the same time. For example:
444 </p>
445 <pre>
452 </pre>
453 <p>This must do what you think it does.
454 </p>
455 <p>Alert members of the audience will immediately notice that buffering
456 is going to make a hash of the output unless special steps are taken.
457 </p>
459 involve doing very little buffering for the standard streams, leaving
460 most of the buffering to the underlying C library. (This kind of
462 The upside is that correctness is ensured. The downside is that
464 performance when the C++ I/O library is layered on top of the C I/O
465 library (as it is for 3.0 by default). Some patches have been applied
466 which improve the situation for 3.1.
467 </p>
468 <p>However, the C and C++ standard streams only need to be kept in sync
469 when both libraries' facilities are in use. If your program only uses
470 C++ I/O, then there's no need to sync with the C streams. The right
471 thing to do in this case is to call
472 </p>
473 <pre>
476 std::ios::sync_with_stdio(false);
477 </pre>
478 <p>You must do this before performing any I/O via the C++ stream objects.
479 Once you call this, the C++ streams will operate independently of the
481 company will become fully buffered on their own.
482 </p>
483 <p>Note, by the way, that the synchronization requirement only applies to
487 objects that you declare yourself have no such requirement and are fully
488 buffered.
489 </p>
491 <hr />
493 <p>I'll assume that you have already read the
495 and the
497 access</a> (you might not think of an I/O stream as a container, but
498 the points made there also hold here). If you have not read them,
499 please do so first.
500 </p>
501 <p>This gets a bit tricky. Please read carefully, and bear with me.
502 </p>
508 </p>
509 <p>A generic locking mechanism is somewhat in place at the filebuf layer,
510 but is not used in the current code. Providing locking at any higher
511 level is akin to providing locking within containers, and is not done
512 for the same reasons (see the links above).
513 </p>
515 <p>The __basic_file type is simply a collection of small wrappers around
516 the C stdio layer (again, see the link under Structure). We do no
519 </p>
521 must be answered with, "is your platform's C library threadsafe
522 for I/O?" Some are by default, some are not; many offer multiple
523 implementations of the C library with varying tradeoffs of threadsafety
524 and efficiency. You, the programmer, are always required to take care
525 with multiple threads.
526 </p>
527 <p>(As an example, the POSIX standard requires that C stdio FILE*
528 operations are atomic. POSIX-conforming C libraries (e.g, on Solaris
529 and GNU/Linux) have an internal mutex to serialize operations on
530 FILE*s. However, you still need to not do stupid things like calling
533 </p>
534 <p>So, if your platform's C library is threadsafe, then your
536 level. For higher-level operations, such as manipulating the data
537 contained in the stream formatting classes (e.g., setting up callbacks
539 like any other critical shared resource.
540 </p>
543 second choice is available for I/O implementations: libio. This is
544 disabled by default, and in fact will not currently work due to other
545 issues. It will be revisited, however.
546 </p>
547 <p>The libio code is a subset of the guts of the GNU libc (glibc) I/O
549 type is basically derived from FILE. (The real situation is more
550 complex than that... it's derived from an internal type used to
551 implement FILE. See libio/libioP.h to see scary things done with
553 to go through; the filebuf makes calls directly into the same
555 and so forth, using internal data structures. (And when I say
557 replaced by a jump into an internal function. Fast but frightening.
558 *grin*)
559 </p>
560 <p>Also, the libio internal locks are used. This requires pulling in
561 large chunks of glibc, such as a pthreads implementation, and is one
562 of the issues preventing widespread use of libio as the libstdc++
563 cstdio implementation.
564 </p>
565 <p>But we plan to make this work, at least as an option if not a future
566 default. Platforms running a copy of glibc with a recent-enough
567 version will see calls from libstdc++ directly into the glibc already
568 installed. For other platforms, a copy of the libio subsection will
569 be built and included in libstdc++.
570 </p>
572 <p>Don't forget that other cstdio implemenations are possible. You could
573 easily write one to perform your own forms of locking, to solve your
575 </p>
577 <hr />
579 <p>To minimize the time you have to wait on the compiler, it's good to
580 only include the headers you really need. Many people simply include
582 your runtime as well.</em> Here are some tips on which header to use
583 for which situations, starting with the simplest.
584 </p>
588 implies, these are forward declarations. (A word to all you fellow
589 old school programmers: trying to forward declare classes like
591 you'd like to know why.) For example,
592 </p>
593 <pre>
596 class MyClass
597 {
598 ....
599 std::ifstream input_file;
600 };
603 </pre>
606 counting types std::streamoff and std::streamsize, the file
607 positioning type std::fpos, and the various manipulators like
608 std::hex, std::fixed, std::noshowbase, and so forth.
609 </p>
610 <p>The ios_base class is what holds the format flags, the state flags,
611 and the functions which change them (setf(), width(), precision(),
612 etc). You can also store extra data and register callback functions
613 through ios_base, but that has been historically underused. Anything
614 which doesn't depend on the type of characters stored is consolidated
615 here.
616 </p>
617 <p>The template class basic_ios is the highest template class in the
618 hierarchy; it is the first one depending on the character type, and
619 holds all general state associated with that type: the pointer to the
620 polymorphic stream buffer, the facet information, etc.
621 </p>
623 basic_streambuf, and two standard instantiations, streambuf and
624 wstreambuf. If you need to work with the vastly useful and capable
625 stream buffer classes, e.g., to create a new form of storage
626 transport, this header is the one to include.
627 </p>
630 interface, or any of the other abstract stream formatting functions.
631 For example,
632 </p>
633 <pre>
637 {
639 }
640 </pre>
641 <p>The std::istream and std::ostream classes are the abstract parents of
642 the various concrete implementations. If you are only using the
643 interfaces, then you only need to use the appropriate interface header.
644 </p>
646 that alter information maintained by class ios_base and its dervied
647 classes," such as std::setprecision and std::setw. If you need
650 </p>
652 declare the six stringstream and fstream classes. As they are the
653 standard concrete descendants of istream and ostream, you will already
654 know about them.
655 </p>
657 global objects (cin, cout, etc). To do this correctly, this header
659 headers, but nothing else. The contents of this header look like
660 </p>
661 <pre>
665 namespace std
666 {
667 extern istream cin;
668 extern ostream cout;
669 ....
671 // this is explained below
673 }
674 </pre>
675 <p>Now, the runtime penalty mentioned previously: the global objects
676 must be initialized before any of your own code uses them; this is
677 guaranteed by the standard. Like any other global object, they must
678 be initialized once and only once. This is typically done with a
679 construct like the one above, and the nested class ios_base::Init is
680 specified in the standard for just this reason.
681 </p>
682 <p>How does it work? Because the header is included before any of your
684 your objects. (Global objects are built in the order in which they
685 are declared, and destroyed in reverse order.) The first time the
686 constructor runs, the eight stream objects are set up.
687 </p>
691 of construction across object files (it's one of those pesky NP
692 problems that make life so interesting), so one copy in each object
693 file means that the stream objects are guaranteed to be set up before
694 any of your code which uses them could run, thereby meeting the
695 requirements of the standard.
696 </p>
697 <p>The penalty, of course, is that after the first copy of
699 processor time. The time spent is merely for an increment-and-test
700 inside a function call, but over several dozen or hundreds of object
701 files, that time can add up. (It's not in a tight loop, either.)
702 </p>
704 the standard objects in that source file; you'll pay less startup
705 time. Only include the header files you need to in general; your
706 compile times will go down when there's less parsing work to do.
707 </p>
710 <hr />
712 <!-- referenced by ext/howto.html#2, update link if numbering changes -->
713 <p>The v2 library included non-standard extensions to construct
716 Today the recommended way to use stdio types with libstdc++-v3
718 but earlier releases provided slightly different mechanisms.
719 </p>
720 <ul>
722 <br />
724 <br />This comes in very handy in a number of places, such as
725 attaching Unix sockets, pipes, and anything else which uses file
726 descriptors, into the IOStream buffering classes. The three
727 arguments are as follows:
728 <ul>
730 // the __c_file_type typedef usually boils down to stdio's FILE
731 </li>
733 // same as all the other uses of openmode
734 </li>
736 // buffer size, defaults to BUFSIZ if not specified
737 </li>
738 </ul>
739 For those wanting to use file descriptors instead of FILE*'s, I
741 </li>
744 integer returned from this function can be used for whatever file
745 descriptors can be used for on your platform. Naturally, the
746 library cannot track what you do on your own with a file descriptor,
747 so if you perform any I/O directly, don't expect the library to be
748 aware of it.
749 </li>
753 a derived class called
754 <a href="http://gcc.gnu.org/onlinedocs/libstdc++/latest-doxygen/class____gnu__cxx_1_1stdio__filebuf.html"><code>__gnu_cxx::stdio_filebuf</code></a>.
757 </li>
758 </ul>
761 call) then you might be interested in Henry Suter's
763 class.
764 </p>
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