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<h1 class="centered"><a name="top">Chapter 27:  Input/Output</a></h1>
 
<p>Chapter 27 deals with iostreams and all their subcomponents
   and extensions.  All <em>kinds</em> of fun stuff.
</p>
 
 
<!-- ####################################################### -->
<hr />
<h1>Contents</h1>
<ul>
   <li><a href="#1">Copying a file</a></li>
   <li><a href="#2">The buffering is screwing up my program!</a></li>
   <li><a href="#3">Binary I/O</a></li>
   <li><a href="#5">What is this &lt;sstream&gt;/stringstreams thing?</a></li>
   <li><a href="#6">Deriving a stream buffer</a></li>
   <li><a href="#7">More on binary I/O</a></li>
   <li><a href="#8">Pathetic performance?  Ditch C.</a></li>
   <li><a href="#9">Threads and I/O</a></li>
   <li><a href="#10">Which header?</a></li>
   <li><a href="#11">Using FILE*s and file descriptors with IOStreams</a></li>
</ul>
 
<hr />
 
<!-- ####################################################### -->
 
<h2><a name="1">Copying a file</a></h2>
   <p>So you want to copy a file quickly and easily, and most important,
      completely portably.  And since this is C++, you have an open
      ifstream (call it IN) and an open ofstream (call it OUT):
   </p>
   <pre>
   #include &lt;fstream&gt;
 
   std::ifstream  IN ("input_file");
   std::ofstream  OUT ("output_file"); </pre>
   <p>Here's the easiest way to get it completely wrong:
   </p>
   <pre>
   OUT &lt;&lt; IN;</pre>
   <p>For those of you who don't already know why this doesn't work
      (probably from having done it before), I invite you to quickly
      create a simple text file called &quot;input_file&quot; containing
      the sentence
   </p>
      <pre>
      The quick brown fox jumped over the lazy dog.</pre>
   <p>surrounded by blank lines.  Code it up and try it.  The contents
      of &quot;output_file&quot; may surprise you.
   </p>
   <p>Seriously, go do it.  Get surprised, then come back.  It's worth it.
   </p>
   <hr width="60%" />
   <p>The thing to remember is that the <code>basic_[io]stream</code> classes
      handle formatting, nothing else.  In particular, they break up on
      whitespace.  The actual reading, writing, and storing of data is
      handled by the <code>basic_streambuf</code> family.  Fortunately, the
      <code>operator&lt;&lt;</code> is overloaded to take an ostream and
      a pointer-to-streambuf, in order to help with just this kind of
      &quot;dump the data verbatim&quot; situation.
   </p>
   <p>Why a <em>pointer</em> to streambuf and not just a streambuf?  Well,
      the [io]streams hold pointers (or references, depending on the
      implementation) to their buffers, not the actual
      buffers.  This allows polymorphic behavior on the part of the buffers
      as well as the streams themselves.  The pointer is easily retrieved
      using the <code>rdbuf()</code> member function.  Therefore, the easiest
      way to copy the file is:
   </p>
   <pre>
   OUT &lt;&lt; IN.rdbuf();</pre>
   <p>So what <em>was</em> happening with OUT&lt;&lt;IN?  Undefined
      behavior, since that particular &lt;&lt; isn't defined by the Standard.
      I have seen instances where it is implemented, but the character
      extraction process removes all the whitespace, leaving you with no
      blank lines and only &quot;Thequickbrownfox...&quot;.  With
      libraries that do not define that operator, IN (or one of IN's
      member pointers) sometimes gets converted to a void*, and the output
      file then contains a perfect text representation of a hexidecimal
      address (quite a big surprise).  Others don't compile at all.
   </p>
   <p>Also note that none of this is specific to o<b>*f*</b>streams.
      The operators shown above are all defined in the parent
      basic_ostream class and are therefore available with all possible
      descendents.
   </p>
   <p>Return <a href="#top">to top of page</a> or
      <a href="../faq/index.html">to the FAQ</a>.
   </p>
 
<hr />
<h2><a name="2">The buffering is screwing up my program!</a></h2>
<!--
  This is not written very well.  I need to redo this section.
-->
   <p>First, are you sure that you understand buffering?  Particularly
      the fact that C++ may not, in fact, have anything to do with it?
   </p>
   <p>The rules for buffering can be a little odd, but they aren't any
      different from those of C.  (Maybe that's why they can be a bit
      odd.)  Many people think that writing a newline to an output
      stream automatically flushes the output buffer.  This is true only
      when the output stream is, in fact, a terminal and not a file
      or some other device -- and <em>that</em> may not even be true
      since C++ says nothing about files nor terminals.  All of that is
      system-dependent.  (The &quot;newline-buffer-flushing only occurring
      on terminals&quot; thing is mostly true on Unix systems, though.)
   </p>
   <p>Some people also believe that sending <code>endl</code> down an
      output stream only writes a newline.  This is incorrect; after a
      newline is written, the buffer is also flushed.  Perhaps this
      is the effect you want when writing to a screen -- get the text
      out as soon as possible, etc -- but the buffering is largely
      wasted when doing this to a file:
   </p>
   <pre>
   output &lt;&lt; &quot;a line of text&quot; &lt;&lt; endl;
   output &lt;&lt; some_data_variable &lt;&lt; endl;
   output &lt;&lt; &quot;another line of text&quot; &lt;&lt; endl; </pre>
   <p>The proper thing to do in this case to just write the data out
      and let the libraries and the system worry about the buffering.
      If you need a newline, just write a newline:
   </p>
   <pre>
   output &lt;&lt; &quot;a line of text\n&quot;
          &lt;&lt; some_data_variable &lt;&lt; '\n'
          &lt;&lt; &quot;another line of text\n&quot;; </pre>
   <p>I have also joined the output statements into a single statement.
      You could make the code prettier by moving the single newline to
      the start of the quoted text on the last line, for example.
   </p>
   <p>If you do need to flush the buffer above, you can send an
      <code>endl</code> if you also need a newline, or just flush the buffer
      yourself:
   </p>
   <pre>
   output &lt;&lt; ...... &lt;&lt; flush;    // can use std::flush manipulator
   output.flush();               // or call a member fn </pre>
   <p>On the other hand, there are times when writing to a file should
      be like writing to standard error; no buffering should be done
      because the data needs to appear quickly (a prime example is a
      log file for security-related information).  The way to do this is
      just to turn off the buffering <em>before any I/O operations at
      all</em> have been done (note that opening counts as an I/O operation):
   </p>
   <pre>
   std::ofstream    os;
   std::ifstream    is;
   int   i;
 
   os.rdbuf()-&gt;pubsetbuf(0,0);
   is.rdbuf()-&gt;pubsetbuf(0,0);
 
   os.open(&quot;/foo/bar/baz&quot;);
   is.open(&quot;/qux/quux/quuux&quot;);
   ...
   os &lt;&lt; &quot;this data is written immediately\n&quot;;
   is &gt;&gt; i;   // and this will probably cause a disk read </pre>
   <p>Since all aspects of buffering are handled by a streambuf-derived
      member, it is necessary to get at that member with <code>rdbuf()</code>.
      Then the public version of <code>setbuf</code> can be called.  The
      arguments are the same as those for the Standard C I/O Library
      function (a buffer area followed by its size).
   </p>
   <p>A great deal of this is implementation-dependent.  For example,
      <code>streambuf</code> does not specify any actions for its own
      <code>setbuf()</code>-ish functions; the classes derived from
      <code>streambuf</code> each define behavior that &quot;makes
      sense&quot; for that class:  an argument of (0,0) turns off buffering
      for <code>filebuf</code> but does nothing at all for its siblings
      <code>stringbuf</code> and <code>strstreambuf</code>, and specifying
      anything other than (0,0) has varying effects.
      User-defined classes derived from <code>streambuf</code> can
      do whatever they want.  (For <code>filebuf</code> and arguments for
      <code>(p,s)</code> other than zeros, libstdc++ does what you'd expect:
      the first <code>s</code> bytes of <code>p</code> are used as a buffer,
      which you must allocate and deallocate.)
   </p>
   <p>A last reminder:  there are usually more buffers involved than
      just those at the language/library level.  Kernel buffers, disk
      buffers, and the like will also have an effect.  Inspecting and
      changing those are system-dependent.
   </p>
   <p>Return <a href="#top">to top of page</a> or
      <a href="../faq/index.html">to the FAQ</a>.
   </p>
 
<hr />
<h2><a name="3">Binary I/O</a></h2>
   <p>The first and most important thing to remember about binary I/O is
      that opening a file with <code>ios::binary</code> is not, repeat
      <em>not</em>, the only thing you have to do.  It is not a silver
      bullet, and will not allow you to use the <code>&lt;&lt;/&gt;&gt;</code>
      operators of the normal fstreams to do binary I/O.
   </p>
   <p>Sorry.  Them's the breaks.
   </p>
   <p>This isn't going to try and be a complete tutorial on reading and
      writing binary files (because &quot;binary&quot;
      <a href="#7">covers a lot of ground)</a>, but we will try and clear
      up a couple of misconceptions and common errors.
   </p>
   <p>First, <code>ios::binary</code> has exactly one defined effect, no more
      and no less.  Normal text mode has to be concerned with the newline
      characters, and the runtime system will translate between (for
      example) '\n' and the appropriate end-of-line sequence (LF on Unix,
      CRLF on DOS, CR on Macintosh, etc).  (There are other things that
      normal mode does, but that's the most obvious.)  Opening a file in
      binary mode disables this conversion, so reading a CRLF sequence
      under Windows won't accidentally get mapped to a '\n' character, etc.
      Binary mode is not supposed to suddenly give you a bitstream, and
      if it is doing so in your program then you've discovered a bug in
      your vendor's compiler (or some other part of the C++ implementation,
      possibly the runtime system).
   </p>
   <p>Second, using <code>&lt;&lt;</code> to write and <code>&gt;&gt;</code> to
      read isn't going to work with the standard file stream classes, even
      if you use <code>skipws</code> during reading.  Why not?  Because
      ifstream and ofstream exist for the purpose of <em>formatting</em>,
      not reading and writing.  Their job is to interpret the data into
      text characters, and that's exactly what you don't want to happen
      during binary I/O.
   </p>
   <p>Third, using the <code>get()</code> and <code>put()/write()</code> member
      functions still aren't guaranteed to help you.  These are
      &quot;unformatted&quot; I/O functions, but still character-based.
      (This may or may not be what you want, see below.)
   </p>
   <p>Notice how all the problems here are due to the inappropriate use
      of <em>formatting</em> functions and classes to perform something
      which <em>requires</em> that formatting not be done?  There are a
      seemingly infinite number of solutions, and a few are listed here:
   </p>
   <ul>
      <li>&quot;Derive your own fstream-type classes and write your own
          &lt;&lt;/&gt;&gt; operators to do binary I/O on whatever data
          types you're using.&quot;  This is a Bad Thing, because while
          the compiler would probably be just fine with it, other humans
          are going to be confused.  The overloaded bitshift operators
          have a well-defined meaning (formatting), and this breaks it.
      </li>
      <li>&quot;Build the file structure in memory, then <code>mmap()</code>
          the file and copy the structure.&quot;  Well, this is easy to
          make work, and easy to break, and is pretty equivalent to
          using <code>::read()</code> and <code>::write()</code> directly, and
          makes no use of the iostream library at all...
      </li>
      <li>&quot;Use streambufs, that's what they're there for.&quot;
          While not trivial for the beginner, this is the best of all
          solutions.  The streambuf/filebuf layer is the layer that is
          responsible for actual I/O.  If you want to use the C++
          library for binary I/O, this is where you start.
      </li>
   </ul>
   <p>How to go about using streambufs is a bit beyond the scope of this
      document (at least for now), but while streambufs go a long way,
      they still leave a couple of things up to you, the programmer.
      As an example, byte ordering is completely between you and the
      operating system, and you have to handle it yourself.
   </p>
   <p>Deriving a streambuf or filebuf
      class from the standard ones, one that is specific to your data
      types (or an abstraction thereof) is probably a good idea, and
      lots of examples exist in journals and on Usenet.  Using the
      standard filebufs directly (either by declaring your own or by
      using the pointer returned from an fstream's <code>rdbuf()</code>)
      is certainly feasible as well.
   </p>
   <p>One area that causes problems is trying to do bit-by-bit operations
      with filebufs.  C++ is no different from C in this respect:  I/O
      must be done at the byte level.  If you're trying to read or write
      a few bits at a time, you're going about it the wrong way.  You
      must read/write an integral number of bytes and then process the
      bytes.  (For example, the streambuf functions take and return
      variables of type <code>int_type</code>.)
   </p>
   <p>Another area of problems is opening text files in binary mode.
      Generally, binary mode is intended for binary files, and opening
      text files in binary mode means that you now have to deal with all of
      those end-of-line and end-of-file problems that we mentioned before.
      An instructive thread from comp.lang.c++.moderated delved off into
      this topic starting more or less at
      <a href="http://groups.google.com/groups?oi=djq&selm=an_436187505">this</a>
      article and continuing to the end of the thread.  (You'll have to
      sort through some flames every couple of paragraphs, but the points
      made are good ones.)
   </p>
 
<hr />
<h2><a name="5">What is this &lt;sstream&gt;/stringstreams thing?</a></h2>
   <p>Stringstreams (defined in the header <code>&lt;sstream&gt;</code>)
      are in this author's opinion one of the coolest things since
      sliced time.  An example of their use is in the Received Wisdom
      section for Chapter 21 (Strings),
      <a href="../21_strings/howto.html#1.1internal"> describing how to
      format strings</a>.
   </p>
   <p>The quick definition is:  they are siblings of ifstream and ofstream,
      and they do for <code>std::string</code> what their siblings do for
      files.  All that work you put into writing <code>&lt;&lt;</code> and
      <code>&gt;&gt;</code> functions for your classes now pays off
      <em>again!</em>  Need to format a string before passing the string
      to a function?  Send your stuff via <code>&lt;&lt;</code> to an
      ostringstream.  You've read a string as input and need to parse it?
      Initialize an istringstream with that string, and then pull pieces
      out of it with <code>&gt;&gt;</code>.  Have a stringstream and need to
      get a copy of the string inside?  Just call the <code>str()</code>
      member function.
   </p>
   <p>This only works if you've written your
      <code>&lt;&lt;</code>/<code>&gt;&gt;</code> functions correctly, though,
      and correctly means that they take istreams and ostreams as
      parameters, not i<b>f</b>streams and o<b>f</b>streams.  If they
      take the latter, then your I/O operators will work fine with
      file streams, but with nothing else -- including stringstreams.
   </p>
   <p>If you are a user of the strstream classes, you need to update
      your code.  You don't have to explicitly append <code>ends</code> to
      terminate the C-style character array, you don't have to mess with
      &quot;freezing&quot; functions, and you don't have to manage the
      memory yourself.  The strstreams have been officially deprecated,
      which means that 1) future revisions of the C++ Standard won't
      support them, and 2) if you use them, people will laugh at you.
   </p>
 
<hr />
<h2><a name="6">Deriving a stream buffer</a></h2>
   <p>Creating your own stream buffers for I/O can be remarkably easy.
      If you are interested in doing so, we highly recommend two very
      excellent books:
      <a href="http://www.langer.camelot.de/iostreams.html">Standard C++
      IOStreams and Locales</a> by Langer and Kreft, ISBN 0-201-18395-1, and
      <a href="http://www.josuttis.com/libbook/">The C++ Standard Library</a>
      by Nicolai Josuttis, ISBN 0-201-37926-0.  Both are published by
      Addison-Wesley, who isn't paying us a cent for saying that, honest.
   </p>
   <p>Here is a simple example, io/outbuf1, from the Josuttis text.  It
      transforms everything sent through it to uppercase.  This version
      assumes many things about the nature of the character type being
      used (for more information, read the books or the newsgroups):
   </p>
   <pre>
    #include &lt;iostream&gt;
    #include &lt;streambuf&gt;
    #include &lt;locale&gt;
    #include &lt;cstdio&gt;
 
    class outbuf : public std::streambuf
    {
      protected:
        /* central output function
         * - print characters in uppercase mode
         */
        virtual int_type overflow (int_type c) {
            if (c != EOF) {
                // convert lowercase to uppercase
                c = std::toupper(static_cast&lt;char&gt;(c),getloc());
 
                // and write the character to the standard output
                if (putchar(c) == EOF) {
                    return EOF;
                }
            }
            return c;
        }
    };
 
    int main()
    {
        // create special output buffer
        outbuf ob;
        // initialize output stream with that output buffer
        std::ostream out(&amp;ob);
 
        out &lt;&lt; "31 hexadecimal: "
            &lt;&lt; std::hex &lt;&lt; 31 &lt;&lt; std::endl;
        return 0;
    }
   </pre>
   <p>Try it yourself!  More examples can be found in 3.1.x code, in
      <code>include/ext/*_filebuf.h</code>, and on
      <a href="http://www.informatik.uni-konstanz.de/~kuehl/c++/iostream/">Dietmar
      K&uuml;hl's IOStreams page</a>.
   </p>
 
<hr />
<h2><a name="7">More on binary I/O</a></h2>
   <p>Towards the beginning of February 2001, the subject of
      &quot;binary&quot; I/O was brought up in a couple of places at the
      same time.  One notable place was Usenet, where James Kanze and
      Dietmar K&uuml;hl separately posted articles on why attempting
      generic binary I/O was not a good idea.  (Here are copies of
      <a href="binary_iostreams_kanze.txt">Kanze's article</a> and
      <a href="binary_iostreams_kuehl.txt">K&uuml;hl's article</a>.)
   </p>
   <p>Briefly, the problems of byte ordering and type sizes mean that
      the unformatted functions like <code>ostream::put()</code> and
      <code>istream::get()</code> cannot safely be used to communicate
      between arbitrary programs, or across a network, or from one
      invocation of a program to another invocation of the same program
      on a different platform, etc.
   </p>
   <p>The entire Usenet thread is instructive, and took place under the
      subject heading &quot;binary iostreams&quot; on both comp.std.c++
      and comp.lang.c++.moderated in parallel.  Also in that thread,
      Dietmar K&uuml;hl mentioned that he had written a pair of stream
      classes that would read and write XDR, which is a good step towards
      a portable binary format.
   </p>
 
<hr />
<h2><a name="8">Pathetic performance?  Ditch C.</a></h2>
   <p>It sounds like a flame on C, but it isn't.  Really.  Calm down.
      I'm just saying it to get your attention.
   </p>
   <p>Because the C++ library includes the C library, both C-style and
      C++-style I/O have to work at the same time.  For example:
   </p>
   <pre>
     #include &lt;iostream&gt;
     #include &lt;cstdio&gt;
 
     std::cout &lt;&lt; &quot;Hel&quot;;
     std::printf (&quot;lo, worl&quot;);
     std::cout &lt;&lt; &quot;d!\n&quot;;
   </pre>
   <p>This must do what you think it does.
   </p>
   <p>Alert members of the audience will immediately notice that buffering
      is going to make a hash of the output unless special steps are taken.
   </p>
   <p>The special steps taken by libstdc++, at least for version 3.0,
      involve doing very little buffering for the standard streams, leaving
      most of the buffering to the underlying C library.  (This kind of
      thing is <a href="../explanations.html#cstdio">tricky to get right</a>.)
      The upside is that correctness is ensured.  The downside is that
      writing through <code>cout</code> can quite easily lead to awful
      performance when the C++ I/O library is layered on top of the C I/O
      library (as it is for 3.0 by default).  Some patches have been applied
      which improve the situation for 3.1.
   </p>
   <p>However, the C and C++ standard streams only need to be kept in sync
      when both libraries' facilities are in use.  If your program only uses
      C++ I/O, then there's no need to sync with the C streams.  The right
      thing to do in this case is to call
   </p>
   <pre>
     #include <em>any of the I/O headers such as ios, iostream, etc</em>
 
     std::ios::sync_with_stdio(false);
   </pre>
   <p>You must do this before performing any I/O via the C++ stream objects.
      Once you call this, the C++ streams will operate independently of the
      (unused) C streams.  For GCC 3.x, this means that <code>cout</code> and
      company will become fully buffered on their own.
   </p>
   <p>Note, by the way, that the synchronization requirement only applies to
      the standard streams (<code>cin</code>, <code>cout</code>,
      <code>cerr</code>,
      <code>clog</code>, and their wide-character counterparts).  File stream
      objects that you declare yourself have no such requirement and are fully
      buffered.
   </p>
 
<hr />
<h2><a name="9">Threads and I/O</a></h2>
   <p>I'll assume that you have already read the
      <a href="../17_intro/howto.html#3">general notes on library threads</a>,
      and the
      <a href="../23_containers/howto.html#3">notes on threaded container
      access</a> (you might not think of an I/O stream as a container, but
      the points made there also hold here).  If you have not read them,
      please do so first.
   </p>
   <p>This gets a bit tricky.  Please read carefully, and bear with me.
   </p>
   <h3>Structure</h3>
   <p>As described <a href="../explanations.html#cstdio">here</a>, a wrapper
      type called <code>__basic_file</code> provides our abstraction layer
      for the <code>std::filebuf</code> classes.  Nearly all decisions dealing
      with actual input and output must be made in <code>__basic_file</code>.
   </p>
   <p>A generic locking mechanism is somewhat in place at the filebuf layer,
      but is not used in the current code.  Providing locking at any higher
      level is akin to providing locking within containers, and is not done
      for the same reasons (see the links above).
   </p>
   <h3>The defaults for 3.0.x</h3>
   <p>The __basic_file type is simply a collection of small wrappers around
      the C stdio layer (again, see the link under Structure).  We do no
      locking ourselves, but simply pass through to calls to <code>fopen</code>,
      <code>fwrite</code>, and so forth.
   </p>
   <p>So, for 3.0, the question of &quot;is multithreading safe for I/O&quot;
      must be answered with, &quot;is your platform's C library threadsafe
      for I/O?&quot;  Some are by default, some are not; many offer multiple
      implementations of the C library with varying tradeoffs of threadsafety
      and efficiency.  You, the programmer, are always required to take care
      with multiple threads.
   </p>
   <p>(As an example, the POSIX standard requires that C stdio FILE*
       operations are atomic.  POSIX-conforming C libraries (e.g, on Solaris
       and GNU/Linux) have an internal mutex to serialize operations on
       FILE*s.  However, you still need to not do stupid things like calling
       <code>fclose(fs)</code> in one thread followed by an access of
       <code>fs</code> in another.)
   </p>
   <p>So, if your platform's C library is threadsafe, then your
      <code>fstream</code> I/O operations will be threadsafe at the lowest
      level.  For higher-level operations, such as manipulating the data
      contained in the stream formatting classes (e.g., setting up callbacks
      inside an <code>std::ofstream</code>), you need to guard such accesses
      like any other critical shared resource.
   </p>
   <h3>The future</h3>
   <p>As already mentioned <a href="../explanations.html#cstdio">here</a>, a
      second choice is available for I/O implementations:  libio.  This is
      disabled by default, and in fact will not currently work due to other
      issues.  It will be revisited, however.
   </p>
   <p>The libio code is a subset of the guts of the GNU libc (glibc) I/O
      implementation.  When libio is in use, the <code>__basic_file</code>
      type is basically derived from FILE.  (The real situation is more
      complex than that... it's derived from an internal type used to
      implement FILE.  See libio/libioP.h to see scary things done with
      vtbls.)  The result is that there is no &quot;layer&quot; of C stdio
      to go through; the filebuf makes calls directly into the same
      functions used to implement <code>fread</code>, <code>fwrite</code>,
      and so forth, using internal data structures.  (And when I say
      &quot;makes calls directly,&quot; I mean the function is literally
      replaced by a jump into an internal function.  Fast but frightening.
      *grin*)
   </p>
   <p>Also, the libio internal locks are used.  This requires pulling in
      large chunks of glibc, such as a pthreads implementation, and is one
      of the issues preventing widespread use of libio as the libstdc++
      cstdio implementation.
   </p>
   <p>But we plan to make this work, at least as an option if not a future
      default.  Platforms running a copy of glibc with a recent-enough
      version will see calls from libstdc++ directly into the glibc already
      installed.  For other platforms, a copy of the libio subsection will
      be built and included in libstdc++.
   </p>
   <h3>Alternatives</h3>
   <p>Don't forget that other cstdio implemenations are possible.  You could
      easily write one to perform your own forms of locking, to solve your
      &quot;interesting&quot; problems.
   </p>
 
<hr />
<h2><a name="10">Which header?</a></h2>
   <p>To minimize the time you have to wait on the compiler, it's good to
      only include the headers you really need.  Many people simply include
      &lt;iostream&gt; when they don't need to -- and that can <em>penalize
      your runtime as well.</em>  Here are some tips on which header to use
      for which situations, starting with the simplest.
   </p>
   <p><strong>&lt;iosfwd&gt;</strong> should be included whenever you simply
      need the <em>name</em> of an I/O-related class, such as
      &quot;ofstream&quot; or &quot;basic_streambuf&quot;.  Like the name
      implies, these are forward declarations.  (A word to all you fellow
      old school programmers:  trying to forward declare classes like
      &quot;class istream;&quot; won't work.  Look in the iosfwd header if
      you'd like to know why.)  For example,
   </p>
   <pre>
    #include &lt;iosfwd&gt;
 
    class MyClass
    {
        ....
        std::ifstream&amp;   input_file;
    };
 
    extern std::ostream&amp; operator&lt;&lt; (std::ostream&amp;, MyClass&amp;);
   </pre>
   <p><strong>&lt;ios&gt;</strong> declares the base classes for the entire
      I/O stream hierarchy, std::ios_base and std::basic_ios&lt;charT&gt;, the
      counting types std::streamoff and std::streamsize, the file
      positioning type std::fpos, and the various manipulators like
      std::hex, std::fixed, std::noshowbase, and so forth.
   </p>
   <p>The ios_base class is what holds the format flags, the state flags,
      and the functions which change them (setf(), width(), precision(),
      etc).  You can also store extra data and register callback functions
      through ios_base, but that has been historically underused.  Anything
      which doesn't depend on the type of characters stored is consolidated
      here.
   </p>
   <p>The template class basic_ios is the highest template class in the
      hierarchy; it is the first one depending on the character type, and
      holds all general state associated with that type:  the pointer to the
      polymorphic stream buffer, the facet information, etc.
   </p>
   <p><strong>&lt;streambuf&gt;</strong> declares the template class
      basic_streambuf, and two standard instantiations, streambuf and
      wstreambuf.  If you need to work with the vastly useful and capable
      stream buffer classes, e.g., to create a new form of storage
      transport, this header is the one to include.
   </p>
   <p><strong>&lt;istream&gt;</strong>/<strong>&lt;ostream&gt;</strong> are
      the headers to include when you are using the &gt;&gt;/&lt;&lt;
      interface, or any of the other abstract stream formatting functions.
      For example,
   </p>
   <pre>
    #include &lt;istream&gt;
 
    std::ostream&amp; operator&lt;&lt; (std::ostream&amp; os, MyClass&amp; c)
    {
       return os &lt;&lt; c.data1() &lt;&lt; c.data2();
    }
   </pre>
   <p>The std::istream and std::ostream classes are the abstract parents of
      the various concrete implementations.  If you are only using the
      interfaces, then you only need to use the appropriate interface header.
   </p>
   <p><strong>&lt;iomanip&gt;</strong> provides &quot;extractors and inserters
      that alter information maintained by class ios_base and its dervied
      classes,&quot; such as std::setprecision and std::setw.  If you need
      to write expressions like <code>os &lt;&lt; setw(3);</code> or
      <code>is &gt;&gt; setbase(8);</code>, you must include &lt;iomanip&gt;.
   </p>
   <p><strong>&lt;sstream&gt;</strong>/<strong>&lt;fstream&gt;</strong>
      declare the six stringstream and fstream classes.  As they are the
      standard concrete descendants of istream and ostream, you will already
      know about them.
   </p>
   <p>Finally, <strong>&lt;iostream&gt;</strong> provides the eight standard
      global objects (cin, cout, etc).  To do this correctly, this header
      also provides the contents of the &lt;istream&gt; and &lt;ostream&gt;
      headers, but nothing else.  The contents of this header look like
   </p>
   <pre>
    #include &lt;ostream&gt;
    #include &lt;istream&gt;
 
    namespace std
    {
        extern istream cin;
        extern ostream cout;
        ....
 
        // this is explained below
        <strong>static ios_base::Init __foo;</strong>    // not its real name
    }
   </pre>
   <p>Now, the runtime penalty mentioned previously:  the global objects
      must be initialized before any of your own code uses them; this is
      guaranteed by the standard.  Like any other global object, they must
      be initialized once and only once.  This is typically done with a
      construct like the one above, and the nested class ios_base::Init is
      specified in the standard for just this reason.
   </p>
   <p>How does it work?  Because the header is included before any of your
      code, the <strong>__foo</strong> object is constructed before any of
      your objects.  (Global objects are built in the order in which they
      are declared, and destroyed in reverse order.)  The first time the
      constructor runs, the eight stream objects are set up.
   </p>
   <p>The <code>static</code> keyword means that each object file compiled
      from a source file containing &lt;iostream&gt; will have its own
      private copy of <strong>__foo</strong>.  There is no specified order
      of construction across object files (it's one of those pesky NP
      problems that make life so interesting), so one copy in each object
      file means that the stream objects are guaranteed to be set up before
      any of your code which uses them could run, thereby meeting the
      requirements of the standard.
   </p>
   <p>The penalty, of course, is that after the first copy of
      <strong>__foo</strong> is constructed, all the others are just wasted
      processor time.  The time spent is merely for an increment-and-test
      inside a function call, but over several dozen or hundreds of object
      files, that time can add up.  (It's not in a tight loop, either.)
   </p>
   <p>The lesson?  Only include &lt;iostream&gt; when you need to use one of
      the standard objects in that source file; you'll pay less startup
      time.  Only include the header files you need to in general; your
      compile times will go down when there's less parsing work to do.
   </p>
 
 
<hr />
<h2><a name="11">Using FILE*s and file descriptors with IOStreams</a></h2>
   <!-- referenced by ext/howto.html#2, update link if numbering changes -->
   <p>The v2 library included non-standard extensions to construct
      <code>std::filebuf</code>s from C stdio types such as
      <code>FILE*</code>s and POSIX file descriptors.
      Today the recommended way to use stdio types with libstdc++-v3
      IOStreams is via the <code>stdio_filebuf</code> class (see below),
      but earlier releases provided slightly different mechanisms.
   </p>
   <ul>
     <li>3.0.x <code>filebuf</code>s have another ctor with this signature:
         <br />
        <code>basic_filebuf(__c_file_type*, ios_base::openmode, int_type);</code>
         <br />This comes in very handy in a number of places, such as
         attaching Unix sockets, pipes, and anything else which uses file
         descriptors, into the IOStream buffering classes.  The three
         arguments are as follows:
         <ul>
          <li><code>__c_file_type*      F   </code>
              // the __c_file_type typedef usually boils down to stdio's FILE
          </li>
          <li><code>ios_base::openmode  M   </code>
              // same as all the other uses of openmode
          </li>
          <li><code>int_type            B   </code>
              // buffer size, defaults to BUFSIZ if not specified
          </li>
         </ul>
         For those wanting to use file descriptors instead of FILE*'s, I
         invite you to contemplate the mysteries of C's <code>fdopen()</code>.
     </li>
     <li>In library snapshot 3.0.95 and later, <code>filebuf</code>s bring
         back an old extension:  the <code>fd()</code> member function.  The
         integer returned from this function can be used for whatever file
         descriptors can be used for on your platform.  Naturally, the
         library cannot track what you do on your own with a file descriptor,
         so if you perform any I/O directly, don't expect the library to be
         aware of it.
     </li>
     <li>Beginning with 3.1, the extra <code>filebuf</code> constructor and
         the <code>fd()</code> function were removed from the standard
         filebuf.  Instead, <code>&lt;ext/stdio_filebuf.h&gt;</code> contains
         a derived class called
         <a href="http://gcc.gnu.org/onlinedocs/libstdc++/latest-doxygen/class____gnu__cxx_1_1stdio__filebuf.html"><code>__gnu_cxx::stdio_filebuf</code></a>.
         This class can be constructed from a C <code>FILE*</code> or a file
         descriptor, and provides the <code>fd()</code> function.
     </li>
   </ul>
   <p>If you want to access a <code>filebuf</code>s file descriptor to
      implement file locking (e.g. using the <code>fcntl()</code> system
      call) then you might be interested in Henry Suter's
      <a href="http://suter.home.cern.ch/suter/RWLock.html">RWLock</a>
      class.
   </p>
 
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<p class="fineprint"><em>
See <a href="../17_intro/license.html">license.html</a> for copying conditions.
Comments and suggestions are welcome, and may be sent to
<a href="mailto:libstdc++@gcc.gnu.org">the libstdc++ mailing list</a>.
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