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  11     "><link rel="home" href="../index.html" title="The GNU C++ Library"><link rel="up" href="io.html" title="Chapter 13.  Input and Output"><link rel="prev" href="stringstreams.html" title="Memory Based Streams"><link rel="next" href="io_and_c.html" title="Interacting with C"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">File Based Streams</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="stringstreams.html">Prev</a> </td><th width="60%" align="center">Chapter 13.
  12   Input and Output
  13
  14 </th><td width="20%" align="right"> <a accesskey="n" href="io_and_c.html">Next</a></td></tr></table><hr></div><div class="section" title="File Based Streams"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="std.io.filestreams"></a>File Based Streams</h2></div></div></div><div class="section" title="Copying a File"><div class="titlepage"><div><div><h3 class="title"><a name="std.io.filestreams.copying_a_file"></a>Copying a File</h3></div></div></div><p>
  15   </p><p>So you want to copy a file quickly and easily, and most important,
  16       completely portably.  And since this is C++, you have an open
  17       ifstream (call it IN) and an open ofstream (call it OUT):
  18    </p><pre class="programlisting">
  19    #include &lt;fstream&gt;
  20
  21    std::ifstream  IN ("input_file");
  22    std::ofstream  OUT ("output_file"); </pre><p>Here's the easiest way to get it completely wrong:
  23    </p><pre class="programlisting">
  24    OUT &lt;&lt; IN;</pre><p>For those of you who don't already know why this doesn't work
  25       (probably from having done it before), I invite you to quickly
  26       create a simple text file called "input_file" containing
  27       the sentence
  28    </p><pre class="programlisting">
  29       The quick brown fox jumped over the lazy dog.</pre><p>surrounded by blank lines.  Code it up and try it.  The contents
  30       of "output_file" may surprise you.
  31    </p><p>Seriously, go do it.  Get surprised, then come back.  It's worth it.
  32    </p><p>The thing to remember is that the <code class="code">basic_[io]stream</code> classes
  33       handle formatting, nothing else.  In chaptericular, they break up on
  34       whitespace.  The actual reading, writing, and storing of data is
  35       handled by the <code class="code">basic_streambuf</code> family.  Fortunately, the
  36       <code class="code">operator&lt;&lt;</code> is overloaded to take an ostream and
  37       a pointer-to-streambuf, in order to help with just this kind of
  38       "dump the data verbatim" situation.
  39    </p><p>Why a <span class="emphasis"><em>pointer</em></span> to streambuf and not just a streambuf?  Well,
  40       the [io]streams hold pointers (or references, depending on the
  41       implementation) to their buffers, not the actual
  42       buffers.  This allows polymorphic behavior on the chapter of the buffers
  43       as well as the streams themselves.  The pointer is easily retrieved
  44       using the <code class="code">rdbuf()</code> member function.  Therefore, the easiest
  45       way to copy the file is:
  46    </p><pre class="programlisting">
  47    OUT &lt;&lt; IN.rdbuf();</pre><p>So what <span class="emphasis"><em>was</em></span> happening with OUT&lt;&lt;IN?  Undefined
  48       behavior, since that chaptericular &lt;&lt; isn't defined by the Standard.
  49       I have seen instances where it is implemented, but the character
  50       extraction process removes all the whitespace, leaving you with no
  51       blank lines and only "Thequickbrownfox...".  With
  52       libraries that do not define that operator, IN (or one of IN's
  53       member pointers) sometimes gets converted to a void*, and the output
  54       file then contains a perfect text representation of a hexadecimal
  55       address (quite a big surprise).  Others don't compile at all.
  56    </p><p>Also note that none of this is specific to o<span class="emphasis"><em>*f*</em></span>streams.
  57       The operators shown above are all defined in the parent
  58       basic_ostream class and are therefore available with all possible
  59       descendants.
  60    </p></div><div class="section" title="Binary Input and Output"><div class="titlepage"><div><div><h3 class="title"><a name="std.io.filestreams.binary"></a>Binary Input and Output</h3></div></div></div><p>
  61     </p><p>The first and most important thing to remember about binary I/O is
  62       that opening a file with <code class="code">ios::binary</code> is not, repeat
  63       <span class="emphasis"><em>not</em></span>, the only thing you have to do.  It is not a silver
  64       bullet, and will not allow you to use the <code class="code">&lt;&lt;/&gt;&gt;</code>
  65       operators of the normal fstreams to do binary I/O.
  66    </p><p>Sorry.  Them's the breaks.
  67    </p><p>This isn't going to try and be a complete tutorial on reading and
  68       writing binary files (because "binary"
  69       covers a lot of ground), but we will try and clear
  70       up a couple of misconceptions and common errors.
  71    </p><p>First, <code class="code">ios::binary</code> has exactly one defined effect, no more
  72       and no less.  Normal text mode has to be concerned with the newline
  73       characters, and the runtime system will translate between (for
  74       example) '\n' and the appropriate end-of-line sequence (LF on Unix,
  75       CRLF on DOS, CR on Macintosh, etc).  (There are other things that
  76       normal mode does, but that's the most obvious.)  Opening a file in
  77       binary mode disables this conversion, so reading a CRLF sequence
  78       under Windows won't accidentally get mapped to a '\n' character, etc.
  79       Binary mode is not supposed to suddenly give you a bitstream, and
  80       if it is doing so in your program then you've discovered a bug in
  81       your vendor's compiler (or some other chapter of the C++ implementation,
  82       possibly the runtime system).
  83    </p><p>Second, using <code class="code">&lt;&lt;</code> to write and <code class="code">&gt;&gt;</code> to
  84       read isn't going to work with the standard file stream classes, even
  85       if you use <code class="code">skipws</code> during reading.  Why not?  Because
  86       ifstream and ofstream exist for the purpose of <span class="emphasis"><em>formatting</em></span>,
  87       not reading and writing.  Their job is to interpret the data into
  88       text characters, and that's exactly what you don't want to happen
  89       during binary I/O.
  90    </p><p>Third, using the <code class="code">get()</code> and <code class="code">put()/write()</code> member
  91       functions still aren't guaranteed to help you.  These are
  92       "unformatted" I/O functions, but still character-based.
  93       (This may or may not be what you want, see below.)
  94    </p><p>Notice how all the problems here are due to the inappropriate use
  95       of <span class="emphasis"><em>formatting</em></span> functions and classes to perform something
  96       which <span class="emphasis"><em>requires</em></span> that formatting not be done?  There are a
  97       seemingly infinite number of solutions, and a few are listed here:
  98    </p><div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem"><p><span class="quote">“<span class="quote">Derive your own fstream-type classes and write your own
  99           &lt;&lt;/&gt;&gt; operators to do binary I/O on whatever data
 100           types you're using.</span>”</span>
 101         </p><p>
 102           This is a Bad Thing, because while
 103           the compiler would probably be just fine with it, other humans
 104           are going to be confused.  The overloaded bitshift operators
 105           have a well-defined meaning (formatting), and this breaks it.
 106         </p></li><li class="listitem"><p>
 107           <span class="quote">“<span class="quote">Build the file structure in memory, then
 108           <code class="code">mmap()</code> the file and copy the
 109           structure.
 110         </span>”</span>
 111         </p><p>
 112           Well, this is easy to make work, and easy to break, and is
 113           pretty equivalent to using <code class="code">::read()</code> and
 114           <code class="code">::write()</code> directly, and makes no use of the
 115           iostream library at all...
 116           </p></li><li class="listitem"><p>
 117           <span class="quote">“<span class="quote">Use streambufs, that's what they're there for.</span>”</span>
 118         </p><p>
 119           While not trivial for the beginner, this is the best of all
 120           solutions.  The streambuf/filebuf layer is the layer that is
 121           responsible for actual I/O.  If you want to use the C++
 122           library for binary I/O, this is where you start.
 123         </p></li></ul></div><p>How to go about using streambufs is a bit beyond the scope of this
 124       document (at least for now), but while streambufs go a long way,
 125       they still leave a couple of things up to you, the programmer.
 126       As an example, byte ordering is completely between you and the
 127       operating system, and you have to handle it yourself.
 128    </p><p>Deriving a streambuf or filebuf
 129       class from the standard ones, one that is specific to your data
 130       types (or an abstraction thereof) is probably a good idea, and
 131       lots of examples exist in journals and on Usenet.  Using the
 132       standard filebufs directly (either by declaring your own or by
 133       using the pointer returned from an fstream's <code class="code">rdbuf()</code>)
 134       is certainly feasible as well.
 135    </p><p>One area that causes problems is trying to do bit-by-bit operations
 136       with filebufs.  C++ is no different from C in this respect:  I/O
 137       must be done at the byte level.  If you're trying to read or write
 138       a few bits at a time, you're going about it the wrong way.  You
 139       must read/write an integral number of bytes and then process the
 140       bytes.  (For example, the streambuf functions take and return
 141       variables of type <code class="code">int_type</code>.)
 142    </p><p>Another area of problems is opening text files in binary mode.
 143       Generally, binary mode is intended for binary files, and opening
 144       text files in binary mode means that you now have to deal with all of
 145       those end-of-line and end-of-file problems that we mentioned before.
 146    </p><p>
 147       An instructive thread from comp.lang.c++.moderated delved off into
 148       this topic starting more or less at
 149       <a class="link" href="http://groups.google.com/group/comp.std.c++/browse_thread/thread/f87b4abd7954a87/946a3eb9921e382d?q=comp.std.c%2B%2B+binary+iostream#946a3eb9921e382d" target="_top">this</a>
 150       post and continuing to the end of the thread. (The subject heading is "binary iostreams" on both comp.std.c++
 151       and comp.lang.c++.moderated.) Take special note of the replies by James Kanze and Dietmar Kühl.
 152    </p><p>Briefly, the problems of byte ordering and type sizes mean that
 153       the unformatted functions like <code class="code">ostream::put()</code> and
 154       <code class="code">istream::get()</code> cannot safely be used to communicate
 155       between arbitrary programs, or across a network, or from one
 156       invocation of a program to another invocation of the same program
 157       on a different platform, etc.
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