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+<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
+<html xmlns="http://www.w3.org/1999/xhtml"><head><title>File Based Streams</title><meta name="generator" content="DocBook XSL-NS Stylesheets V1.76.1"/><meta name="keywords" content="&#10; ISO C++&#10; , &#10; library&#10; "/><link rel="home" href="../spine.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><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">File Based Streams</th></tr><tr><td align="left"><a accesskey="p" href="stringstreams.html">Prev</a> </td><th width="60%" align="center">Chapter 13. 
+ Input and Output
+
+</th><td 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"><a id="std.io.filestreams"/>File Based Streams</h2></div></div></div><div class="section" title="Copying a File"><div class="titlepage"><div><div><h3 class="title"><a id="std.io.filestreams.copying_a_file"/>Copying a File</h3></div></div></div><p>
+ </p><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 class="programlisting">
+ #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 class="programlisting">
+ 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 "input_file" containing
+ the sentence
+ </p><pre class="programlisting">
+ The quick brown fox jumped over the lazy dog.</pre><p>surrounded by blank lines. Code it up and try it. The contents
+ of "output_file" may surprise you.
+ </p><p>Seriously, go do it. Get surprised, then come back. It's worth it.
+ </p><p>The thing to remember is that the <code class="code">basic_[io]stream</code> classes
+ handle formatting, nothing else. In chaptericular, they break up on
+ whitespace. The actual reading, writing, and storing of data is
+ handled by the <code class="code">basic_streambuf</code> family. Fortunately, the
+ <code class="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
+ "dump the data verbatim" situation.
+ </p><p>Why a <span class="emphasis"><em>pointer</em></span> 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 chapter of the buffers
+ as well as the streams themselves. The pointer is easily retrieved
+ using the <code class="code">rdbuf()</code> member function. Therefore, the easiest
+ way to copy the file is:
+ </p><pre class="programlisting">
+ OUT &lt;&lt; IN.rdbuf();</pre><p>So what <span class="emphasis"><em>was</em></span> happening with OUT&lt;&lt;IN? Undefined
+ behavior, since that chaptericular &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 "Thequickbrownfox...". 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 hexadecimal
+ address (quite a big surprise). Others don't compile at all.
+ </p><p>Also note that none of this is specific to o<span class="emphasis"><em>*f*</em></span>streams.
+ The operators shown above are all defined in the parent
+ basic_ostream class and are therefore available with all possible
+ descendants.
+ </p></div><div class="section" title="Binary Input and Output"><div class="titlepage"><div><div><h3 class="title"><a id="std.io.filestreams.binary"/>Binary Input and Output</h3></div></div></div><p>
+ </p><p>The first and most important thing to remember about binary I/O is
+ that opening a file with <code class="code">ios::binary</code> is not, repeat
+ <span class="emphasis"><em>not</em></span>, the only thing you have to do. It is not a silver
+ bullet, and will not allow you to use the <code class="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 "binary"
+ covers a lot of ground), but we will try and clear
+ up a couple of misconceptions and common errors.
+ </p><p>First, <code class="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 chapter of the C++ implementation,
+ possibly the runtime system).
+ </p><p>Second, using <code class="code">&lt;&lt;</code> to write and <code class="code">&gt;&gt;</code> to
+ read isn't going to work with the standard file stream classes, even
+ if you use <code class="code">skipws</code> during reading. Why not? Because
+ ifstream and ofstream exist for the purpose of <span class="emphasis"><em>formatting</em></span>,
+ 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 class="code">get()</code> and <code class="code">put()/write()</code> member
+ functions still aren't guaranteed to help you. These are
+ "unformatted" 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 <span class="emphasis"><em>formatting</em></span> functions and classes to perform something
+ which <span class="emphasis"><em>requires</em></span> that formatting not be done? There are a
+ seemingly infinite number of solutions, and a few are listed here:
+ </p><div class="itemizedlist"><ul class="itemizedlist"><li class="listitem"><p><span class="quote">“<span class="quote">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.</span>”</span>
+ </p><p>
+ 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.
+ </p></li><li class="listitem"><p>
+ <span class="quote">“<span class="quote">Build the file structure in memory, then
+ <code class="code">mmap()</code> the file and copy the
+ structure.
+ </span>”</span>
+ </p><p>
+ Well, this is easy to make work, and easy to break, and is
+ pretty equivalent to using <code class="code">::read()</code> and
+ <code class="code">::write()</code> directly, and makes no use of the
+ iostream library at all...
+ </p></li><li class="listitem"><p>
+ <span class="quote">“<span class="quote">Use streambufs, that's what they're there for.</span>”</span>
+ </p><p>
+ 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.
+ </p></li></ul></div><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 class="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 class="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.
+ </p><p>
+ An instructive thread from comp.lang.c++.moderated delved off into
+ this topic starting more or less at
+ <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">this</a>
+ post and continuing to the end of the thread. (The subject heading is "binary iostreams" on both comp.std.c++
+ and comp.lang.c++.moderated.) Take special note of the replies by James Kanze and Dietmar Kühl.
+ </p><p>Briefly, the problems of byte ordering and type sizes mean that
+ the unformatted functions like <code class="code">ostream::put()</code> and
+ <code class="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></div></div><div class="navfooter"><hr/><table width="100%" summary="Navigation footer"><tr><td align="left"><a accesskey="p" href="stringstreams.html">Prev</a> </td><td align="center"><a accesskey="u" href="io.html">Up</a></td><td align="right"> <a accesskey="n" href="io_and_c.html">Next</a></td></tr><tr><td align="left" valign="top">Memory Based Streams </td><td align="center"><a accesskey="h" href="../spine.html">Home</a></td><td align="right" valign="top"> Interacting with C</td></tr></table></div></body></html>