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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!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=" ISO C++ , library "/><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 <fstream>
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 << 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<<</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 << IN.rdbuf();</pre><p>So what <span class="emphasis"><em>was</em></span> happening with OUT<<IN? Undefined
behavior, since that chaptericular << 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"><</>></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"><<</code> to write and <code class="code">>></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
<</>> 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>
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