149 lines
12 KiB
HTML
149 lines
12 KiB
HTML
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
|
||
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>File Based Streams</title><meta name="generator" content="DocBook XSL-NS Stylesheets V1.78.1" /><meta name="keywords" content="ISO C++, library" /><meta name="keywords" content="ISO C++, runtime, library" /><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><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.
|
||
Input and Output
|
||
|
||
</th><td width="20%" align="right"> <a accesskey="n" href="io_and_c.html">Next</a></td></tr></table><hr /></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="std.io.filestreams"></a>File Based Streams</h2></div></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="std.io.filestreams.copying_a_file"></a>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 particular, 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 particular << 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"><div class="titlepage"><div><div><h3 class="title"><a id="std.io.filestreams.binary"></a>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" style="list-style-type: disc; "><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" target="_top">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 width="40%" align="left"><a accesskey="p" href="stringstreams.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="io.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="io_and_c.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Memory Based Streams </td><td width="20%" align="center"><a accesskey="h" href="../index.html">Home</a></td><td width="40%" align="right" valign="top"> Interacting with C</td></tr></table></div></body></html> |