mirror of
https://github.com/autc04/Retro68.git
synced 2024-11-30 19:53:46 +00:00
151 lines
12 KiB
HTML
151 lines
12 KiB
HTML
|
<?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 "/><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 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="../index.html">Home</a></td><td align="right" valign="top"> Interacting with C</td></tr></table></div></body></html>
|