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<!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 Stylesheets Vsnapshot" /><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.
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Input and Output
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</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>
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</p><p>So you want to copy a file quickly and easily, and most important,
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completely portably. And since this is C++, you have an open
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ifstream (call it IN) and an open ofstream (call it OUT):
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</p><pre class="programlisting">
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#include <fstream>
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std::ifstream IN ("input_file");
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std::ofstream OUT ("output_file"); </pre><p>Here's the easiest way to get it completely wrong:
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</p><pre class="programlisting">
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OUT << IN;</pre><p>For those of you who don't already know why this doesn't work
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(probably from having done it before), I invite you to quickly
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create a simple text file called "input_file" containing
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the sentence
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</p><pre class="programlisting">
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The quick brown fox jumped over the lazy dog.</pre><p>surrounded by blank lines. Code it up and try it. The contents
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of "output_file" may surprise you.
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</p><p>Seriously, go do it. Get surprised, then come back. It's worth it.
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</p><p>The thing to remember is that the <code class="code">basic_[io]stream</code> classes
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handle formatting, nothing else. In particular, they break up on
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whitespace. The actual reading, writing, and storing of data is
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handled by the <code class="code">basic_streambuf</code> family. Fortunately, the
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<code class="code">operator<<</code> is overloaded to take an ostream and
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a pointer-to-streambuf, in order to help with just this kind of
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"dump the data verbatim" situation.
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</p><p>Why a <span class="emphasis"><em>pointer</em></span> to streambuf and not just a streambuf? Well,
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the [io]streams hold pointers (or references, depending on the
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implementation) to their buffers, not the actual
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buffers. This allows polymorphic behavior on the chapter of the buffers
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as well as the streams themselves. The pointer is easily retrieved
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using the <code class="code">rdbuf()</code> member function. Therefore, the easiest
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way to copy the file is:
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</p><pre class="programlisting">
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OUT << IN.rdbuf();</pre><p>So what <span class="emphasis"><em>was</em></span> happening with OUT<<IN? Undefined
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behavior, since that particular << isn't defined by the Standard.
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I have seen instances where it is implemented, but the character
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extraction process removes all the whitespace, leaving you with no
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blank lines and only "Thequickbrownfox...". With
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libraries that do not define that operator, IN (or one of IN's
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member pointers) sometimes gets converted to a void*, and the output
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file then contains a perfect text representation of a hexadecimal
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address (quite a big surprise). Others don't compile at all.
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</p><p>Also note that none of this is specific to o<span class="emphasis"><em>*f*</em></span>streams.
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The operators shown above are all defined in the parent
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basic_ostream class and are therefore available with all possible
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descendants.
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</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>
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</p><p>The first and most important thing to remember about binary I/O is
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that opening a file with <code class="code">ios::binary</code> is not, repeat
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<span class="emphasis"><em>not</em></span>, the only thing you have to do. It is not a silver
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bullet, and will not allow you to use the <code class="code"><</>></code>
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operators of the normal fstreams to do binary I/O.
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</p><p>Sorry. Them's the breaks.
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</p><p>This isn't going to try and be a complete tutorial on reading and
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writing binary files (because "binary"
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covers a lot of ground), but we will try and clear
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up a couple of misconceptions and common errors.
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</p><p>First, <code class="code">ios::binary</code> has exactly one defined effect, no more
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and no less. Normal text mode has to be concerned with the newline
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characters, and the runtime system will translate between (for
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example) '\n' and the appropriate end-of-line sequence (LF on Unix,
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CRLF on DOS, CR on Macintosh, etc). (There are other things that
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normal mode does, but that's the most obvious.) Opening a file in
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binary mode disables this conversion, so reading a CRLF sequence
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under Windows won't accidentally get mapped to a '\n' character, etc.
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Binary mode is not supposed to suddenly give you a bitstream, and
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if it is doing so in your program then you've discovered a bug in
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your vendor's compiler (or some other chapter of the C++ implementation,
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possibly the runtime system).
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</p><p>Second, using <code class="code"><<</code> to write and <code class="code">>></code> to
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read isn't going to work with the standard file stream classes, even
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if you use <code class="code">skipws</code> during reading. Why not? Because
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ifstream and ofstream exist for the purpose of <span class="emphasis"><em>formatting</em></span>,
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not reading and writing. Their job is to interpret the data into
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text characters, and that's exactly what you don't want to happen
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during binary I/O.
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</p><p>Third, using the <code class="code">get()</code> and <code class="code">put()/write()</code> member
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functions still aren't guaranteed to help you. These are
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"unformatted" I/O functions, but still character-based.
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(This may or may not be what you want, see below.)
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</p><p>Notice how all the problems here are due to the inappropriate use
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of <span class="emphasis"><em>formatting</em></span> functions and classes to perform something
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which <span class="emphasis"><em>requires</em></span> that formatting not be done? There are a
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seemingly infinite number of solutions, and a few are listed here:
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</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
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<</>> operators to do binary I/O on whatever data
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types you're using.</span>”</span>
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</p><p>
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This is a Bad Thing, because while
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the compiler would probably be just fine with it, other humans
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are going to be confused. The overloaded bitshift operators
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have a well-defined meaning (formatting), and this breaks it.
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</p></li><li class="listitem"><p>
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<span class="quote">“<span class="quote">Build the file structure in memory, then
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<code class="code">mmap()</code> the file and copy the
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structure.
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</span>”</span>
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</p><p>
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Well, this is easy to make work, and easy to break, and is
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pretty equivalent to using <code class="code">::read()</code> and
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<code class="code">::write()</code> directly, and makes no use of the
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iostream library at all...
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</p></li><li class="listitem"><p>
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<span class="quote">“<span class="quote">Use streambufs, that's what they're there for.</span>”</span>
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</p><p>
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While not trivial for the beginner, this is the best of all
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solutions. The streambuf/filebuf layer is the layer that is
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responsible for actual I/O. If you want to use the C++
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library for binary I/O, this is where you start.
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</p></li></ul></div><p>How to go about using streambufs is a bit beyond the scope of this
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document (at least for now), but while streambufs go a long way,
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they still leave a couple of things up to you, the programmer.
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As an example, byte ordering is completely between you and the
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operating system, and you have to handle it yourself.
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</p><p>Deriving a streambuf or filebuf
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class from the standard ones, one that is specific to your data
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types (or an abstraction thereof) is probably a good idea, and
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lots of examples exist in journals and on Usenet. Using the
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standard filebufs directly (either by declaring your own or by
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using the pointer returned from an fstream's <code class="code">rdbuf()</code>)
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is certainly feasible as well.
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</p><p>One area that causes problems is trying to do bit-by-bit operations
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with filebufs. C++ is no different from C in this respect: I/O
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must be done at the byte level. If you're trying to read or write
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a few bits at a time, you're going about it the wrong way. You
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must read/write an integral number of bytes and then process the
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bytes. (For example, the streambuf functions take and return
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variables of type <code class="code">int_type</code>.)
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</p><p>Another area of problems is opening text files in binary mode.
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Generally, binary mode is intended for binary files, and opening
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text files in binary mode means that you now have to deal with all of
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those end-of-line and end-of-file problems that we mentioned before.
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</p><p>
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An instructive thread from comp.lang.c++.moderated delved off into
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this topic starting more or less at
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<a class="link" href="https://groups.google.com/forum/#!topic/comp.std.c++/D4e0q9eVSoc" target="_top">this post</a>
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and continuing to the end of the thread. (The subject heading is "binary iostreams" on both comp.std.c++
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and comp.lang.c++.moderated.) Take special note of the replies by James Kanze and Dietmar Kühl.
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</p><p>Briefly, the problems of byte ordering and type sizes mean that
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the unformatted functions like <code class="code">ostream::put()</code> and
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<code class="code">istream::get()</code> cannot safely be used to communicate
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between arbitrary programs, or across a network, or from one
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invocation of a program to another invocation of the same program
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on a different platform, etc.
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</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> |