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| <html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Chapter 27. File Based Streams</title><meta name="generator" content="DocBook XSL Stylesheets V1.74.0" /><meta name="keywords" content=" ISO C++ , library " /><link rel="home" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="io.html" title="Part XI. Input and Output" /><link rel="prev" href="stringstreams.html" title="Chapter 26. Memory Based Streams" /><link rel="next" href="bk01pt11ch27s02.html" title="Binary Input and Output" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 27. 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">Part XI. |
| Input and Output |
| |
| </th><td width="20%" align="right"> <a accesskey="n" href="bk01pt11ch27s02.html">Next</a></td></tr></table><hr /></div><div class="chapter" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title"><a id="manual.io.filestreams"></a>Chapter 27. File Based Streams</h2></div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="sect1"><a href="fstreams.html#manual.io.filestreams.copying_a_file">Copying a File</a></span></dt><dt><span class="sect1"><a href="bk01pt11ch27s02.html">Binary Input and Output</a></span></dt><dt><span class="sect1"><a href="bk01pt11ch27s03.html">More Binary Input and Output</a></span></dt></dl></div><div class="sect1" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="manual.io.filestreams.copying_a_file"></a>Copying a File</h2></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 part 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><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="bk01pt11ch27s02.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 26. Memory Based Streams </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> Binary Input and Output</td></tr></table></div></body></html> |
