1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html><meta http-equiv="Content-Type" content="text/html; charset=utf-8"> 3<title>A file handle - Boost.Outcome documentation</title> 4<link rel="stylesheet" href="../../../css/boost.css" type="text/css"> 5<meta name="generator" content="Hugo 0.52 with Boostdoc theme"> 6<meta name="viewport" content="width=device-width,initial-scale=1.0"/> 7 8<link rel="icon" href="../../../images/favicon.ico" type="image/ico"/> 9<body><div class="spirit-nav"> 10<a accesskey="p" href="../../../tutorial/advanced/constructors/two-phase-init.html"><img src="../../../images/prev.png" alt="Prev"></a> 11 <a accesskey="u" href="../../../tutorial/advanced/constructors.html"><img src="../../../images/up.png" alt="Up"></a> 12 <a accesskey="h" href="../../../index.html"><img src="../../../images/home.png" alt="Home"></a><a accesskey="n" href="../../../tutorial/advanced/constructors/static-constructor.html"><img src="../../../images/next.png" alt="Next"></a></div><div id="content"> 13 <div class="titlepage"><div><div><h1 style="clear: both">A file handle</h1></div></div></div> 14 <p>Borrowing from <a href="https://ned14.github.io/llfio/classllfio__v2__xxx_1_1file__handle.html"><code>llfio::file_handle</code></a> 15which uses this design pattern<sup class="footnote-ref" id="fnref:1"><a href="#fn:1">1</a></sup>, here is a simplified <code>file_handle</code> implementation:</p> 16 17<div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="k">class</span><span class="err"> </span><span class="nc">file_handle</span> 18<span class="p">{</span> 19 <span class="kt">int</span> <span class="n">_fd</span><span class="p">{</span><span class="o">-</span><span class="mi">1</span><span class="p">};</span> <span class="c1">// file descriptor 20</span><span class="c1"></span> <span class="k">struct</span> <span class="n">stat</span> <span class="n">_stat</span> 21 <span class="p">{</span> 22 <span class="mi">0</span> 23 <span class="p">};</span> <span class="c1">// stat of the fd at open 24</span><span class="c1"></span> 25 <span class="c1">// Phase 1 private constexpr constructor 26</span><span class="c1"></span> <span class="k">constexpr</span> <span class="nf">file_handle</span><span class="p">()</span> <span class="p">{}</span> 27 28<span class="k">public</span><span class="o">:</span> 29 <span class="k">using</span> <span class="n">path_type</span> <span class="o">=</span> <span class="n">filesystem</span><span class="o">::</span><span class="n">path</span><span class="p">;</span> 30 31 <span class="c1">//! The behaviour of the handle: does it read, read and write, or atomic append? 32</span><span class="c1"></span> <span class="k">enum</span> <span class="k">class</span><span class="err"> </span><span class="nc">mode</span> <span class="o">:</span> <span class="kt">unsigned</span> <span class="kt">char</span> <span class="c1">// bit 0 set means writable 33</span><span class="c1"></span> <span class="p">{</span> 34 <span class="n">unchanged</span> <span class="o">=</span> <span class="mi">0</span><span class="p">,</span> 35 <span class="n">none</span> <span class="o">=</span> <span class="mi">2</span><span class="p">,</span> <span class="c1">//!< No ability to read or write anything, but can synchronise (SYNCHRONIZE or 0) 36</span><span class="c1"></span> <span class="n">attr_read</span> <span class="o">=</span> <span class="mi">4</span><span class="p">,</span> <span class="c1">//!< Ability to read attributes (FILE_READ_ATTRIBUTES|SYNCHRONIZE or O_RDONLY) 37</span><span class="c1"></span> <span class="n">attr_write</span> <span class="o">=</span> <span class="mi">5</span><span class="p">,</span> <span class="c1">//!< Ability to read and write attributes (FILE_READ_ATTRIBUTES|FILE_WRITE_ATTRIBUTES|SYNCHRONIZE or O_RDONLY) 38</span><span class="c1"></span> <span class="n">read</span> <span class="o">=</span> <span class="mi">6</span><span class="p">,</span> <span class="c1">//!< Ability to read (READ_CONTROL|FILE_READ_DATA|FILE_READ_ATTRIBUTES|FILE_READ_EA|SYNCHRONISE or O_RDONLY) 39</span><span class="c1"></span> <span class="n">write</span> <span class="o">=</span> <span class="mi">7</span><span class="p">,</span> <span class="c1">//!< Ability to read and write (READ_CONTROL|FILE_READ_DATA|FILE_READ_ATTRIBUTES|FILE_READ_EA|FILE_WRITE_DATA|FILE_WRITE_ATTRIBUTES|FILE_WRITE_EA|FILE_APPEND_DATA|SYNCHRONISE or O_RDWR) 40</span><span class="c1"></span> <span class="n">append</span> <span class="o">=</span> <span class="mi">9</span> <span class="c1">//!< All mainstream OSs and CIFS guarantee this is atomic with respect to all other appenders (FILE_APPEND_DATA|SYNCHRONISE or O_APPEND) 41</span><span class="c1"></span> <span class="p">};</span> 42 43 <span class="c1">// Moves but not copies permitted 44</span><span class="c1"></span> <span class="n">file_handle</span><span class="p">(</span><span class="k">const</span> <span class="n">file_handle</span> <span class="o">&</span><span class="p">)</span> <span class="o">=</span> <span class="k">delete</span><span class="p">;</span> 45 <span class="n">file_handle</span><span class="p">(</span><span class="n">file_handle</span> <span class="o">&&</span><span class="n">o</span><span class="p">)</span> <span class="k">noexcept</span> <span class="o">:</span> <span class="n">_fd</span><span class="p">(</span><span class="n">o</span><span class="p">.</span><span class="n">_fd</span><span class="p">)</span> <span class="p">{</span> <span class="n">o</span><span class="p">.</span><span class="n">_fd</span> <span class="o">=</span> <span class="o">-</span><span class="mi">1</span><span class="p">;</span> <span class="p">}</span> 46 <span class="n">file_handle</span> <span class="o">&</span><span class="k">operator</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">file_handle</span> <span class="o">&</span><span class="p">)</span> <span class="o">=</span> <span class="k">delete</span><span class="p">;</span> 47 <span class="n">file_handle</span> <span class="o">&</span><span class="k">operator</span><span class="o">=</span><span class="p">(</span><span class="n">file_handle</span> <span class="o">&&</span><span class="n">o</span><span class="p">)</span> <span class="k">noexcept</span> 48 <span class="p">{</span> 49 <span class="k">this</span><span class="o">->~</span><span class="n">file_handle</span><span class="p">();</span> 50 <span class="k">new</span><span class="p">(</span><span class="k">this</span><span class="p">)</span> <span class="n">file_handle</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">o</span><span class="p">));</span> 51 <span class="k">return</span> <span class="o">*</span><span class="k">this</span><span class="p">;</span> 52 <span class="p">}</span> 53 <span class="c1">// Destruction closes the handle 54</span><span class="c1"></span> <span class="o">~</span><span class="n">file_handle</span><span class="p">()</span> 55 <span class="p">{</span> 56 <span class="k">if</span><span class="p">(</span><span class="n">_fd</span> <span class="o">!=</span> <span class="o">-</span><span class="mi">1</span><span class="p">)</span> 57 <span class="p">{</span> 58 <span class="k">if</span><span class="p">(</span><span class="o">-</span><span class="mi">1</span> <span class="o">==</span> <span class="o">::</span><span class="n">close</span><span class="p">(</span><span class="n">_fd</span><span class="p">))</span> 59 <span class="p">{</span> 60 <span class="kt">int</span> <span class="n">e</span> <span class="o">=</span> <span class="n">errno</span><span class="p">;</span> 61 <span class="n">std</span><span class="o">::</span><span class="n">cerr</span> <span class="o"><<</span> <span class="s">"FATAL: Closing the fd during destruction failed due to "</span> <span class="o"><<</span> <span class="n">strerror</span><span class="p">(</span><span class="n">e</span><span class="p">)</span> <span class="o"><<</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span> 62 <span class="n">std</span><span class="o">::</span><span class="n">terminate</span><span class="p">();</span> 63 <span class="p">}</span> 64 <span class="n">_fd</span> <span class="o">=</span> <span class="o">-</span><span class="mi">1</span><span class="p">;</span> 65 <span class="p">}</span> 66 <span class="p">}</span> 67 68 <span class="c1">// Phase 2 static member constructor function, which cannot throw 69</span><span class="c1"></span> <span class="k">static</span> <span class="kr">inline</span> <span class="n">outcome</span><span class="o">::</span><span class="n">result</span><span class="o"><</span><span class="n">file_handle</span><span class="o">></span> <span class="n">file</span><span class="p">(</span><span class="n">path_type</span> <span class="n">path</span><span class="p">,</span> <span class="n">mode</span> <span class="n">mode</span> <span class="o">=</span> <span class="n">mode</span><span class="o">::</span><span class="n">read</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span> 70<span class="p">};</span> 71</code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/constructors.cpp#L58" class="code-snippet-url" target="_blank">View this code on Github</a></div> 72 73 74<p>Note the default member initialisers, these are particularly convenient for 75implementing phase 1 of construction. Note also the <code>constexpr</code> constructor, 76which thanks to the default member initialisers is otherwise empty.</p> 77 78<p>File handles are very expensive to copy as they involve a syscall to duplicate 79the file descriptor, so we enable moves only.</p> 80 81<p>The destructor closes the file descriptor if it is not -1, and if the close 82fails, seeing as there is nothing else we can do without leaking the file 83descriptor, we fatal exit the process.</p> 84 85<p>Finally we declare the phase 2 constructor which is a static member function.</p> 86<div class="footnotes"> 87 88<hr /> 89 90<ol> 91<li id="fn:1">LLFIO uses Outcome “in anger”, both in Standard and Experimental configurations. If you would like a real world user of Outcome to study the source code of, it can be studied at <a href="https://github.com/ned14/llfio/">https://github.com/ned14/llfio/</a>. 92 <a class="footnote-return" href="#fnref:1"><sup>[return]</sup></a></li> 93</ol> 94</div> 95 96 97 </div><p><small>Last revised: February 08, 2019 at 22:18:08 UTC</small></p> 98<hr> 99<div class="spirit-nav"> 100<a accesskey="p" href="../../../tutorial/advanced/constructors/two-phase-init.html"><img src="../../../images/prev.png" alt="Prev"></a> 101 <a accesskey="u" href="../../../tutorial/advanced/constructors.html"><img src="../../../images/up.png" alt="Up"></a> 102 <a accesskey="h" href="../../../index.html"><img src="../../../images/home.png" alt="Home"></a><a accesskey="n" href="../../../tutorial/advanced/constructors/static-constructor.html"><img src="../../../images/next.png" alt="Next"></a></div></body> 103</html> 104
