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>Auto-throwing filesystem_error - 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/payload/copy_file2.html"><img src="../../../images/prev.png" alt="Prev"></a> 11 <a accesskey="u" href="../../../tutorial/advanced/payload.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.html"><img src="../../../images/next.png" alt="Next"></a></div><div id="content"> 13 <div class="titlepage"><div><div><h1 style="clear: both">Auto-throwing filesystem_error</h1></div></div></div> 14 <p>Something not mentioned at all until now (and properly described in the next 15section, <a href="../../default-actions/">Default actions</a>) is that Outcome can be 16programmed take various actions when the user tries to observe <code>.value()</code> 17when there is no value, and so on for the other possible state observations.</p> 18 19<p>Seeing as we are replacing the throwing overload of <code>copy_file()</code> in the 20Filesystem TS with a <code>result</code> returning edition instead, it would make 21sense if an attempt to observe the value of an unsuccessful <code>fs_result</code> 22threw the exact same <code>filesystem_error</code> as the Filesystem TS does.</p> 23 24<p>Telling Outcome how to throw a <code>filesystem_error</code> with payload of the 25failing paths is easy:</p> 26 27<div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="k">namespace</span> <span class="n">filesystem2</span> 28<span class="p">{</span> 29 <span class="c1">// If we would like Outcome to do something other than the default action (see next 30</span><span class="c1"></span> <span class="c1">// section), we can declare this ADL discovered free function to customise what 31</span><span class="c1"></span> <span class="c1">// to do instead. 32</span><span class="c1"></span> <span class="c1">// 33</span><span class="c1"></span> <span class="c1">// Note that rvalue semantics are propagated internally by Outcome, so if the user 34</span><span class="c1"></span> <span class="c1">// called .value() on a rvalued result, failure_info will be moved rather than 35</span><span class="c1"></span> <span class="c1">// copied from the result. That means we can overload once with value semantics, 36</span><span class="c1"></span> <span class="c1">// and not need to overload for lvalue and rvalue situations unless we really feel 37</span><span class="c1"></span> <span class="c1">// we need to for some reason. 38</span><span class="c1"></span> <span class="kr">inline</span> <span class="kt">void</span> <span class="n">outcome_throw_as_system_error_with_payload</span><span class="p">(</span><span class="n">failure_info</span> <span class="n">fi</span><span class="p">)</span> 39 <span class="p">{</span> 40 <span class="c1">// If the error code is not filesystem related e.g. ENOMEM, throw that as a 41</span><span class="c1"></span> <span class="c1">// standard STL exception. 42</span><span class="c1"></span> <span class="n">outcome</span><span class="o">::</span><span class="n">try_throw_std_exception_from_error</span><span class="p">(</span><span class="n">fi</span><span class="p">.</span><span class="n">ec</span><span class="p">);</span> 43 <span class="c1">// Throw the exact same filesystem_error exception which the throwing copy_file() 44</span><span class="c1"></span> <span class="c1">// edition does. 45</span><span class="c1"></span> <span class="k">throw</span> <span class="nf">filesystem_error</span><span class="p">(</span><span class="n">fi</span><span class="p">.</span><span class="n">ec</span><span class="p">.</span><span class="n">message</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">fi</span><span class="p">.</span><span class="n">path1</span><span class="p">),</span> <span class="c1">// 46</span><span class="c1"></span> <span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">fi</span><span class="p">.</span><span class="n">path2</span><span class="p">),</span> <span class="n">fi</span><span class="p">.</span><span class="n">ec</span><span class="p">);</span> 47 <span class="p">}</span> 48<span class="p">}</span> 49</code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/outcome_payload.cpp#L107" class="code-snippet-url" target="_blank">View this code on Github</a></div> 50 51 52<p>Reference documentation for the above functions:</p> 53 54<ul> 55<li><a href="../../../reference/functions/policy.html">List of builtin <code>outcome_throw_as_system_error_with_payload()</code> overloads</a></li> 56<li><a href="../../../reference/functions/try_throw_std_exception_from_error.html" class="api-reference"><code>void try_throw_std_exception_from_error(std::error_code ec, const std::string &msg = std::string{})</code></a> 57</li> 58</ul> 59 60<p>Usage of our new “upgraded” Filesystem <code>copy_file()</code> might now be as follows:</p> 61 62<div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"> <span class="c1">// Non-throwing use case 63</span><span class="c1"></span> <span class="k">auto</span> <span class="n">o</span> <span class="o">=</span> <span class="n">filesystem2</span><span class="o">::</span><span class="n">copy_file</span><span class="p">(</span><span class="s">"dontexist"</span><span class="p">,</span> <span class="s">"alsodontexist"</span><span class="p">);</span> 64 <span class="k">if</span><span class="p">(</span><span class="o">!</span><span class="n">o</span><span class="p">)</span> 65 <span class="p">{</span> 66 <span class="n">std</span><span class="o">::</span><span class="n">cerr</span> <span class="o"><<</span> <span class="s">"Copy file failed with error "</span> <span class="o"><<</span> <span class="n">o</span><span class="p">.</span><span class="n">error</span><span class="p">().</span><span class="n">ec</span><span class="p">.</span><span class="n">message</span><span class="p">()</span> <span class="c1">// 67</span><span class="c1"></span> <span class="o"><<</span> <span class="s">" (path1 = "</span> <span class="o"><<</span> <span class="n">o</span><span class="p">.</span><span class="n">error</span><span class="p">().</span><span class="n">path1</span> <span class="o"><<</span> <span class="s">", path2 = "</span> <span class="o"><<</span> <span class="n">o</span><span class="p">.</span><span class="n">error</span><span class="p">().</span><span class="n">path2</span> <span class="o"><<</span> <span class="s">")"</span> <span class="c1">// 68</span><span class="c1"></span> <span class="o"><<</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span> 69 <span class="p">}</span> 70 71 <span class="c1">// Throwing use case 72</span><span class="c1"></span> <span class="k">try</span> 73 <span class="p">{</span> 74 <span class="c1">// Try to observe the successful value, thus triggering default actions which invokes 75</span><span class="c1"></span> <span class="c1">// our outcome_throw_as_system_error_with_payload() above which then throws filesystem_error 76</span><span class="c1"></span> <span class="c1">// exactly like the Filesystem TS does for its throwing overload. 77</span><span class="c1"></span> <span class="n">filesystem2</span><span class="o">::</span><span class="n">copy_file</span><span class="p">(</span><span class="s">"dontexist"</span><span class="p">,</span> <span class="s">"alsodontexist"</span><span class="p">).</span><span class="n">value</span><span class="p">();</span> 78 <span class="p">}</span> 79 <span class="k">catch</span><span class="p">(</span><span class="k">const</span> <span class="n">filesystem2</span><span class="o">::</span><span class="n">filesystem_error</span> <span class="o">&</span><span class="n">e</span><span class="p">)</span> 80 <span class="p">{</span> 81 <span class="n">std</span><span class="o">::</span><span class="n">cerr</span> <span class="o"><<</span> <span class="s">"Copy file failed with exception "</span> <span class="o"><<</span> <span class="n">e</span><span class="p">.</span><span class="n">what</span><span class="p">()</span> <span class="c1">// 82</span><span class="c1"></span> <span class="o"><<</span> <span class="s">" (path1 = "</span> <span class="o"><<</span> <span class="n">e</span><span class="p">.</span><span class="n">path1</span><span class="p">()</span> <span class="o"><<</span> <span class="s">", path2 = "</span> <span class="o"><<</span> <span class="n">e</span><span class="p">.</span><span class="n">path2</span><span class="p">()</span> <span class="o"><<</span> <span class="s">")"</span> <span class="c1">// 83</span><span class="c1"></span> <span class="o"><<</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span> 84 <span class="p">}</span> 85 <span class="k">catch</span><span class="p">(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">exception</span> <span class="o">&</span><span class="n">e</span><span class="p">)</span> 86 <span class="p">{</span> 87 <span class="n">std</span><span class="o">::</span><span class="n">cerr</span> <span class="o"><<</span> <span class="s">"Copy file failed with exception "</span> <span class="o"><<</span> <span class="n">e</span><span class="p">.</span><span class="n">what</span><span class="p">()</span> <span class="c1">// 88</span><span class="c1"></span> <span class="o"><<</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span> 89 <span class="p">}</span> 90</code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/outcome_payload.cpp#L134" class="code-snippet-url" target="_blank">View this code on Github</a></div> 91 92 93 94 </div><p><small>Last revised: February 08, 2019 at 22:18:08 UTC</small></p> 95<hr> 96<div class="spirit-nav"> 97<a accesskey="p" href="../../../tutorial/advanced/payload/copy_file2.html"><img src="../../../images/prev.png" alt="Prev"></a> 98 <a accesskey="u" href="../../../tutorial/advanced/payload.html"><img src="../../../images/up.png" alt="Up"></a> 99 <a accesskey="h" href="../../../index.html"><img src="../../../images/home.png" alt="Home"></a><a accesskey="n" href="../../../tutorial/advanced/constructors.html"><img src="../../../images/next.png" alt="Next"></a></div></body> 100</html> 101
