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>Upgrading the Filesystem TS - 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_file.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/payload/copy_file3.html"><img src="../../../images/next.png" alt="Next"></a></div><div id="content"> 13 <div class="titlepage"><div><div><h1 style="clear: both">Upgrading the Filesystem TS</h1></div></div></div> 14 <p>An Outcome based solution to the dual overload problem is straightforward:</p> 15 16<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> 17<span class="p">{</span> 18 <span class="c1">// Error code + paths related to a failure. Also causes ADL discovery to check this namespace. 19</span><span class="c1"></span> <span class="k">struct</span> <span class="n">failure_info</span> 20 <span class="p">{</span> 21 <span class="n">std</span><span class="o">::</span><span class="n">error_code</span> <span class="n">ec</span><span class="p">;</span> 22 <span class="n">path</span> <span class="n">path1</span><span class="p">,</span> <span class="n">path2</span><span class="p">;</span> 23 <span class="p">};</span> 24 25 <span class="c1">// Tell Outcome that failure_info is to be treated as a std::error_code 26</span><span class="c1"></span> <span class="kr">inline</span> <span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">error_code</span> <span class="o">&</span><span class="n">make_error_code</span><span class="p">(</span><span class="k">const</span> <span class="n">failure_info</span> <span class="o">&</span><span class="n">fi</span><span class="p">)</span> <span class="p">{</span> <span class="k">return</span> <span class="n">fi</span><span class="p">.</span><span class="n">ec</span><span class="p">;</span> <span class="p">}</span> 27 28 <span class="c1">// Localise an outcome implementation specific to this namespace. Normally would just 29</span><span class="c1"></span> <span class="c1">// be `result`, but for clarity we'll use `fs_result`. 30</span><span class="c1"></span> <span class="k">template</span> <span class="o"><</span><span class="k">class</span><span class="err"> </span><span class="nc">T</span><span class="o">></span> <span class="k">using</span> <span class="n">fs_result</span> <span class="o">=</span> <span class="n">outcome</span><span class="o">::</span><span class="n">result</span><span class="o"><</span><span class="n">T</span><span class="p">,</span> <span class="n">failure_info</span><span class="o">></span><span class="p">;</span> 31 32 <span class="cm">/*! Copies the file at path `from` to path `to`. 33</span><span class="cm"> \returns Successful if file was successfully copied, otherwise the error code reported 34</span><span class="cm"> by the operating system plus a payload of the paths involved. 35</span><span class="cm"> \throws Never throws. 36</span><span class="cm"> */</span> 37 <span class="n">fs_result</span><span class="o"><</span><span class="kt">void</span><span class="o">></span> <span class="n">copy_file</span><span class="p">(</span><span class="k">const</span> <span class="n">path</span> <span class="o">&</span><span class="n">from</span><span class="p">,</span> <span class="k">const</span> <span class="n">path</span> <span class="o">&</span><span class="n">to</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span> 38<span class="p">}</span> 39</code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/outcome_payload.cpp#L76" class="code-snippet-url" target="_blank">View this code on Github</a></div> 40 41 42<p>Starting at the bottom, there is now a single <code>copy_file()</code> function which returns a <code>fs_result<void></code>. 43As <code>result</code> is either successful or not, there is no longer any point in returning a boolean, so we 44simply return <code>void</code> on success. On failure, as the template alias <code>fs_result<T></code> above it shows, 45we are returning a <code>failure_info</code> structure containing an error code and the same additional information 46as <code>filesystem_error</code> provides.</p> 47 48<p>It is important to note that the fact that <code>failure_info</code> is defined in namespace <code>filesystem2</code> is very 49important. This is because Outcome uses <a href="http://en.cppreference.com/w/cpp/language/adl">Argument Dependent Lookup (ADL)</a> 50to find the <code>make_error_code()</code> 51function, as well as other customisation point free functions. In other words, only the namespaces as 52defined by ADL are searched when finding a free function telling us what to do for <code>failure_info</code>, 53which includes the namespace <code>failure_info</code> is declared into.</p> 54 55 56 </div><p><small>Last revised: February 08, 2019 at 22:18:08 UTC</small></p> 57<hr> 58<div class="spirit-nav"> 59<a accesskey="p" href="../../../tutorial/advanced/payload/copy_file.html"><img src="../../../images/prev.png" alt="Prev"></a> 60 <a accesskey="u" href="../../../tutorial/advanced/payload.html"><img src="../../../images/up.png" alt="Up"></a> 61 <a accesskey="h" href="../../../index.html"><img src="../../../images/home.png" alt="Home"></a><a accesskey="n" href="../../../tutorial/advanced/payload/copy_file3.html"><img src="../../../images/next.png" alt="Next"></a></div></body> 62</html> 63
