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>result<> - 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/essential/before.html"><img src="../../images/prev.png" alt="Prev"></a> 11 <a accesskey="u" href="../../tutorial/essential.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/essential/result/inspecting.html"><img src="../../images/next.png" alt="Next"></a></div><div id="content"> 13 14 <div class="titlepage"><div><div><h1 style="clear: both">result<></h1></div></div></div> 15<p>We will define a function that converts a <code>std::string</code> to an <code>int</code>. This function can fail for a number of reasons; 16if it does we want to communicate the failure reason.</p> 17 18<div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="n">outcome</span><span class="o">::</span><span class="n">result</span><span class="o"><</span><span class="kt">int</span><span class="o">></span> <span class="n">convert</span><span class="p">(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&</span> <span class="n">str</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span> 19</code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/using_result.cpp#L42" class="code-snippet-url" target="_blank">View this code on Github</a></div> 20 21 22<p>Template alias <a href="../../reference/aliases/result.html" class="api-reference"><code>result<T, E = varies, NoValuePolicy = policy::default_policy<T, E, void>></code></a> 23 24has three template parameters, but the last two have default values. The first 25(<code>T</code>) represents the type of the object returned from the function upon success. 26The second (<code>EC</code>) is the type of object containing information about the reason 27for failure when the function fails. A result object stores either a <code>T</code> or an 28<code>EC</code> at any given moment, and is therefore conceptually similar to <code>variant<T, EC></code>. 29<code>EC</code> is defaulted to <code>std::error_code</code> in standalone Outcome, or to <code>boost::system::error_code</code> 30in Boost.Outcome<sup class="footnote-ref" id="fnref:1"><a href="#fn:1">1</a></sup>. The third parameter (<code>NoValuePolicy</code>) is called a 31no-value policy. We will cover it later.</p> 32 33<p>If both <code>T</code> and <code>EC</code> are <a href="https://en.cppreference.com/w/cpp/named_req/TriviallyCopyable">trivially copyable</a>, <code>result<T, EC, NVP></code> is also trivially copyable. 34Triviality, complexity and constexpr-ness of each operation (construction, copy construction, assignment, 35destruction etc) is always the intersection of the corresponding operation in <code>T</code> and <code>EC</code>, 36so for example if both <code>T</code> and <code>EC</code> are <a href="https://en.cppreference.com/w/cpp/named_req/LiteralType">literal types</a>, so will be <code>result<T, EC, NVP></code>. 37Additionally, if both <code>T</code> and <code>EC</code> have <a href="https://en.cppreference.com/w/cpp/named_req/StandardLayoutType">standard layout</a>, <code>result<T, EC, NVP></code> has standard layout; 38thus if both <code>T</code> and <code>EC</code> are C-compatible, so will <code>result<T, EC, NVP></code> be C-compatible.</p> 39 40<p>Now, we will define an enumeration describing different failure situations during conversion.</p> 41 42<div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="k">enum</span> <span class="k">class</span><span class="err"> </span><span class="nc">ConversionErrc</span> 43<span class="p">{</span> 44 <span class="n">Success</span> <span class="o">=</span> <span class="mi">0</span><span class="p">,</span> <span class="c1">// 0 should not represent an error 45</span><span class="c1"></span> <span class="n">EmptyString</span> <span class="o">=</span> <span class="mi">1</span><span class="p">,</span> <span class="c1">// (for rationale, see tutorial on error codes) 46</span><span class="c1"></span> <span class="n">IllegalChar</span> <span class="o">=</span> <span class="mi">2</span><span class="p">,</span> 47 <span class="n">TooLong</span> <span class="o">=</span> <span class="mi">3</span><span class="p">,</span> 48<span class="p">};</span> 49 50<span class="c1">// all boilerplate necessary to plug ConversionErrc 51</span><span class="c1">// into std::error_code framework 52</span><span class="c1"></span></code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/using_result.cpp#L46" class="code-snippet-url" target="_blank">View this code on Github</a></div> 53 54 55<p>Assume we have plugged it into <code>std::error_code</code> framework, as described in <a href="../../motivation/plug_error_code.html">this section</a>.</p> 56 57<p>One notable effect of such plugging is that <code>ConversionErrc</code> is now convertible to <code>std::error_code</code>. 58Now we can implement function <code>convert</code> as follows:</p> 59 60<div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="n">outcome</span><span class="o">::</span><span class="n">result</span><span class="o"><</span><span class="kt">int</span><span class="o">></span> <span class="n">convert</span><span class="p">(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&</span> <span class="n">str</span><span class="p">)</span> <span class="k">noexcept</span> 61<span class="p">{</span> 62 <span class="k">if</span> <span class="p">(</span><span class="n">str</span><span class="p">.</span><span class="n">empty</span><span class="p">())</span> 63 <span class="k">return</span> <span class="n">ConversionErrc</span><span class="o">::</span><span class="n">EmptyString</span><span class="p">;</span> 64 65 <span class="k">if</span> <span class="p">(</span><span class="o">!</span><span class="n">std</span><span class="o">::</span><span class="n">all_of</span><span class="p">(</span><span class="n">str</span><span class="p">.</span><span class="n">begin</span><span class="p">(),</span> <span class="n">str</span><span class="p">.</span><span class="n">end</span><span class="p">(),</span> <span class="o">::</span><span class="n">isdigit</span><span class="p">))</span> 66 <span class="k">return</span> <span class="n">ConversionErrc</span><span class="o">::</span><span class="n">IllegalChar</span><span class="p">;</span> 67 68 <span class="k">if</span> <span class="p">(</span><span class="n">str</span><span class="p">.</span><span class="n">length</span><span class="p">()</span> <span class="o">></span> <span class="mi">9</span><span class="p">)</span> 69 <span class="k">return</span> <span class="n">ConversionErrc</span><span class="o">::</span><span class="n">TooLong</span><span class="p">;</span> 70 71 <span class="k">return</span> <span class="nf">atoi</span><span class="p">(</span><span class="n">str</span><span class="p">.</span><span class="n">c_str</span><span class="p">());</span> 72<span class="p">}</span> 73</code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/using_result.cpp#L65" class="code-snippet-url" target="_blank">View this code on Github</a></div> 74 75 76<p><code>result<T, EC></code> is convertible from any <code>T2</code> convertible to <code>T</code> as well as any <code>EC2</code> convertible to <code>EC</code>, 77provided that there is no constructability possible in either direction between <code>T</code> and <code>EC</code>. If there is, 78all implicit conversion is disabled, and you will need to use one of the tagged constructors:</p> 79 80<div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="n">outcome</span><span class="o">::</span><span class="n">result</span><span class="o"><</span><span class="kt">int</span><span class="o">></span> <span class="n">r</span> <span class="p">{</span><span class="n">outcome</span><span class="o">::</span><span class="n">in_place_type</span><span class="o"><</span><span class="n">std</span><span class="o">::</span><span class="n">error_code</span><span class="o">></span><span class="p">,</span> <span class="n">ConversionErrc</span><span class="o">::</span><span class="n">EmptyString</span><span class="p">};</span> 81<span class="n">outcome</span><span class="o">::</span><span class="n">result</span><span class="o"><</span><span class="kt">int</span><span class="o">></span> <span class="n">s</span> <span class="p">{</span><span class="n">outcome</span><span class="o">::</span><span class="n">in_place_type</span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="p">,</span> <span class="mi">1</span><span class="p">};</span> 82</code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/using_result.cpp#L115" class="code-snippet-url" target="_blank">View this code on Github</a></div> 83 84 85<p>Or use helper functions:</p> 86 87<div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="n">outcome</span><span class="o">::</span><span class="n">result</span><span class="o"><</span><span class="kt">int</span><span class="o">></span> <span class="n">r</span> <span class="o">=</span> <span class="n">outcome</span><span class="o">::</span><span class="n">failure</span><span class="p">(</span><span class="n">ConversionErrc</span><span class="o">::</span><span class="n">EmptyString</span><span class="p">);</span> 88<span class="n">outcome</span><span class="o">::</span><span class="n">result</span><span class="o"><</span><span class="kt">int</span><span class="o">></span> <span class="n">s</span> <span class="o">=</span> <span class="n">outcome</span><span class="o">::</span><span class="n">success</span><span class="p">(</span><span class="mi">1</span><span class="p">);</span> 89</code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/using_result.cpp#L123" class="code-snippet-url" target="_blank">View this code on Github</a></div> 90 91 92<div class="notices note" style="background: url('../../images/note.png') top left no-repeat padding-box padding-box;"> 93<div class="notices heading">note</div> 94<div class="notices message"><p>The functions <a href="../../reference/functions/failure.html" class="api-reference"><code>auto failure(T &&, …)</code></a> 95 and <a href="../../reference/functions/success.html" class="api-reference"><code>auto success(T &&)</code></a> 96 return special 97types implicitly convertible to failed or successful <code>result</code> (and <code>outcome</code>).</p> 98</div> 99</div> 100 101<div class="footnotes"> 102 103<hr /> 104 105<ol> 106<li id="fn:1">You can mandate a choice using <code>std_result<T></code> or <code>boost_result<T></code>. 107 <a class="footnote-return" href="#fnref:1"><sup>[return]</sup></a></li> 108</ol> 109</div> 110 111 112 113 </div><p><small>Last revised: July 07, 2020 at 23:06:43 +0100</small></p> 114<hr> 115<div class="spirit-nav"> 116<a accesskey="p" href="../../tutorial/essential/before.html"><img src="../../images/prev.png" alt="Prev"></a> 117 <a accesskey="u" href="../../tutorial/essential.html"><img src="../../images/up.png" alt="Up"></a> 118 <a accesskey="h" href="../../index.html"><img src="../../images/home.png" alt="Home"></a><a accesskey="n" href="../../tutorial/essential/result/inspecting.html"><img src="../../images/next.png" alt="Next"></a></div></body> 119</html> 120
