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>Inspecting outcome<T, EC, EP> - 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/outcome.html"><img src="../../../images/prev.png" alt="Prev"></a> 11 <a accesskey="u" href="../../../tutorial/essential/outcome.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/no-value.html"><img src="../../../images/next.png" alt="Next"></a></div><div id="content"> 13 <div class="titlepage"><div><div><h1 style="clear: both">Inspecting outcome<T, EC, EP></h1></div></div></div> 14 <p>Continuing with the previous example, in <code>Layer3</code> we have function <code>z</code> which again reports failures via exceptions. 15It will call function <code>h</code> from <code>Layer2_old</code> which returns <code>outcome<int></code> (which may store an <code>int</code> or a <code>std::error_code</code> or a <code>std::exception_ptr</code>). 16The goal is to unpack it to either the successful return value <code>int</code> or to throw an appropriate exception: if we are storing an <code>std::exception_ptr</code>, just rethrow it. 17If we are storing a <code>std::error_code</code> throw it as <code>std::system_error</code>, which is designed to store <code>std::error_code</code>’s:</p> 18 19<div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="k">auto</span> <span class="nf">z</span><span class="p">()</span> <span class="o">-></span> <span class="kt">int</span> <span class="c1">// throws on failure 20</span><span class="c1"></span><span class="p">{</span> 21 <span class="k">if</span> <span class="p">(</span><span class="n">outcome</span><span class="o">::</span><span class="n">outcome</span><span class="o"><</span><span class="kt">int</span><span class="o">></span> <span class="n">o</span> <span class="o">=</span> <span class="n">old</span><span class="o">::</span><span class="n">h</span><span class="p">())</span> 22 <span class="k">return</span> <span class="n">o</span><span class="p">.</span><span class="n">value</span><span class="p">();</span> 23 <span class="k">else</span> <span class="k">if</span> <span class="p">(</span><span class="n">o</span><span class="p">.</span><span class="n">has_exception</span><span class="p">())</span> 24 <span class="n">std</span><span class="o">::</span><span class="n">rethrow_exception</span><span class="p">(</span><span class="n">o</span><span class="p">.</span><span class="n">exception</span><span class="p">());</span> 25 <span class="k">else</span> 26 <span class="k">throw</span> <span class="n">std</span><span class="o">::</span><span class="n">system_error</span><span class="p">{</span><span class="n">o</span><span class="p">.</span><span class="n">error</span><span class="p">()};</span> 27<span class="p">}</span> 28</code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/using_outcome.cpp#L70" class="code-snippet-url" target="_blank">View this code on Github</a></div> 29 30 31<p>Function <code>has_exception()</code> checks if it is <code>EP</code> (<code>std::exception_ptr</code>) that is stored, function <code>exception()</code> accesses it. Similarly, function <code>error()</code> accesses the <code>EC</code> (<code>std::error_code</code>) if it is stored. 32<code>outcome<></code> also has a function <code>has_failure()</code> to check if either <code>EC</code> or <code>EP</code> is being stored.</p> 33 34<p>It would seem obvious that the above pattern of ‘unpacking’ <code>outcome<></code> 35is so common that it ought to be implemented inside function <code>.value()</code>, 36so function <code>z</code> could be written as:</p> 37<div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="k">return</span> <span class="n">old</span><span class="o">::</span><span class="n">h</span><span class="p">().</span><span class="n">value</span><span class="p">();</span> 38</code></pre></div> 39<p>And this is exactly what the aforementioned no-value policy template argument is for, which is described in 40the next section.</p> 41 42 43 </div><p><small>Last revised: June 19, 2020 at 15:42:26 +0100</small></p> 44<hr> 45<div class="spirit-nav"> 46<a accesskey="p" href="../../../tutorial/essential/outcome.html"><img src="../../../images/prev.png" alt="Prev"></a> 47 <a accesskey="u" href="../../../tutorial/essential/outcome.html"><img src="../../../images/up.png" alt="Up"></a> 48 <a accesskey="h" href="../../../index.html"><img src="../../../images/home.png" alt="Home"></a><a accesskey="n" href="../../../tutorial/essential/no-value.html"><img src="../../../images/next.png" alt="Next"></a></div></body> 49</html> 50
