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>Hook outcome - 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/hooks/poke_exception.html"><img src="../../../images/prev.png" alt="Prev"></a> 11 <a accesskey="u" href="../../../tutorial/advanced/hooks.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/interop.html"><img src="../../../images/next.png" alt="Next"></a></div><div id="content"> 13 <div class="titlepage"><div><div><h1 style="clear: both">Hook outcome</h1></div></div></div> 14 <p>The final step is to add ADL discovered event hooks for the very specific case of 15when our localised <code>outcome</code> is copy or move constructed from our localised <code>result</code>.</p> 16 17<p>You ought to be very careful that the <code>noexcept</code>-ness of these matches the <code>noexcept</code>-ness 18of the types in the <code>outcome</code>. You may have noticed that <code>poke_exception()</code> creates 19a <code>std::string</code> and appends to it. This can throw an exception. If the copy and/or 20move constructors of <code>T</code>, <code>EC</code> and <code>EP</code> are <code>noexcept</code>, then so will be <code>outcome</code>’s 21copy and/or move constructor. Thus if <code>poke_exception()</code> throws, instant program 22termination would occur, which is bad.</p> 23 24<p>We avoid that problem in this case by wrapping <code>poke_exception()</code> in a <code>try...catch</code> 25which throws away any exceptions thrown.</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">error_code_extended</span> 28<span class="p">{</span> 29 <span class="c1">// Specialise the outcome copy and move conversion hook for when our localised result 30</span><span class="c1"></span> <span class="c1">// is used as the source for copy construction our localised outcome 31</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="p">,</span> <span class="k">class</span><span class="err"> </span><span class="nc">U</span><span class="o">></span> <span class="kr">inline</span> <span class="kt">void</span> <span class="n">hook_outcome_copy_construction</span><span class="p">(</span><span class="n">outcome</span><span class="o"><</span><span class="n">T</span><span class="o">></span> <span class="o">*</span><span class="n">res</span><span class="p">,</span> <span class="k">const</span> <span class="n">result</span><span class="o"><</span><span class="n">U</span><span class="o">></span> <span class="o">&</span> <span class="cm">/*unused*/</span><span class="p">)</span> <span class="k">noexcept</span> 32 <span class="p">{</span> 33 <span class="k">try</span> 34 <span class="p">{</span> 35 <span class="c1">// when copy constructing from a result<T>, poke in an exception 36</span><span class="c1"></span> <span class="n">poke_exception</span><span class="p">(</span><span class="n">res</span><span class="p">);</span> 37 <span class="p">}</span> 38 <span class="k">catch</span><span class="p">(...)</span> 39 <span class="p">{</span> 40 <span class="c1">// Do nothing 41</span><span class="c1"></span> <span class="p">}</span> 42 <span class="p">}</span> 43 <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="p">,</span> <span class="k">class</span><span class="err"> </span><span class="nc">U</span><span class="o">></span> <span class="kr">inline</span> <span class="kt">void</span> <span class="n">hook_outcome_move_construction</span><span class="p">(</span><span class="n">outcome</span><span class="o"><</span><span class="n">T</span><span class="o">></span> <span class="o">*</span><span class="n">res</span><span class="p">,</span> <span class="n">result</span><span class="o"><</span><span class="n">U</span><span class="o">></span> <span class="o">&&</span> <span class="cm">/*unused*/</span><span class="p">)</span> <span class="k">noexcept</span> 44 <span class="p">{</span> 45 <span class="k">try</span> 46 <span class="p">{</span> 47 <span class="c1">// when move constructing from a result<T>, poke in an exception 48</span><span class="c1"></span> <span class="n">poke_exception</span><span class="p">(</span><span class="n">res</span><span class="p">);</span> 49 <span class="p">}</span> 50 <span class="k">catch</span><span class="p">(...)</span> 51 <span class="p">{</span> 52 <span class="c1">// Do nothing 53</span><span class="c1"></span> <span class="p">}</span> 54 <span class="p">}</span> 55<span class="p">}</span> 56</code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/error_code_extended.cpp#L182" class="code-snippet-url" target="_blank">View this code on Github</a></div> 57 58 59 60 </div><p><small>Last revised: February 08, 2019 at 22:18:08 UTC</small></p> 61<hr> 62<div class="spirit-nav"> 63<a accesskey="p" href="../../../tutorial/advanced/hooks/poke_exception.html"><img src="../../../images/prev.png" alt="Prev"></a> 64 <a accesskey="u" href="../../../tutorial/advanced/hooks.html"><img src="../../../images/up.png" alt="Up"></a> 65 <a accesskey="h" href="../../../index.html"><img src="../../../images/home.png" alt="Home"></a><a accesskey="n" href="../../../tutorial/advanced/interop.html"><img src="../../../images/next.png" alt="Next"></a></div></body> 66</html> 67
