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>Hooking events - 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/constructors/metaprogrammg3.html"><img src="../../images/prev.png" alt="Prev"></a> 11 <a accesskey="u" href="../../tutorial/advanced.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/hooks/keeping_state.html"><img src="../../images/next.png" alt="Next"></a></div><div id="content"> 13 14 <div class="titlepage"><div><div><h1 style="clear: both">Hooking events</h1></div></div></div> 15<p>Outcome provides multiple methods for user code to intercept various events which occur. 16The deepest method is simply to inherit from <code>basic_result</code> or <code>basic_outcome</code>, and override member functions, 17for which you will need to study the source code as that form of customisation is out of scope for this tutorial.</p> 18 19<p>Another option is to supply a custom <code>NoValuePolicy</code> which can get you surprisingly 20far into customisation (<a href="../../tutorial/essential/no-value/custom.html">see preceding section</a>).</p> 21 22<p>The final option, which this section covers, is to use the ADL discovered event hooks 23which tell you when a namespace-localised <code>basic_outcome</code> or <code>basic_result</code> has been:</p> 24 25<ul> 26<li>Constructed 27 28<ul> 29<li><a href="../../reference/functions/hooks/hook_result_construction.html" class="api-reference"><code>void hook_result_construction(T *, U &&) noexcept</code></a> 30</li> 31<li><a href="../../reference/functions/hooks/hook_outcome_construction.html" class="api-reference"><code>void hook_outcome_construction(T *, U &&) noexcept</code></a> 32</li> 33</ul></li> 34<li>In-place constructed 35 36<ul> 37<li><a href="../../reference/functions/hooks/hook_result_in_place_construction.html" class="api-reference"><code>void hook_result_in_place_construction(T *, in_place_type_t<U>, Args &&...) noexcept</code></a> 38</li> 39<li><a href="../../reference/functions/hooks/hook_outcome_in_place_construction.html" class="api-reference"><code>void hook_outcome_in_place_construction(T *, in_place_type_t<U>, Args &&...) noexcept</code></a> 40</li> 41</ul></li> 42<li>Copied 43 44<ul> 45<li><a href="../../reference/functions/hooks/hook_result_copy_construction.html" class="api-reference"><code>void hook_result_copy_construction(T *, U &&) noexcept</code></a> 46</li> 47<li><a href="../../reference/functions/hooks/hook_outcome_copy_construction.html" class="api-reference"><code>void hook_outcome_copy_construction(T *, U &&) noexcept</code></a> 48</li> 49</ul></li> 50<li>Moved 51 52<ul> 53<li><a href="../../reference/functions/hooks/hook_result_move_construction.html" class="api-reference"><code>void hook_result_move_construction(T *, U &&) noexcept</code></a> 54</li> 55<li><a href="../../reference/functions/hooks/hook_outcome_move_construction.html" class="api-reference"><code>void hook_outcome_move_construction(T *, U &&) noexcept</code></a> 56</li> 57</ul></li> 58</ul> 59 60<p>One criticism often levelled against library-based exception throw alternatives is that they do 61not provide as rich a set of facilities as C++ exception throws. This section shows 62you how to configure Outcome, using the ADL event hooks, to take a stack backtrace on 63construction of an errored <code>result<T, error_code></code>, 64and if that <code>result<T, error_code></code> should ever be converted into an <code>outcome<T, error_code, std::exception_ptr></code>, 65a custom <code>std::exception_ptr</code> will be just-in-time synthesised consisting of the <code>std::system_error</code> 66for the error code, plus an expanded message string containing the stack backtrace of where 67the error originally occurred.</p> 68 69<p>One can see the use case for such a configuration where low-level, deterministic, 70fixed latency code is built with <code>result</code>, and it dovetails into higher-level 71application code built with <code>outcome</code> where execution time guarantees are not 72important, and thus where a <code>malloc</code> is okay. One effectively has constructed a 73“lazy indeterminism”, or “just-in-time indeterminism” mechanism for handling 74failure, but with all the rich information of throwing C++ exceptions.</p> 75 76 77 78 </div><p><small>Last revised: February 08, 2019 at 22:18:08 UTC</small></p> 79<hr> 80<div class="spirit-nav"> 81<a accesskey="p" href="../../tutorial/advanced/constructors/metaprogrammg3.html"><img src="../../images/prev.png" alt="Prev"></a> 82 <a accesskey="u" href="../../tutorial/advanced.html"><img src="../../images/up.png" alt="Up"></a> 83 <a accesskey="h" href="../../index.html"><img src="../../images/home.png" alt="Home"></a><a accesskey="n" href="../../tutorial/advanced/hooks/keeping_state.html"><img src="../../images/next.png" alt="Next"></a></div></body> 84</html> 85
