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>A custom no-value policy - 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/no-value.html"><img src="../../../images/prev.png" alt="Prev"></a> 11 <a accesskey="u" href="../../../tutorial/essential/no-value.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/builtin.html"><img src="../../../images/next.png" alt="Next"></a></div><div id="content"> 13 <div class="titlepage"><div><div><h1 style="clear: both">A custom no-value policy</h1></div></div></div> 14 <p>If you want your <code>basic_outcome<></code> or <code>basic_result<></code> instances to call 15<code>std::abort()</code> whenever <code>.value()</code> is called on an object that does not contain a value, or <code>.error()</code> is called on an object that does not contain an error, you will need to define your own no-value policy as follows:</p> 16 17<div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="k">struct</span> <span class="nl">abort_policy</span> <span class="p">:</span> <span class="n">outcome</span><span class="o">::</span><span class="n">policy</span><span class="o">::</span><span class="n">base</span> 18<span class="p">{</span> 19 <span class="k">template</span> <span class="o"><</span><span class="k">class</span><span class="err"> </span><span class="nc">Impl</span><span class="o">></span> <span class="k">static</span> <span class="k">constexpr</span> <span class="kt">void</span> <span class="n">wide_value_check</span><span class="p">(</span><span class="n">Impl</span> <span class="o">&&</span><span class="n">self</span><span class="p">)</span> 20 <span class="p">{</span> 21 <span class="k">if</span><span class="p">(</span><span class="o">!</span><span class="n">base</span><span class="o">::</span><span class="n">_has_value</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">forward</span><span class="o"><</span><span class="n">Impl</span><span class="o">></span><span class="p">(</span><span class="n">self</span><span class="p">)))</span> 22 <span class="n">std</span><span class="o">::</span><span class="n">abort</span><span class="p">();</span> 23 <span class="p">}</span> 24 25 <span class="k">template</span> <span class="o"><</span><span class="k">class</span><span class="err"> </span><span class="nc">Impl</span><span class="o">></span> <span class="k">static</span> <span class="k">constexpr</span> <span class="kt">void</span> <span class="n">wide_error_check</span><span class="p">(</span><span class="n">Impl</span> <span class="o">&&</span><span class="n">self</span><span class="p">)</span> 26 <span class="p">{</span> 27 <span class="k">if</span><span class="p">(</span><span class="o">!</span><span class="n">base</span><span class="o">::</span><span class="n">_has_error</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">forward</span><span class="o"><</span><span class="n">Impl</span><span class="o">></span><span class="p">(</span><span class="n">self</span><span class="p">)))</span> 28 <span class="n">std</span><span class="o">::</span><span class="n">abort</span><span class="p">();</span> 29 <span class="p">}</span> 30 31 <span class="k">template</span> <span class="o"><</span><span class="k">class</span><span class="err"> </span><span class="nc">Impl</span><span class="o">></span> <span class="k">static</span> <span class="k">constexpr</span> <span class="kt">void</span> <span class="n">wide_exception_check</span><span class="p">(</span><span class="n">Impl</span> <span class="o">&&</span><span class="n">self</span><span class="p">)</span> 32 <span class="p">{</span> 33 <span class="k">if</span><span class="p">(</span><span class="o">!</span><span class="n">base</span><span class="o">::</span><span class="n">_has_exception</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">forward</span><span class="o"><</span><span class="n">Impl</span><span class="o">></span><span class="p">(</span><span class="n">self</span><span class="p">)))</span> 34 <span class="n">std</span><span class="o">::</span><span class="n">abort</span><span class="p">();</span> 35 <span class="p">}</span> 36<span class="p">};</span> 37</code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/policies.cpp#L36" class="code-snippet-url" target="_blank">View this code on Github</a></div> 38 39 40<p>All policies ought to inherit from <a href="../../../reference/policies/base.html" class="api-reference"><code>base</code></a> 41 in order to provide your policy implementation with 42the internal policy API for accessing and manipulating <code>result</code> and <code>outcome</code> state.</p> 43 44<p>Once the policy is defined, you have to specify it when providing your own 45<code>basic_outcome</code> specialization:</p> 46 47<div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="k">template</span> <span class="o"><</span><span class="k">typename</span> <span class="n">T</span><span class="o">></span> 48<span class="k">using</span> <span class="n">strictOutcome</span> <span class="o">=</span> <span class="c1">// 49</span><span class="c1"></span><span class="n">outcome</span><span class="o">::</span><span class="n">basic_outcome</span><span class="o"><</span><span class="n">T</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">error_code</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">exception_ptr</span><span class="p">,</span> <span class="n">abort_policy</span><span class="o">></span><span class="p">;</span> 50</code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/policies.cpp#L94" class="code-snippet-url" target="_blank">View this code on Github</a></div> 51 52 53 54 </div><p><small>Last revised: February 08, 2019 at 22:18:08 UTC</small></p> 55<hr> 56<div class="spirit-nav"> 57<a accesskey="p" href="../../../tutorial/essential/no-value.html"><img src="../../../images/prev.png" alt="Prev"></a> 58 <a accesskey="u" href="../../../tutorial/essential/no-value.html"><img src="../../../images/up.png" alt="Up"></a> 59 <a accesskey="h" href="../../../index.html"><img src="../../../images/home.png" alt="Home"></a><a accesskey="n" href="../../../tutorial/essential/no-value/builtin.html"><img src="../../../images/next.png" alt="Next"></a></div></body> 60</html> 61
