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>TRY is greedy - 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/result/try.html"><img src="../../../images/prev.png" alt="Prev"></a> 11 <a accesskey="u" href="../../../tutorial/essential/result.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/outcome.html"><img src="../../../images/next.png" alt="Next"></a></div><div id="content"> 13 <div class="titlepage"><div><div><h1 style="clear: both">TRY is greedy</h1></div></div></div> 14 <p><a href="../../../reference/macros/tryv.html" class="api-reference"><code>BOOST_OUTCOME_TRYV(expr)/BOOST_OUTCOME_TRY(expr)</code></a> 15 has ‘greedier’ implicit conversion semantics than 16<a href="../../../reference/types/basic_result.html" class="api-reference"><code>basic_result<T, E, NoValuePolicy></code></a> 17. For example, this code won’t compile:</p> 18<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="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">error_code</span><span class="o">></span> <span class="n">test</span><span class="p">(</span><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="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">errc</span><span class="o">></span> <span class="n">r</span><span class="p">)</span> 19<span class="p">{</span> 20 <span class="k">return</span> <span class="n">r</span><span class="p">;</span> <span class="c1">// you need to use explicit construction here 21</span><span class="c1"></span> <span class="c1">// i.e. return outcome::result<int>(r); 22</span><span class="c1"></span><span class="p">}</span> 23</code></pre></div> 24<p>This is chosen because there is a non-trivial conversion between <code>std::errc</code> and <code>std::error_code</code>, 25so even though that conversion is implicit for <code>std::error_code</code>, Outcome does not expose the 26implicitness here in order to keep the implicit constructor count low (implicit constructors 27add significantly to build times).</p> 28 29<p>The <code>TRY</code> operation is more greedy though:</p> 30<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="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">error_code</span><span class="o">></span> <span class="n">test</span><span class="p">(</span><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="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">errc</span><span class="o">></span> <span class="n">r</span><span class="p">)</span> 31<span class="p">{</span> 32 <span class="n">BOOST_OUTCOME_TRY</span><span class="p">(</span><span class="n">r</span><span class="p">);</span> <span class="c1">// no explicit conversion needed 33</span><span class="c1"></span> <span class="k">return</span> <span class="n">r</span><span class="p">.</span><span class="n">value</span><span class="p">();</span> 34<span class="p">}</span> 35</code></pre></div> 36<p>This is because <code>result<int, std::error_code></code> will implicitly construct from anything which 37either <code>int</code> or <code>std::error_code</code> will implicitly construct from. However, 38<code>result<int, std::error_code></code> will not implicitly construct from <code>result<int, std::errc></code>.</p> 39 40<p>Thus bear this in mind during usage: <code>TRY</code> is greedier for implicit conversions than the Outcome 41types themselves.</p> 42 43 44 </div><p><small>Last revised: February 03, 2020 at 12:05:24 UTC</small></p> 45<hr> 46<div class="spirit-nav"> 47<a accesskey="p" href="../../../tutorial/essential/result/try.html"><img src="../../../images/prev.png" alt="Prev"></a> 48 <a accesskey="u" href="../../../tutorial/essential/result.html"><img src="../../../images/up.png" alt="Up"></a> 49 <a accesskey="h" href="../../../index.html"><img src="../../../images/home.png" alt="Home"></a><a accesskey="n" href="../../../tutorial/essential/outcome.html"><img src="../../../images/next.png" alt="Next"></a></div></body> 50</html> 51
