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>construct<T> - 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/metaprogrammg1.html"><img src="../../../images/prev.png" alt="Prev"></a> 11 <a accesskey="u" href="../../../tutorial/advanced/constructors.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/constructors/metaprogrammg3.html"><img src="../../../images/next.png" alt="Next"></a></div><div id="content"> 13 <div class="titlepage"><div><div><h1 style="clear: both">construct<T></h1></div></div></div> 14 <p>First, we need a base definition for <code>make<T></code>:</p> 15 16<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">class</span><span class="err"> </span><span class="nc">T</span><span class="o">></span> <span class="k">struct</span> <span class="n">make</span> 17<span class="p">{</span> 18 <span class="n">outcome</span><span class="o">::</span><span class="n">result</span><span class="o"><</span><span class="n">T</span><span class="o">></span> <span class="k">operator</span><span class="p">()()</span> <span class="k">const</span> <span class="k">noexcept</span> 19 <span class="p">{</span> <span class="c1">// 20</span><span class="c1"></span> <span class="k">static_assert</span><span class="p">(</span><span class="o">!</span><span class="n">std</span><span class="o">::</span><span class="n">is_same</span><span class="o"><</span><span class="n">T</span><span class="p">,</span> <span class="n">T</span><span class="o">>::</span><span class="n">value</span><span class="p">,</span> <span class="c1">// 21</span><span class="c1"></span> <span class="s">"make<T>() was not specialised for the type T supplied"</span><span class="p">);</span> 22 <span class="p">}</span> 23<span class="p">};</span> 24</code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/constructors.cpp#L158" class="code-snippet-url" target="_blank">View this code on Github</a></div> 25 26 27<p>This fails a static assert if the type is ever instantiated unspecialised.</p> 28 29<p>We then specialise for <code>make<file_handle></code>:</p> 30 31<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">struct</span> <span class="n">make</span><span class="o"><</span><span class="n">file_handle</span><span class="o">></span> 32<span class="p">{</span> 33 <span class="n">file_handle</span><span class="o">::</span><span class="n">path_type</span> <span class="n">_path</span><span class="p">;</span> 34 <span class="n">file_handle</span><span class="o">::</span><span class="n">mode</span> <span class="n">_mode</span><span class="p">{</span><span class="n">file_handle</span><span class="o">::</span><span class="n">mode</span><span class="o">::</span><span class="n">read</span><span class="p">};</span> 35 <span class="c1">// Any other args, default initialised if necessary, follow here ... 36</span><span class="c1"></span> 37 <span class="n">outcome</span><span class="o">::</span><span class="n">result</span><span class="o"><</span><span class="n">file_handle</span><span class="o">></span> <span class="k">operator</span><span class="p">()()</span> <span class="k">const</span> <span class="k">noexcept</span> <span class="c1">// 38</span><span class="c1"></span> <span class="p">{</span> 39 <span class="k">return</span> <span class="n">file_handle</span><span class="o">::</span><span class="n">file</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">_path</span><span class="p">));</span> 40 <span class="p">}</span> 41<span class="p">};</span> 42</code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/constructors.cpp#L169" class="code-snippet-url" target="_blank">View this code on Github</a></div> 43 44 45<p>Because this is a struct, we can list initialise <code>make</code>, and use 46default member initialisers to implement default arguments. This can get 47you surprisingly far before you need to start writing custom constructors.</p> 48 49<p>But in more complex code, you will usually provide all the initialisation overloads that 50you would for the constructors of your main type. You then implement a single phase 2 constructing 51function which accepts <code>make<YOURTYPE></code> as input, and construct solely from 52that source.</p> 53 54 55 </div><p><small>Last revised: February 08, 2019 at 22:18:08 UTC</small></p> 56<hr> 57<div class="spirit-nav"> 58<a accesskey="p" href="../../../tutorial/advanced/constructors/metaprogrammg1.html"><img src="../../../images/prev.png" alt="Prev"></a> 59 <a accesskey="u" href="../../../tutorial/advanced/constructors.html"><img src="../../../images/up.png" alt="Up"></a> 60 <a accesskey="h" href="../../../index.html"><img src="../../../images/home.png" alt="Home"></a><a accesskey="n" href="../../../tutorial/advanced/constructors/metaprogrammg3.html"><img src="../../../images/next.png" alt="Next"></a></div></body> 61</html> 62
