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>Phase 3 - 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/static-constructor.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/metaprogrammg2.html"><img src="../../../images/next.png" alt="Next"></a></div><div id="content"> 13 <div class="titlepage"><div><div><h1 style="clear: both">Phase 3</h1></div></div></div> 14 <p>We have built our first two phases of construction for <code>file_handle</code>, 15and for some users they might be happy writing:</p> 16 17<div class="code-snippet"><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="n">file_handle</span><span class="o">></span> <span class="n">fh1</span> <span class="o">=</span> <span class="n">file_handle</span><span class="o">::</span><span class="n">file</span><span class="p">(</span><span class="s">"hello"</span> <span class="cm">/*, file_handle::mode::read */</span><span class="p">);</span> 18 <span class="k">if</span><span class="p">(</span><span class="o">!</span><span class="n">fh1</span><span class="p">)</span> 19 <span class="p">{</span> 20 <span class="n">std</span><span class="o">::</span><span class="n">cerr</span> <span class="o"><<</span> <span class="s">"Opening file 'hello' failed with "</span> <span class="o"><<</span> <span class="n">fh1</span><span class="p">.</span><span class="n">error</span><span class="p">().</span><span class="n">message</span><span class="p">()</span> <span class="o"><<</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span> 21 <span class="p">}</span> 22</code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/constructors.cpp#L185" class="code-snippet-url" target="_blank">View this code on Github</a></div> 23 24 25<p>… and be done with it.</p> 26 27<p>But wouldn’t it be nicer if we could instead write:</p> 28 29<div class="code-snippet"><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="n">file_handle</span><span class="o">></span> <span class="n">fh2</span> <span class="o">=</span> <span class="n">make</span><span class="o"><</span><span class="n">file_handle</span><span class="o">></span><span class="p">{</span><span class="s">"hello"</span> <span class="cm">/*, file_handle::mode::read */</span><span class="p">}();</span> 30 <span class="k">if</span><span class="p">(</span><span class="o">!</span><span class="n">fh2</span><span class="p">)</span> 31 <span class="p">{</span> 32 <span class="n">std</span><span class="o">::</span><span class="n">cerr</span> <span class="o"><<</span> <span class="s">"Opening file 'hello' failed with "</span> <span class="o"><<</span> <span class="n">fh2</span><span class="p">.</span><span class="n">error</span><span class="p">().</span><span class="n">message</span><span class="p">()</span> <span class="o"><<</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span> 33 <span class="p">}</span> 34</code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/constructors.cpp#L192" class="code-snippet-url" target="_blank">View this code on Github</a></div> 35 36 37<p>The eye is immediately drawn to the two-stage invocation pattern, so we are 38constructing a type <code>make<file_handle></code> using the arguments with which we wish 39to invoke the <code>file_handle</code> constructor with, and then invoking the 40call operator on that <code>make<file_handle></code> instance to do the 41actual construction.</p> 42 43<p>It may seem a bit clunky to use brace initialisation for parameters followed 44by a “spurious” set of empty brackets, but we think that this is a better 45approach than alternatives. We shall briefly cover those at the end of this section.</p> 46 47 48 </div><p><small>Last revised: February 08, 2019 at 22:18:08 UTC</small></p> 49<hr> 50<div class="spirit-nav"> 51<a accesskey="p" href="../../../tutorial/advanced/constructors/static-constructor.html"><img src="../../../images/prev.png" alt="Prev"></a> 52 <a accesskey="u" href="../../../tutorial/advanced/constructors.html"><img src="../../../images/up.png" alt="Up"></a> 53 <a accesskey="h" href="../../../index.html"><img src="../../../images/home.png" alt="Home"></a><a accesskey="n" href="../../../tutorial/advanced/constructors/metaprogrammg2.html"><img src="../../../images/next.png" alt="Next"></a></div></body> 54</html> 55
