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  1. <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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  2. <html><meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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  3. <title>Phase 3 - Boost.Outcome documentation</title>
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    13. 

  13.   <div class="titlepage"><div><div><h1 style="clear: both">Phase 3</h1></div></div></div>
    14. 

  14.   <p>We have built our first two phases of construction for <code>file_handle</code>,
    15. 

  15. and for some users they might be happy writing:</p>
    16. 

  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">&lt;</span><span class="n">file_handle</span><span class="o">&gt;</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">&#34;hello&#34;</span> <span class="cm">/*, file_handle::mode::read */</span><span class="p">);</span>
    18. 

  18.   <span class="k">if</span><span class="p">(</span><span class="o">!</span><span class="n">fh1</span><span class="p">)</span>
    19. 

  19.   <span class="p">{</span>
    20. 

  20.     <span class="n">std</span><span class="o">::</span><span class="n">cerr</span> <span class="o">&lt;&lt;</span> <span class="s">&#34;Opening file &#39;hello&#39; failed with &#34;</span> <span class="o">&lt;&lt;</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">&lt;&lt;</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
    21. 

  21.   <span class="p">}</span>
    22. 

  22. </code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/constructors.cpp#L179" class="code-snippet-url" target="_blank">View this code on Github</a></div>
    23. 

  23. 

  24. 

  25. <p>&hellip; and be done with it.</p>
    26. 

  26. 

  27. <p>But wouldn&rsquo;t it be nicer if we could instead write:</p>
    28. 

  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">&lt;</span><span class="n">file_handle</span><span class="o">&gt;</span> <span class="n">fh2</span> <span class="o">=</span> <span class="n">make</span><span class="o">&lt;</span><span class="n">file_handle</span><span class="o">&gt;</span><span class="p">{</span><span class="s">&#34;hello&#34;</span> <span class="cm">/*, file_handle::mode::read */</span><span class="p">}();</span>
    30. 

  30.   <span class="k">if</span><span class="p">(</span><span class="o">!</span><span class="n">fh2</span><span class="p">)</span>
    31. 

  31.   <span class="p">{</span>
    32. 

  32.     <span class="n">std</span><span class="o">::</span><span class="n">cerr</span> <span class="o">&lt;&lt;</span> <span class="s">&#34;Opening file &#39;hello&#39; failed with &#34;</span> <span class="o">&lt;&lt;</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">&lt;&lt;</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
    33. 

  33.   <span class="p">}</span>
    34. 

  34. </code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/constructors.cpp#L186" class="code-snippet-url" target="_blank">View this code on Github</a></div>
    35. 

  35. 

  36. 

  37. <p>The eye is immediately drawn to the two-stage invocation pattern, so we are
    38. 

  38. constructing a type <code>make&lt;file_handle&gt;</code> using the arguments with which we wish
    39. 

  39. to invoke the <code>file_handle</code> constructor with, and then invoking the
    40. 

  40. call operator on that <code>make&lt;file_handle&gt;</code> instance to do the
    41. 

  41. actual construction.</p>
    42. 

  42. 

  43. <p>It may seem a bit clunky to use brace initialisation for parameters followed
    44. 

  44. by a &ldquo;spurious&rdquo; set of empty brackets, but we think that this is a better
    45. 

  45. approach than alternatives. We shall briefly cover those at the end of this section.</p>
    46. 

  46. 

  47. 

  48.         </div><p><small>Last revised: February 08, 2019 at 22:18:08 UTC</small></p>
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