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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>Tying it all together - Boost.Outcome documentation</title>
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  10. <a accesskey="p" href="../experimental/worked-example/implicit_conversion.html"><img src="../images/prev.png" alt="Prev"></a>
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    13. 

  13.   <div class="titlepage"><div><div><h1 style="clear: both">Tying it all together</h1></div></div></div>
    14. 

  14.   
    15. 

  15. 

  16. <p>Firstly let&rsquo;s alias a more convenient form of <code>status_result</code>:</p>
    17. 

  17. 

  18. <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">&lt;</span><span class="k">class</span><span class="err"> </span><span class="nc">T</span><span class="p">,</span> <span class="k">class</span><span class="err"> </span><span class="nc">E</span> <span class="o">=</span> <span class="n">outcome_e</span><span class="o">::</span><span class="n">error</span><span class="o">&gt;</span>
    19. 

  19. <span class="k">using</span> <span class="n">result</span> <span class="o">=</span>  <span class="c1">//
    20. 

  20. </span><span class="c1"></span><span class="n">outcome_e</span><span class="o">::</span><span class="n">status_result</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="p">,</span> <span class="n">outcome_e</span><span class="o">::</span><span class="n">policy</span><span class="o">::</span><span class="n">default_status_result_policy</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">&gt;&gt;</span><span class="p">;</span>
    21. 

  21. </code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/experimental_status_code.cpp#L196" class="code-snippet-url" target="_blank">View this code on Github</a></div>
    22. 

  22. 

  23. 

  24. <p>(The defaulting of <code>default_result_policy</code> is superfluous, it&rsquo;s already the default)</p>
    25. 

  25. 

  26. <p>What follows now is very standard Outcome code. Indeed, it would compile
    27. 

  27. just fine under standard Outcome with only a few typedefs.</p>
    28. 

  28. 

  29. <div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="n">result</span><span class="o">&lt;</span><span class="n">file_handle</span><span class="p">,</span> <span class="n">file_io_error</span><span class="o">&gt;</span> <span class="n">open_file</span><span class="p">(</span><span class="k">const</span> <span class="kt">char</span> <span class="o">*</span><span class="n">path</span><span class="p">)</span>  <span class="c1">// models throws(file_io_error)
    30. 

  30. </span><span class="c1"></span><span class="p">{</span>
    31. 

  31.   <span class="n">file_handle</span> <span class="n">ret</span><span class="p">(</span><span class="o">::</span><span class="n">fopen</span><span class="p">(</span><span class="n">path</span><span class="p">,</span> <span class="s">&#34;r&#34;</span><span class="p">));</span>
    32. 

  32.   <span class="k">if</span><span class="p">(</span><span class="n">ret</span><span class="p">)</span>
    33. 

  33.     <span class="k">return</span> <span class="n">ret</span><span class="p">;</span>
    34. 

  34.   <span class="k">return</span> <span class="nf">file_io_error</span><span class="p">({</span><span class="n">errno</span><span class="p">,</span> <span class="n">__LINE__</span><span class="p">,</span> <span class="n">__FILE__</span><span class="p">});</span>
    35. 

  35. <span class="p">}</span>
    36. 

  36. 

  37. <span class="n">result</span><span class="o">&lt;</span><span class="kt">void</span><span class="o">&gt;</span> <span class="n">open_resource</span><span class="p">()</span>  <span class="c1">// models throws(std::error)
    38. 

  38. </span><span class="c1"></span><span class="p">{</span>
    39. 

  39.   <span class="k">for</span><span class="p">(;;)</span>
    40. 

  40.   <span class="p">{</span>
    41. 

  41.     <span class="n">result</span><span class="o">&lt;</span><span class="n">file_handle</span><span class="p">,</span> <span class="n">file_io_error</span><span class="o">&gt;</span> <span class="n">r</span> <span class="o">=</span> <span class="n">open_file</span><span class="p">(</span><span class="s">&#34;some file&#34;</span><span class="p">);</span>
    42. 

  42.     <span class="k">if</span><span class="p">(</span><span class="n">r</span><span class="p">)</span>
    43. 

  43.       <span class="k">break</span><span class="p">;</span>
    44. 

  44.     <span class="n">file_io_error</span> <span class="n">e</span> <span class="o">=</span> <span class="n">r</span><span class="p">.</span><span class="n">error</span><span class="p">();</span>
    45. 

  45.     <span class="k">if</span><span class="p">(</span><span class="n">e</span> <span class="o">!=</span> <span class="n">outcome_e</span><span class="o">::</span><span class="n">errc</span><span class="o">::</span><span class="n">resource_unavailable_try_again</span><span class="p">)</span>
    46. 

  46.     <span class="p">{</span>
    47. 

  47.       <span class="c1">// NOTE this implicitly converts from `file_io_error` to `error` via the
    48. 

  48. </span><span class="c1"></span>      <span class="c1">// `make_status_code()` free function customisation point defined above.
    49. 

  49. </span><span class="c1"></span>      <span class="k">return</span> <span class="n">e</span><span class="p">;</span>
    50. 

  50.     <span class="p">}</span>
    51. 

  51.   <span class="p">}</span>
    52. 

  52.   <span class="c1">// success continues here ...
    53. 

  53. </span><span class="c1"></span>  <span class="k">return</span> <span class="n">outcome_e</span><span class="o">::</span><span class="n">success</span><span class="p">();</span>
    54. 

  54. <span class="p">}</span>
    55. 

  55. 

  56. <span class="kt">int</span> <span class="n">main</span><span class="p">(</span><span class="kt">void</span><span class="p">)</span>
    57. 

  57. <span class="p">{</span>
    58. 

  58.   <span class="n">result</span><span class="o">&lt;</span><span class="kt">void</span><span class="o">&gt;</span> <span class="n">r</span> <span class="o">=</span> <span class="n">open_resource</span><span class="p">();</span>
    59. 

  59.   <span class="k">if</span><span class="p">(</span><span class="n">r</span><span class="p">)</span>
    60. 

  60.     <span class="n">printf</span><span class="p">(</span><span class="s">&#34;Success!</span><span class="se">\n</span><span class="s">&#34;</span><span class="p">);</span>
    61. 

  61.   <span class="k">else</span>
    62. 

  62.   <span class="p">{</span>
    63. 

  63.     <span class="k">auto</span> <span class="n">e</span> <span class="o">=</span> <span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">r</span><span class="p">).</span><span class="n">error</span><span class="p">();</span>
    64. 

  64.     <span class="c1">// A quick demonstration that the indirection works as indicated
    65. 

  65. </span><span class="c1"></span>    <span class="n">printf</span><span class="p">(</span><span class="s">&#34;Returned error has a code domain of &#39;%s&#39;, a message of &#39;%s&#39;</span><span class="se">\n</span><span class="s">&#34;</span><span class="p">,</span> <span class="n">e</span><span class="p">.</span><span class="n">domain</span><span class="p">().</span><span class="n">name</span><span class="p">().</span><span class="n">c_str</span><span class="p">(),</span> <span class="n">e</span><span class="p">.</span><span class="n">message</span><span class="p">().</span><span class="n">c_str</span><span class="p">());</span>
    66. 

  66.     <span class="n">printf</span><span class="p">(</span><span class="s">&#34;</span><span class="se">\n</span><span class="s">And semantically comparing it to &#39;errc::no_such_file_or_directory&#39; = %d</span><span class="se">\n</span><span class="s">&#34;</span><span class="p">,</span> <span class="n">e</span> <span class="o">==</span> <span class="n">outcome_e</span><span class="o">::</span><span class="n">errc</span><span class="o">::</span><span class="n">no_such_file_or_directory</span><span class="p">);</span>
    67. 

  67.   <span class="p">}</span>
    68. 

  68. <span class="p">}</span>
    69. 

  69. </code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/experimental_status_code.cpp#L201" class="code-snippet-url" target="_blank">View this code on Github</a></div>
    70. 

  70. 

  71. 

  72. <p>And running this program yields:</p>
    73. 

  73. 

  74. <pre><code>Returned error has a code domain of 'file i/o error domain', a message of 'No such file or directory (c:\users\ned\documents\boostish\outcome\doc\src\snippets\experimental_status_code.cpp:195)'
    75. 

  75. 

  76. And semantically comparing it to 'errc::no_such_file_or_directory' = 1
    77. 

  77. </code></pre>
    78. 

  78. 

  79. <h3 id="conclusion">Conclusion</h3>
    80. 

  80. 

  81. <p>Once you get used to <code>&lt;system_error2&gt;</code> and the fact that any <code>result</code> with
    82. 

  82. <code>E = error</code> is always move-only, using experimental Outcome is just like
    83. 

  83. using normal Outcome. Except that codegen will be better, custom domains
    84. 

  84. are safe to use in headers, semantic comparisons have guaranteed complexity
    85. 

  85. bounds, and build times are much reduced.</p>
    86. 

  86. 

  87. <p>What&rsquo;s not to like? :)</p>
    88. 

  88. 

  89. <p>Finally, if you have feedback on using experimental Outcome which you think
    90. 

  90. would be of use to the standards committee when evaluating possible
    91. 

  91. implementations of <a href="http://wg21.link/P0709">P0709 <em>Zero overhead exceptions: Throwing values</em></a>,
    92. 

  92. please do get in touch! This <strong>especially</strong> includes successful experiences!!!</p>
    93. 

  93. 

  94. 

  95.         </div><p><small>Last revised: February 05, 2019 at 17:14:18 UTC</small></p>
    96. 

  96. <hr>
    97. 

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    98. 

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    102.