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    2. 

  2. <html><meta http-equiv="Content-Type" content="text/html; charset=utf-8">
    3. 

  3. <title>Auto-throwing filesystem_error - Boost.Outcome documentation</title>
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  10. <a accesskey="p" href="../../../tutorial/advanced/payload/copy_file2.html"><img src="../../../images/prev.png" alt="Prev"></a>
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    13. 

  13.   <div class="titlepage"><div><div><h1 style="clear: both">Auto-throwing filesystem_error</h1></div></div></div>
    14. 

  14.   <p>Something not mentioned at all until now (and properly described in the next
    15. 

  15. section, <a href="../../default-actions/">Default actions</a>) is that Outcome can be
    16. 

  16. programmed take various actions when the user tries to observe <code>.value()</code>
    17. 

  17. when there is no value, and so on for the other possible state observations.</p>
    18. 

  18. 

  19. <p>Seeing as we are replacing the throwing overload of <code>copy_file()</code> in the
    20. 

  20. Filesystem TS with a <code>result</code> returning edition instead, it would make
    21. 

  21. sense if an attempt to observe the value of an unsuccessful <code>fs_result</code>
    22. 

  22. threw the exact same <code>filesystem_error</code> as the Filesystem TS does.</p>
    23. 

  23. 

  24. <p>Telling Outcome how to throw a <code>filesystem_error</code> with payload of the
    25. 

  25. failing paths is easy:</p>
    26. 

  26. 

  27. <div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="k">namespace</span> <span class="n">filesystem2</span>
    28. 

  28. <span class="p">{</span>
    29. 

  29.   <span class="c1">// If we would like Outcome to do something other than the default action (see next
    30. 

  30. </span><span class="c1"></span>  <span class="c1">// section), we can declare this ADL discovered free function to customise what
    31. 

  31. </span><span class="c1"></span>  <span class="c1">// to do instead.
    32. 

  32. </span><span class="c1"></span>  <span class="c1">//
    33. 

  33. </span><span class="c1"></span>  <span class="c1">// Note that rvalue semantics are propagated internally by Outcome, so if the user
    34. 

  34. </span><span class="c1"></span>  <span class="c1">// called .value() on a rvalued result, failure_info will be moved rather than
    35. 

  35. </span><span class="c1"></span>  <span class="c1">// copied from the result. That means we can overload once with value semantics,
    36. 

  36. </span><span class="c1"></span>  <span class="c1">// and not need to overload for lvalue and rvalue situations unless we really feel
    37. 

  37. </span><span class="c1"></span>  <span class="c1">// we need to for some reason.
    38. 

  38. </span><span class="c1"></span>  <span class="kr">inline</span> <span class="kt">void</span> <span class="n">outcome_throw_as_system_error_with_payload</span><span class="p">(</span><span class="n">failure_info</span> <span class="n">fi</span><span class="p">)</span>
    39. 

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

  40.     <span class="c1">// If the error code is not filesystem related e.g. ENOMEM, throw that as a
    41. 

  41. </span><span class="c1"></span>    <span class="c1">// standard STL exception.
    42. 

  42. </span><span class="c1"></span>    <span class="n">outcome</span><span class="o">::</span><span class="n">try_throw_std_exception_from_error</span><span class="p">(</span><span class="n">fi</span><span class="p">.</span><span class="n">ec</span><span class="p">);</span>
    43. 

  43.     <span class="c1">// Throw the exact same filesystem_error exception which the throwing copy_file()
    44. 

  44. </span><span class="c1"></span>    <span class="c1">// edition does.
    45. 

  45. </span><span class="c1"></span>    <span class="k">throw</span> <span class="nf">filesystem_error</span><span class="p">(</span><span class="n">fi</span><span class="p">.</span><span class="n">ec</span><span class="p">.</span><span class="n">message</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">fi</span><span class="p">.</span><span class="n">path1</span><span class="p">),</span>  <span class="c1">//
    46. 

  46. </span><span class="c1"></span>                           <span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">fi</span><span class="p">.</span><span class="n">path2</span><span class="p">),</span> <span class="n">fi</span><span class="p">.</span><span class="n">ec</span><span class="p">);</span>
    47. 

  47.   <span class="p">}</span>
    48. 

  48. <span class="p">}</span>
    49. 

  49. </code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/outcome_payload.cpp#L100" class="code-snippet-url" target="_blank">View this code on Github</a></div>
    50. 

  50. 

  51. 

  52. <p>Reference documentation for the above functions:</p>
    53. 

  53. 

  54. <ul>
    55. 

  55. <li><a href="../../../reference/functions/policy.html">List of builtin <code>outcome_throw_as_system_error_with_payload()</code> overloads</a></li>
    56. 

  56. <li><a href="../../../reference/functions/try_throw_std_exception_from_error.html" class="api-reference"><code>void try_throw_std_exception_from_error(std::error_code ec, const std::string &amp;msg = std::string{})</code></a>
    57. 

  57. </li>
    58. 

  58. </ul>
    59. 

  59. 

  60. <p>Usage of our new &ldquo;upgraded&rdquo; Filesystem <code>copy_file()</code> might now be as follows:</p>
    61. 

  61. 

  62. <div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++">  <span class="c1">// Non-throwing use case
    63. 

  63. </span><span class="c1"></span>  <span class="k">auto</span> <span class="n">o</span> <span class="o">=</span> <span class="n">filesystem2</span><span class="o">::</span><span class="n">copy_file</span><span class="p">(</span><span class="s">&#34;dontexist&#34;</span><span class="p">,</span> <span class="s">&#34;alsodontexist&#34;</span><span class="p">);</span>
    64. 

  64.   <span class="k">if</span><span class="p">(</span><span class="o">!</span><span class="n">o</span><span class="p">)</span>
    65. 

  65.   <span class="p">{</span>
    66. 

  66.     <span class="n">std</span><span class="o">::</span><span class="n">cerr</span> <span class="o">&lt;&lt;</span> <span class="s">&#34;Copy file failed with error &#34;</span> <span class="o">&lt;&lt;</span> <span class="n">o</span><span class="p">.</span><span class="n">error</span><span class="p">().</span><span class="n">ec</span><span class="p">.</span><span class="n">message</span><span class="p">()</span>                   <span class="c1">//
    67. 

  67. </span><span class="c1"></span>              <span class="o">&lt;&lt;</span> <span class="s">&#34; (path1 = &#34;</span> <span class="o">&lt;&lt;</span> <span class="n">o</span><span class="p">.</span><span class="n">error</span><span class="p">().</span><span class="n">path1</span> <span class="o">&lt;&lt;</span> <span class="s">&#34;, path2 = &#34;</span> <span class="o">&lt;&lt;</span> <span class="n">o</span><span class="p">.</span><span class="n">error</span><span class="p">().</span><span class="n">path2</span> <span class="o">&lt;&lt;</span> <span class="s">&#34;)&#34;</span>  <span class="c1">//
    68. 

  68. </span><span class="c1"></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>
    69. 

  69.   <span class="p">}</span>
    70. 

  70. 

  71.   <span class="c1">// Throwing use case
    72. 

  72. </span><span class="c1"></span>  <span class="k">try</span>
    73. 

  73.   <span class="p">{</span>
    74. 

  74.     <span class="c1">// Try to observe the successful value, thus triggering default actions which invokes
    75. 

  75. </span><span class="c1"></span>    <span class="c1">// our outcome_throw_as_system_error_with_payload() above which then throws filesystem_error
    76. 

  76. </span><span class="c1"></span>    <span class="c1">// exactly like the Filesystem TS does for its throwing overload.
    77. 

  77. </span><span class="c1"></span>    <span class="n">filesystem2</span><span class="o">::</span><span class="n">copy_file</span><span class="p">(</span><span class="s">&#34;dontexist&#34;</span><span class="p">,</span> <span class="s">&#34;alsodontexist&#34;</span><span class="p">).</span><span class="n">value</span><span class="p">();</span>
    78. 

  78.   <span class="p">}</span>
    79. 

  79.   <span class="k">catch</span><span class="p">(</span><span class="k">const</span> <span class="n">filesystem2</span><span class="o">::</span><span class="n">filesystem_error</span> <span class="o">&amp;</span><span class="n">e</span><span class="p">)</span>
    80. 

  80.   <span class="p">{</span>
    81. 

  81.     <span class="n">std</span><span class="o">::</span><span class="n">cerr</span> <span class="o">&lt;&lt;</span> <span class="s">&#34;Copy file failed with exception &#34;</span> <span class="o">&lt;&lt;</span> <span class="n">e</span><span class="p">.</span><span class="n">what</span><span class="p">()</span>                 <span class="c1">//
    82. 

  82. </span><span class="c1"></span>              <span class="o">&lt;&lt;</span> <span class="s">&#34; (path1 = &#34;</span> <span class="o">&lt;&lt;</span> <span class="n">e</span><span class="p">.</span><span class="n">path1</span><span class="p">()</span> <span class="o">&lt;&lt;</span> <span class="s">&#34;, path2 = &#34;</span> <span class="o">&lt;&lt;</span> <span class="n">e</span><span class="p">.</span><span class="n">path2</span><span class="p">()</span> <span class="o">&lt;&lt;</span> <span class="s">&#34;)&#34;</span>  <span class="c1">//
    83. 

  83. </span><span class="c1"></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>
    84. 

  84.   <span class="p">}</span>
    85. 

  85.   <span class="k">catch</span><span class="p">(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">exception</span> <span class="o">&amp;</span><span class="n">e</span><span class="p">)</span>
    86. 

  86.   <span class="p">{</span>
    87. 

  87.     <span class="n">std</span><span class="o">::</span><span class="n">cerr</span> <span class="o">&lt;&lt;</span> <span class="s">&#34;Copy file failed with exception &#34;</span> <span class="o">&lt;&lt;</span> <span class="n">e</span><span class="p">.</span><span class="n">what</span><span class="p">()</span>  <span class="c1">//
    88. 

  88. </span><span class="c1"></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>
    89. 

  89.   <span class="p">}</span>
    90. 

  90. </code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/outcome_payload.cpp#L127" class="code-snippet-url" target="_blank">View this code on Github</a></div>
    91. 

  91. 

  92. 

  93. 

  94.         </div><p><small>Last revised: February 08, 2019 at 22:18:08 UTC</small></p>
    95. 

  95. <hr>
    96. 

  96. <div class="spirit-nav">
    97. 

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    101.