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app-map-httplib1.html

app-map-httplib1.html 11 KB
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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>Mapping the HTTP library into the Application &lt;sup&gt;1&lt;/sup&gt;&frasl;&lt;sub&gt;2&lt;/sub&gt; - Boost.Outcome documentation</title>
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  9. <body><div class="spirit-nav">
    10. 

  10. <a accesskey="p" href="../../../tutorial/advanced/interop/app.html"><img src="../../../images/prev.png" alt="Prev"></a>
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  11.     <a accesskey="u" href="../../../tutorial/advanced/interop.html"><img src="../../../images/up.png" alt="Up"></a>
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  12.     <a accesskey="h" href="../../../index.html"><img src="../../../images/home.png" alt="Home"></a><a accesskey="n" href="../../../tutorial/advanced/interop/app-map-httplib2.html"><img src="../../../images/next.png" alt="Next"></a></div><div id="content">
    13. 

  13.   <div class="titlepage"><div><div><h1 style="clear: both">Mapping the HTTP library into the Application <code>1/2</code></h1></div></div></div>
    14. 

  14.   <p>Firstly, remember that we are the application writer who has the problem of
    15. 

  15. integrating three third party libraries into our application&rsquo;s Outcome-based
    16. 

  16. failure handling mechanism. We cannot modify those third party library
    17. 

  17. sources; we must be <em>non-intrusive</em>.</p>
    18. 

  18. 

  19. <p>We start by dealing with the HTTP library. We will integrate this
    20. 

  20. into our application by wrapping up <code>httplib::failure</code> into a custom
    21. 

  21. STL exception type. We then type erase it into an <code>exception_ptr</code>
    22. 

  22. instance. Please note that this code is exclusively defined in the <code>app</code> namespace:</p>
    23. 

  23. 

  24. <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">app</span>
    25. 

  25. <span class="p">{</span>
    26. 

  26.   <span class="c1">// Specialise an exception type for httplib errors
    27. 

  27. </span><span class="c1"></span>  <span class="k">struct</span> <span class="nl">httplib_error</span> <span class="p">:</span> <span class="n">std</span><span class="o">::</span><span class="n">runtime_error</span>
    28. 

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

  29.     <span class="c1">// passthrough
    30. 

  30. </span><span class="c1"></span>    <span class="k">using</span> <span class="n">std</span><span class="o">::</span><span class="n">runtime_error</span><span class="o">::</span><span class="n">runtime_error</span><span class="p">;</span>
    31. 

  31.     <span class="n">httplib_error</span><span class="p">(</span><span class="n">httplib</span><span class="o">::</span><span class="n">failure</span> <span class="n">_failure</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="n">msg</span><span class="p">)</span>
    32. 

  32.         <span class="o">:</span> <span class="n">std</span><span class="o">::</span><span class="n">runtime_error</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">msg</span><span class="p">))</span>
    33. 

  33.         <span class="p">,</span> <span class="n">failure</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">_failure</span><span class="p">))</span>
    34. 

  34.     <span class="p">{</span>
    35. 

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

  36. 

  37.     <span class="c1">// the original failure
    38. 

  38. </span><span class="c1"></span>    <span class="n">httplib</span><span class="o">::</span><span class="n">failure</span> <span class="n">failure</span><span class="p">;</span>
    39. 

  39.   <span class="p">};</span>
    40. 

  40. 

  41.   <span class="c1">// Type erase httplib::result&lt;U&gt; into a httplib_error exception ptr
    42. 

  42. </span><span class="c1"></span>  <span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span><span class="err"> </span><span class="nc">U</span><span class="o">&gt;</span>  <span class="c1">//
    43. 

  43. </span><span class="c1"></span>  <span class="kr">inline</span> <span class="n">std</span><span class="o">::</span><span class="n">exception_ptr</span> <span class="n">make_httplib_exception</span><span class="p">(</span><span class="k">const</span> <span class="n">httplib</span><span class="o">::</span><span class="n">result</span><span class="o">&lt;</span><span class="n">U</span><span class="o">&gt;</span> <span class="o">&amp;</span><span class="n">src</span><span class="p">)</span>
    44. 

  44.   <span class="p">{</span>
    45. 

  45.     <span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="n">str</span><span class="p">(</span><span class="s">&#34;httplib failed with error &#34;</span><span class="p">);</span>
    46. 

  46.     <span class="k">switch</span><span class="p">(</span><span class="n">src</span><span class="p">.</span><span class="n">error</span><span class="p">().</span><span class="n">status</span><span class="p">)</span>
    47. 

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

  48.     <span class="k">case</span> <span class="n">httplib</span><span class="o">::</span><span class="n">status_code</span><span class="o">::</span><span class="nl">success</span><span class="p">:</span>
    49. 

  49.       <span class="n">str</span><span class="p">.</span><span class="n">append</span><span class="p">(</span><span class="s">&#34;success&#34;</span><span class="p">);</span>
    50. 

  50.       <span class="k">break</span><span class="p">;</span>
    51. 

  51.     <span class="k">case</span> <span class="n">httplib</span><span class="o">::</span><span class="n">status_code</span><span class="o">::</span><span class="nl">bad_request</span><span class="p">:</span>
    52. 

  52.       <span class="n">str</span><span class="p">.</span><span class="n">append</span><span class="p">(</span><span class="s">&#34;bad request&#34;</span><span class="p">);</span>
    53. 

  53.       <span class="k">break</span><span class="p">;</span>
    54. 

  54.     <span class="k">case</span> <span class="n">httplib</span><span class="o">::</span><span class="n">status_code</span><span class="o">::</span><span class="nl">access_denied</span><span class="p">:</span>
    55. 

  55.       <span class="n">str</span><span class="p">.</span><span class="n">append</span><span class="p">(</span><span class="s">&#34;access denied&#34;</span><span class="p">);</span>
    56. 

  56.       <span class="k">break</span><span class="p">;</span>
    57. 

  57.     <span class="k">case</span> <span class="n">httplib</span><span class="o">::</span><span class="n">status_code</span><span class="o">::</span><span class="nl">logon_failed</span><span class="p">:</span>
    58. 

  58.       <span class="n">str</span><span class="p">.</span><span class="n">append</span><span class="p">(</span><span class="s">&#34;logon failed&#34;</span><span class="p">);</span>
    59. 

  59.       <span class="k">break</span><span class="p">;</span>
    60. 

  60.     <span class="k">case</span> <span class="n">httplib</span><span class="o">::</span><span class="n">status_code</span><span class="o">::</span><span class="nl">forbidden</span><span class="p">:</span>
    61. 

  61.       <span class="n">str</span><span class="p">.</span><span class="n">append</span><span class="p">(</span><span class="s">&#34;forbidden&#34;</span><span class="p">);</span>
    62. 

  62.       <span class="k">break</span><span class="p">;</span>
    63. 

  63.     <span class="k">case</span> <span class="n">httplib</span><span class="o">::</span><span class="n">status_code</span><span class="o">::</span><span class="nl">not_found</span><span class="p">:</span>
    64. 

  64.       <span class="n">str</span><span class="p">.</span><span class="n">append</span><span class="p">(</span><span class="s">&#34;not found&#34;</span><span class="p">);</span>
    65. 

  65.       <span class="k">break</span><span class="p">;</span>
    66. 

  66.     <span class="k">case</span> <span class="n">httplib</span><span class="o">::</span><span class="n">status_code</span><span class="o">::</span><span class="nl">internal_error</span><span class="p">:</span>
    67. 

  67.       <span class="n">str</span><span class="p">.</span><span class="n">append</span><span class="p">(</span><span class="s">&#34;internal error&#34;</span><span class="p">);</span>
    68. 

  68.       <span class="k">break</span><span class="p">;</span>
    69. 

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

  70.     <span class="n">str</span><span class="p">.</span><span class="n">append</span><span class="p">(</span><span class="s">&#34; [url was &#34;</span><span class="p">);</span>
    71. 

  71.     <span class="n">str</span><span class="p">.</span><span class="n">append</span><span class="p">(</span><span class="n">src</span><span class="p">.</span><span class="n">error</span><span class="p">().</span><span class="n">url</span><span class="p">);</span>
    72. 

  72.     <span class="n">str</span><span class="p">.</span><span class="n">append</span><span class="p">(</span><span class="s">&#34;]&#34;</span><span class="p">);</span>
    73. 

  73.     <span class="k">return</span> <span class="n">std</span><span class="o">::</span><span class="n">make_exception_ptr</span><span class="p">(</span><span class="n">httplib_error</span><span class="p">(</span><span class="n">src</span><span class="p">.</span><span class="n">error</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">str</span><span class="p">)));</span>
    74. 

  74.   <span class="p">}</span>
    75. 

  75. <span class="p">}</span>  <span class="c1">// namespace app
    76. 

  76. </span><span class="c1"></span></code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/finale.cpp#L195" class="code-snippet-url" target="_blank">View this code on Github</a></div>
    77. 

  77. 

  78. 

  79. <p>Most of the complexity in this code fragment is driven by the need to create
    80. 

  80. some sort of descriptive string for <code>std::runtime_error</code>
    81. 

  81. so its <code>.what()</code> returns a useful summary of the original failure. This
    82. 

  82. is the main purpose of the <code>app::make_httplib_exception()</code> function.</p>
    83. 

  83. 

  84. <p>(Note that if you have Reflection in your C++ compiler, it may be possible to script
    85. 

  85. the conversion of enum values to string representations)</p>
    86. 

  86. 

  87. <p>The only real thing to note about <code>app::httplib_error</code> is that it squirrels away
    88. 

  88. the original <code>httplib::failure</code> in case that is ever needed.</p>
    89. 

  89. 

  90. 

  91.         </div><p><small>Last revised: February 11, 2019 at 13:38:04 UTC</small></p>
    92. 

  92. <hr>
    93. 

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    98.