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message.html

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  3. <title>Redefining message() - Boost.Outcome documentation</title>
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    13. 

  13.   <div class="titlepage"><div><div><h1 style="clear: both">Redefining <code>message()</code></h1></div></div></div>
    14. 

  14.   <p>You may remember that our custom <code>_file_io_error_domain</code> inherits from
    15. 

  15. <code>outcome_e::posix_code::domain_type</code>, and thus does not have to
    16. 

  16. implement the many pure virtual functions required by <code>outcome_e::status_code_domain</code>.</p>
    17. 

  17. 

  18. <p>What we do need to do is reimplement <code>_do_message()</code> to append the
    19. 

  19. file and line information to the POSIX error description string
    20. 

  20. returned by <code>outcome_e::posix_code::domain_type</code>. This causes
    21. 

  21. the status code&rsquo;s <code>.message()</code> observer to return a string
    22. 

  22. with the extra payload information represented in text.</p>
    23. 

  23. 

  24. <div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++">  <span class="c1">// Return a string describing a specific code. We will return the
    25. 

  25. </span><span class="c1"></span>  <span class="c1">// string returned by our POSIX code base domain, with the source
    26. 

  26. </span><span class="c1"></span>  <span class="c1">// file and line number appended
    27. 

  27. </span><span class="c1"></span>  <span class="k">virtual</span> <span class="n">_base</span><span class="o">::</span><span class="n">string_ref</span> <span class="n">_do_message</span><span class="p">(</span><span class="k">const</span> <span class="n">outcome_e</span><span class="o">::</span><span class="n">status_code</span><span class="o">&lt;</span><span class="kt">void</span><span class="o">&gt;</span> <span class="o">&amp;</span><span class="n">code</span><span class="p">)</span> <span class="k">const</span> <span class="k">noexcept</span> <span class="k">override</span> <span class="k">final</span>  <span class="c1">// NOLINT
    28. 

  28. </span><span class="c1"></span>  <span class="p">{</span>
    29. 

  29.     <span class="n">assert</span><span class="p">(</span><span class="n">code</span><span class="p">.</span><span class="n">domain</span><span class="p">()</span> <span class="o">==</span> <span class="o">*</span><span class="k">this</span><span class="p">);</span>
    30. 

  30. 

  31.     <span class="c1">// Fetch message from base domain (POSIX)
    32. 

  32. </span><span class="c1"></span>    <span class="k">auto</span> <span class="n">msg</span> <span class="o">=</span> <span class="n">_base</span><span class="o">::</span><span class="n">_do_message</span><span class="p">(</span><span class="n">code</span><span class="p">);</span>
    33. 

  33.     <span class="k">const</span> <span class="k">auto</span> <span class="o">&amp;</span><span class="n">c1</span> <span class="o">=</span> <span class="k">static_cast</span><span class="o">&lt;</span><span class="k">const</span> <span class="n">file_io_error</span> <span class="o">&amp;&gt;</span><span class="p">(</span><span class="n">code</span><span class="p">);</span>  <span class="c1">// NOLINT
    34. 

  34. </span><span class="c1"></span>    <span class="k">const</span> <span class="n">value_type</span> <span class="o">&amp;</span><span class="n">v</span> <span class="o">=</span> <span class="n">c1</span><span class="p">.</span><span class="n">value</span><span class="p">();</span>
    35. 

  35. 

  36.     <span class="c1">// Append my source file and line number
    37. 

  37. </span><span class="c1"></span>    <span class="k">if</span><span class="p">(</span><span class="n">v</span><span class="p">.</span><span class="n">file</span> <span class="o">==</span> <span class="k">nullptr</span><span class="p">)</span>
    38. 

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

  39.       <span class="k">return</span> <span class="n">msg</span><span class="p">;</span>
    40. 

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

  41.     <span class="n">size_t</span> <span class="n">length</span> <span class="o">=</span> <span class="n">strlen</span><span class="p">(</span><span class="n">v</span><span class="p">.</span><span class="n">file</span><span class="p">)</span> <span class="o">+</span> <span class="mi">16</span> <span class="o">+</span> <span class="n">msg</span><span class="p">.</span><span class="n">size</span><span class="p">();</span>
    42. 

  42.     <span class="k">auto</span> <span class="o">*</span><span class="n">p</span> <span class="o">=</span> <span class="k">static_cast</span><span class="o">&lt;</span><span class="kt">char</span> <span class="o">*&gt;</span><span class="p">(</span><span class="n">malloc</span><span class="p">(</span><span class="n">length</span><span class="p">));</span>  <span class="c1">// NOLINT
    43. 

  43. </span><span class="c1"></span>    <span class="k">if</span><span class="p">(</span><span class="n">p</span> <span class="o">==</span> <span class="k">nullptr</span><span class="p">)</span>
    44. 

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

  45.       <span class="k">return</span> <span class="n">_base</span><span class="o">::</span><span class="n">string_ref</span><span class="p">(</span><span class="s">&#34;failed to get message from system&#34;</span><span class="p">);</span>
    46. 

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

  47.     <span class="n">sprintf</span><span class="p">(</span><span class="n">p</span><span class="p">,</span> <span class="s">&#34;%s (%s:%d)&#34;</span><span class="p">,</span> <span class="n">msg</span><span class="p">.</span><span class="n">data</span><span class="p">(),</span> <span class="n">v</span><span class="p">.</span><span class="n">file</span><span class="p">,</span> <span class="n">v</span><span class="p">.</span><span class="n">lineno</span><span class="p">);</span>
    48. 

  48. 

  49.     <span class="c1">// Return as atomically reference counted string
    50. 

  50. </span><span class="c1"></span>    <span class="k">return</span> <span class="n">_base</span><span class="o">::</span><span class="n">atomic_refcounted_string_ref</span><span class="p">(</span><span class="n">p</span><span class="p">,</span> <span class="n">length</span><span class="p">);</span>
    51. 

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

  52. <span class="p">};</span>
    53. 

  53. </code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/experimental_status_code.cpp#L111" class="code-snippet-url" target="_blank">View this code on Github</a></div>
    54. 

  54. 

  55. 

  56. 

  57.         </div><p><small>Last revised: January 26, 2019 at 23:38:56 UTC</small></p>
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