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<div class="titlepage"><div><div><h1 style="clear: both">Redefining <code>message()</code></h1></div></div></div>
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<p>You may remember that our custom <code>_file_io_error_domain</code> inherits from
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<code>outcome_e::posix_code::domain_type</code>, and thus does not have to
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implement the many pure virtual functions required by <code>outcome_e::status_code_domain</code>.</p>
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<p>What we do need to do is reimplement <code>_do_message()</code> to append the
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file and line information to the POSIX error description string
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returned by <code>outcome_e::posix_code::domain_type</code>. This causes
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the status code’s <code>.message()</code> observer to return a string
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with the extra payload information represented in text.</p>
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<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
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</span><span class="c1"></span> <span class="c1">// string returned by our POSIX code base domain, with the source
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</span><span class="c1"></span> <span class="c1">// file and line number appended
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</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"><</span><span class="kt">void</span><span class="o">></span> <span class="o">&</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
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</span><span class="c1"></span> <span class="p">{</span>
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<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>
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<span class="c1">// Fetch message from base domain (POSIX)
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</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>
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<span class="k">const</span> <span class="k">auto</span> <span class="o">&</span><span class="n">c1</span> <span class="o">=</span> <span class="k">static_cast</span><span class="o"><</span><span class="k">const</span> <span class="n">file_io_error</span> <span class="o">&></span><span class="p">(</span><span class="n">code</span><span class="p">);</span> <span class="c1">// NOLINT
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</span><span class="c1"></span> <span class="k">const</span> <span class="n">value_type</span> <span class="o">&</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>
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<span class="c1">// Append my source file and line number
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</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>
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<span class="p">{</span>
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<span class="k">return</span> <span class="n">msg</span><span class="p">;</span>
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<span class="p">}</span>
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<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>
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<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"><</span><span class="kt">char</span> <span class="o">*></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
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</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>
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<span class="p">{</span>
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<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">"failed to get message from system"</span><span class="p">);</span>
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<span class="p">}</span>
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<span class="n">sprintf</span><span class="p">(</span><span class="n">p</span><span class="p">,</span> <span class="s">"%s (%s:%d)"</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>
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<span class="c1">// Return as atomically reference counted string
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</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>
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<span class="p">}</span>
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<span class="p">};</span>
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</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>
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</div><p><small>Last revised: January 26, 2019 at 23:38:56 UTC</small></p>
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