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<title>Mapping the HTTP library into the Application <sup>1</sup>⁄<sub>2</sub> - Boost.Outcome documentation</title>
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<div class="titlepage"><div><div><h1 style="clear: both">Mapping the HTTP library into the Application <code>1/2</code></h1></div></div></div>
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<p>Firstly, remember that we are the application writer who has the problem of
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integrating three third party libraries into our application’s Outcome-based
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failure handling mechanism. We cannot modify those third party library
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sources; we must be <em>non-intrusive</em>.</p>
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<p>We start by dealing with the HTTP library. We will integrate this
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into our application by wrapping up <code>httplib::failure</code> into a custom
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STL exception type. We then type erase it into an <code>exception_ptr</code>
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instance. Please note that this code is exclusively defined in the <code>app</code> namespace:</p>
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<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>
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<span class="p">{</span>
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<span class="c1">// Specialise an exception type for httplib errors
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</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>
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<span class="p">{</span>
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<span class="c1">// passthrough
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</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>
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<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>
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<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>
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<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>
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<span class="p">{</span>
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<span class="p">}</span>
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<span class="c1">// the original failure
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</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>
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<span class="p">};</span>
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<span class="c1">// Type erase httplib::result<U> into a httplib_error exception ptr
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</span><span class="c1"></span> <span class="k">template</span> <span class="o"><</span><span class="k">class</span><span class="err"> </span><span class="nc">U</span><span class="o">></span> <span class="c1">//
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</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"><</span><span class="n">U</span><span class="o">></span> <span class="o">&</span><span class="n">src</span><span class="p">)</span>
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<span class="p">{</span>
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<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">"httplib failed with error "</span><span class="p">);</span>
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<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>
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<span class="p">{</span>
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<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>
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<span class="n">str</span><span class="p">.</span><span class="n">append</span><span class="p">(</span><span class="s">"success"</span><span class="p">);</span>
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<span class="k">break</span><span class="p">;</span>
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<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>
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<span class="n">str</span><span class="p">.</span><span class="n">append</span><span class="p">(</span><span class="s">"bad request"</span><span class="p">);</span>
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<span class="k">break</span><span class="p">;</span>
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<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>
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<span class="n">str</span><span class="p">.</span><span class="n">append</span><span class="p">(</span><span class="s">"access denied"</span><span class="p">);</span>
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<span class="k">break</span><span class="p">;</span>
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<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>
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<span class="n">str</span><span class="p">.</span><span class="n">append</span><span class="p">(</span><span class="s">"logon failed"</span><span class="p">);</span>
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<span class="k">break</span><span class="p">;</span>
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<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>
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<span class="n">str</span><span class="p">.</span><span class="n">append</span><span class="p">(</span><span class="s">"forbidden"</span><span class="p">);</span>
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<span class="k">break</span><span class="p">;</span>
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<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>
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<span class="n">str</span><span class="p">.</span><span class="n">append</span><span class="p">(</span><span class="s">"not found"</span><span class="p">);</span>
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<span class="k">break</span><span class="p">;</span>
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<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>
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<span class="n">str</span><span class="p">.</span><span class="n">append</span><span class="p">(</span><span class="s">"internal error"</span><span class="p">);</span>
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<span class="k">break</span><span class="p">;</span>
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<span class="p">}</span>
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<span class="n">str</span><span class="p">.</span><span class="n">append</span><span class="p">(</span><span class="s">" [url was "</span><span class="p">);</span>
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<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>
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<span class="n">str</span><span class="p">.</span><span class="n">append</span><span class="p">(</span><span class="s">"]"</span><span class="p">);</span>
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<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>
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<span class="p">}</span>
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<span class="p">}</span> <span class="c1">// namespace app
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</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>
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<p>Most of the complexity in this code fragment is driven by the need to create
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some sort of descriptive string for <code>std::runtime_error</code>
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so its <code>.what()</code> returns a useful summary of the original failure. This
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is the main purpose of the <code>app::make_httplib_exception()</code> function.</p>
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<p>(Note that if you have Reflection in your C++ compiler, it may be possible to script
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the conversion of enum values to string representations)</p>
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<p>The only real thing to note about <code>app::httplib_error</code> is that it squirrels away
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the original <code>httplib::failure</code> in case that is ever needed.</p>
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</div><p><small>Last revised: February 11, 2019 at 13:38:04 UTC</small></p>
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