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<div class="titlepage"><div><div><h2 class="title" style="clear: both">Mapping the HTTP library into the Application 2/2</h2></div></div></div>
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<p>If you remember the tutorial section on the <a href="../value-or-error"><code>ValueOrError</code> Concept</a>,
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this is an example of how to implement a custom <code>ValueOrError</code> Concept converter
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in Outcome:</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">// Inject custom ValueOrError conversion
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</span><span class="c1"></span><span class="n">BOOST_OUTCOME_V2_NAMESPACE_BEGIN</span>
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<span class="k">namespace</span> <span class="n">convert</span>
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<span class="p">{</span>
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<span class="c1">// Provide custom ValueOrError conversion from httplib::result<U> into any app::outcome<T>
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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">T</span><span class="p">,</span> <span class="k">class</span><span class="err"> </span><span class="nc">U</span><span class="o">></span> <span class="k">struct</span> <span class="n">value_or_error</span><span class="o"><</span><span class="n">app</span><span class="o">::</span><span class="n">outcome</span><span class="o"><</span><span class="n">T</span><span class="o">></span><span class="p">,</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>
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<span class="p">{</span>
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<span class="c1">// False to indicate that this converter wants `result`/`outcome` to NOT reject all other `result`
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</span><span class="c1"></span> <span class="k">static</span> <span class="k">constexpr</span> <span class="kt">bool</span> <span class="n">enable_result_inputs</span> <span class="o">=</span> <span class="nb">true</span><span class="p">;</span>
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<span class="c1">// False to indicate that this converter wants `outcome` to NOT reject all other `outcome`
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</span><span class="c1"></span> <span class="k">static</span> <span class="k">constexpr</span> <span class="kt">bool</span> <span class="n">enable_outcome_inputs</span> <span class="o">=</span> <span class="nb">true</span><span class="p">;</span>
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<span class="k">template</span> <span class="o"><</span><span class="k">class</span><span class="err"> </span><span class="nc">X</span><span class="p">,</span> <span class="c1">//
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</span><span class="c1"></span> <span class="k">typename</span> <span class="o">=</span> <span class="n">std</span><span class="o">::</span><span class="n">enable_if_t</span><span class="o"><</span><span class="n">std</span><span class="o">::</span><span class="n">is_same</span><span class="o"><</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="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">decay_t</span><span class="o"><</span><span class="n">X</span><span class="o">>>::</span><span class="n">value</span> <span class="c1">//
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</span><span class="c1"></span> <span class="o">&&</span> <span class="n">std</span><span class="o">::</span><span class="n">is_constructible</span><span class="o"><</span><span class="n">T</span><span class="p">,</span> <span class="n">U</span><span class="o">>::</span><span class="n">value</span><span class="o">>></span> <span class="c1">//
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</span><span class="c1"></span> <span class="k">constexpr</span> <span class="n">app</span><span class="o">::</span><span class="n">outcome</span><span class="o"><</span><span class="n">T</span><span class="o">></span>
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<span class="k">operator</span><span class="p">()(</span><span class="n">X</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="c1">// Forward any successful value, else synthesise an exception ptr
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</span><span class="c1"></span> <span class="k">return</span> <span class="n">src</span><span class="p">.</span><span class="n">has_value</span><span class="p">()</span> <span class="o">?</span> <span class="c1">//
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</span><span class="c1"></span> <span class="n">app</span><span class="o">::</span><span class="n">outcome</span><span class="o"><</span><span class="n">T</span><span class="o">></span><span class="p">{</span><span class="n">std</span><span class="o">::</span><span class="n">forward</span><span class="o"><</span><span class="n">X</span><span class="o">></span><span class="p">(</span><span class="n">src</span><span class="p">).</span><span class="n">value</span><span class="p">()}</span> <span class="c1">//
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</span><span class="c1"></span> <span class="o">:</span>
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<span class="n">app</span><span class="o">::</span><span class="n">outcome</span><span class="o"><</span><span class="n">T</span><span class="o">></span><span class="p">{</span><span class="n">app</span><span class="o">::</span><span class="n">make_httplib_exception</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">forward</span><span class="o"><</span><span class="n">X</span><span class="o">></span><span class="p">(</span><span class="n">src</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="p">}</span>
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<span class="n">BOOST_OUTCOME_V2_NAMESPACE_END</span>
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</code></pre></div><a href="https://github.com/ned14/boost-outcome/tree/master/doc/src/snippets/finale.cpp#L238" class="code-snippet-url" target="_blank">View this code on Github</a></div>
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<p>The first thing that you should note is that these custom converters must be injected
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directly into the <code>BOOST_OUTCOME_V2_NAMESPACE::convert</code> namespace, and they must partially
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or completely specialise <code>struct value_or_error<DEST, SRC></code>.</p>
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<p>The second thing to note is that you need to set <code>enable_result_inputs</code> and <code>enable_outcome_inputs</code>
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appropriately. The reason that these are here is because of a chicken-or-egg problem
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whereby the <code>ValueOrError</code> infrastructure needs to be defined before <code>result</code> can be defined, and
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we also wish to permit inclusion of <code>result</code> without needing to include <code>outcome</code>.
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So this is a slightly irritating workaround, but one with low impact.</p>
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<p>The third thing to note is the SFINAE on <code>operator()</code>. If the SFINAE fails, the <code>ValueOrError</code>
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converting constructor disables. Obviously, if you are using Concept requirements,
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that works too. Note the requirement that SFINAEd <code>X</code> matches <code>httplib::result<U></code>, and that
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<code>T</code> is constructible from <code>U</code>.</p>
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<p>If <code>operator()</code> is available, it naturally converts a <code>httplib::result<U></code> into an
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<code>app::outcome<T></code> by either forwarding any success as-is, or calling <code>app::make_httplib_exception()</code>
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to type erase the <code>httplib::failure</code> into an <code>app::httplib_error</code>.</p>
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</div><p><small>Last revised: December 05, 2018 at 14:18:52 UTC</small></p>
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