1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html><meta http-equiv="Content-Type" content="text/html; charset=utf-8"> 3<title>Mapping the HTTP library into the Application <sup>2</sup>⁄<sub>2</sub> - Boost.Outcome documentation</title> 4<link rel="stylesheet" href="../../../css/boost.css" type="text/css"> 5<meta name="generator" content="Hugo 0.52 with Boostdoc theme"> 6<meta name="viewport" content="width=device-width,initial-scale=1.0"/> 7 8<link rel="icon" href="../../../images/favicon.ico" type="image/ico"/> 9<body><div class="spirit-nav"> 10<a accesskey="p" href="../../../tutorial/advanced/interop/app-map-httplib1.html"><img src="../../../images/prev.png" alt="Prev"></a> 11 <a accesskey="u" href="../../../tutorial/advanced/interop.html"><img src="../../../images/up.png" alt="Up"></a> 12 <a accesskey="h" href="../../../index.html"><img src="../../../images/home.png" alt="Home"></a><a accesskey="n" href="../../../tutorial/advanced/interop/app-map-filelib.html"><img src="../../../images/next.png" alt="Next"></a></div><div id="content"> 13 <div class="titlepage"><div><div><h1 style="clear: both">Mapping the HTTP library into the Application <code>2/2</code></h1></div></div></div> 14 <p>If you remember the tutorial section on the <a href="../value-or-error"><code>ValueOrError</code> Concept</a>, 15this is an example of how to implement a custom <code>ValueOrError</code> Concept converter 16in Outcome:</p> 17 18<div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="c1">// Inject custom ValueOrError conversion 19</span><span class="c1"></span><span class="n">BOOST_OUTCOME_V2_NAMESPACE_BEGIN</span> 20<span class="k">namespace</span> <span class="n">convert</span> 21<span class="p">{</span> 22 <span class="c1">// Provide custom ValueOrError conversion from 23</span><span class="c1"></span> <span class="c1">// httplib::result<U> into any app::outcome<T> 24</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="c1">// 25</span><span class="c1"></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> 26 <span class="p">{</span> 27 <span class="c1">// False to indicate that this converter wants `result`/`outcome` 28</span><span class="c1"></span> <span class="c1">// to NOT reject all other `result` 29</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> 30 <span class="c1">// False to indicate that this converter wants `outcome` to NOT 31</span><span class="c1"></span> <span class="c1">// reject all other `outcome` 32</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> 33 34 <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">// 35</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">// 36</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">// 37</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> <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> 38 <span class="p">{</span> 39 <span class="c1">// Forward any successful value, else synthesise an exception ptr 40</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">// 41</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">// 42</span><span class="c1"></span> <span class="o">:</span> 43 <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> 44 <span class="p">}</span> 45 <span class="p">};</span> 46<span class="p">}</span> <span class="c1">// namespace convert 47</span><span class="c1"></span><span class="n">BOOST_OUTCOME_V2_NAMESPACE_END</span> 48</code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/finale.cpp#L257" class="code-snippet-url" target="_blank">View this code on Github</a></div> 49 50 51<p>The first thing that you should note is that these custom converters must be injected 52directly into the <code>BOOST_OUTCOME_V2_NAMESPACE::convert</code> namespace, and they must partially 53or completely specialise <a href="../../../reference/converters/value_or_error.html" class="api-reference"><code>value_or_error<T, U></code></a> 54. Here we specialise the 55converter for <code>ValueOrError</code> conversions from <code>httplib::result<U></code> to <code>app::outcome<T></code> 56i.e. from our third party HTTP library’s error type into our application’s <code>outcome</code> 57type (which is unique to our application, as we hard code an <code>app</code>-local error type).</p> 58 59<p>The second thing to note is that you need to set <code>enable_result_inputs</code> and <code>enable_outcome_inputs</code> 60appropriately, otherwise <code>result</code> and <code>outcome</code> inputs will not be matched by this 61converter<sup class="footnote-ref" id="fnref:1"><a href="#fn:1">1</a></sup>. In this converter, we really do wish to convert other <code>result</code> and 62<code>outcome</code> inputs, so we mark these booleans as <code>true</code>.</p> 63 64<p>The third thing to note is the requirements on <code>operator()</code>. If the requirements are 65not met, the <code>ValueOrError</code> converting constructor in <code>basic_result</code> and <code>basic_outcome</code> 66disables. Note the requirement that the decayed <code>operator()</code> input <code>X</code> matches 67<code>httplib::result<U></code>, and that <code>T</code> is constructible from <code>U</code>. This means that the 68<a href="../../../reference/types/basic_result/explicit_valueorerror_converting_constructor.html" class="api-reference"><code>explicit basic_result(ValueOrError<T, E> &&)</code></a> 69 and <a href="../../../reference/types/basic_outcome/explicit_valueorerror_converting_constructor.html" class="api-reference"><code>explicit basic_outcome(ValueOrError<T, E> &&)</code></a> 70 71constructors are available if, and only if, the input type is a <code>httplib::result<U></code>, 72and the result’s value type is constructible from the input’s value type.</p> 73 74<p>If <code>operator()</code> is available, it naturally converts a <code>httplib::result<U></code> into an 75<code>app::outcome<T></code> by either forwarding any success as-is, or calling <code>app::make_httplib_exception()</code> 76to type erase the <code>httplib::failure</code> into an <code>app::httplib_error</code>.</p> 77<div class="footnotes"> 78 79<hr /> 80 81<ol> 82<li id="fn:1">Here we refer to <code>result</code> and <code>outcome</code> as defined by this specific Outcome library. If <code>result</code> or <code>outcome</code> from another Outcome implementation is seen, those <strong>always</strong> must get parsed via the <code>ValueOrError</code> matching conversion framework. 83 <a class="footnote-return" href="#fnref:1"><sup>[return]</sup></a></li> 84</ol> 85</div> 86 87 88 </div><p><small>Last revised: February 11, 2019 at 13:38:04 UTC</small></p> 89<hr> 90<div class="spirit-nav"> 91<a accesskey="p" href="../../../tutorial/advanced/interop/app-map-httplib1.html"><img src="../../../images/prev.png" alt="Prev"></a> 92 <a accesskey="u" href="../../../tutorial/advanced/interop.html"><img src="../../../images/up.png" alt="Up"></a> 93 <a accesskey="h" href="../../../index.html"><img src="../../../images/home.png" alt="Home"></a><a accesskey="n" href="../../../tutorial/advanced/interop/app-map-filelib.html"><img src="../../../images/next.png" alt="Next"></a></div></body> 94</html> 95
