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Edouard DUPIN d0115b733d [DEV] add v1.76.0
2021-10-05 21:37:46 +02:00

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<div class="title">boost::hana::when&lt; condition &gt; Struct Template Reference<div class="ingroups"><a class="el" href="group__group-core.html">Core</a></div></div> </div>
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<a name="details" id="details"></a><h2 class="groupheader">Description</h2>
<div class="textblock"><h3>template&lt;bool condition&gt;<br />
struct boost::hana::when&lt; condition &gt;</h3>
<p>Enable a partial specialization only if a boolean condition is true. </p>
<p>You might also want to take a look at <code>when_valid</code>, which provides similar functionality but enables a specialziation only when some expression is well-formed.</p>
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<p></p>
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<h4><a class="anchor" id="autotoc_md202"></a>
Rationale for using &lt;tt&gt;when&lt;/tt&gt; instead of &lt;tt&gt;std::enable_if&lt;/tt&gt;</h4>
<p><code>when</code> is used to control the priority of partial specializations in a finer grained manner than what can be achieved with the usual <code>typename Enable = void</code> and <code>std::enable_if</code> pattern. For example, a partially specialized tag-dispatched method will have a higher priority than an equivalent specialization that uses <code>when</code>. For more details, see the tutorial section on <a class="el" href="index.html#tutorial-core-tag_dispatching">tag-dispatching</a>. </p>
<h2><a class="anchor" id="autotoc_md203"></a>
Example</h2>
<div class="fragment"><div class="line"><span class="comment">// Copyright Louis Dionne 2013-2017</span></div>
<div class="line"><span class="comment">// Distributed under the Boost Software License, Version 1.0.</span></div>
<div class="line"><span class="comment">// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)</span></div>
<div class="line"> </div>
<div class="line"><span class="preprocessor">#include &lt;<a class="code" href="core_2when_8hpp.html">boost/hana/core/when.hpp</a>&gt;</span></div>
<div class="line"> </div>
<div class="line"><span class="preprocessor">#include &lt;type_traits&gt;</span></div>
<div class="line"><span class="keyword">namespace </span>hana = <a class="code" href="namespaceboost_1_1hana.html">boost::hana</a>;</div>
<div class="line"> </div>
<div class="line"> </div>
<div class="line"><span class="keyword">template</span> &lt;<span class="keyword">typename</span> T, <span class="keyword">typename</span> = hana::when&lt;true&gt;&gt;</div>
<div class="line"><span class="keyword">struct </span>base_template;</div>
<div class="line"> </div>
<div class="line"><span class="keyword">template</span> &lt;<span class="keyword">typename</span> T&gt;</div>
<div class="line"><span class="keyword">struct </span>base_template&lt;T, hana::when&lt;std::is_integral&lt;T&gt;::value&gt;&gt; {</div>
<div class="line"> <span class="comment">// something useful...</span></div>
<div class="line">};</div>
<div class="line"> </div>
<div class="line"><span class="keywordtype">int</span> main() { }</div>
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<div class="ttc" id="acore_2when_8hpp_html"><div class="ttname"><a href="core_2when_8hpp.html">when.hpp</a></div><div class="ttdoc">Defines boost::hana::when and boost::hana::when_valid.</div></div>
<div class="ttc" id="anamespaceboost_1_1hana_html"><div class="ttname"><a href="namespaceboost_1_1hana.html">boost::hana</a></div><div class="ttdoc">Namespace containing everything in the library.</div><div class="ttdef"><b>Definition:</b> accessors.hpp:20</div></div>
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