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 125 <div class="title">boost::hana::tag_of&lt; T, enabler &gt; Struct Template Reference<div class="ingroups"><a class="el" href="group__group-core.html">Core</a></div></div>  </div>
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 127 <div class="contents">
 128 <a name="details" id="details"></a><h2 class="groupheader">Description</h2>
 129 <div class="textblock"><h3>template&lt;typename T, optional when-based enabler&gt;<br />
 130 struct boost::hana::tag_of&lt; T, enabler &gt;</h3>
 131
 132 <p>Metafunction returning the tag associated to <code>T</code>. </p>
 133 <p>There are several ways to specify the tag of a C++ type. If it's a user-defined type, one can define a nested <code>hana_tag</code> alias: </p><div class="fragment"><div class="line"><span class="keyword">struct </span>MyUserDefinedType {</div><div class="line">    <span class="keyword">using</span> hana_tag = MyTag;</div><div class="line">};</div></div><!-- fragment --><p>Sometimes, however, the C++ type can't be modified (if it's in a foreign library) or simply can't have nested types (if it's not a struct or class). In those cases, using a nested alias is impossible and so ad-hoc customization is also supported by specializing <code><a class="el" href="structboost_1_1hana_1_1tag__of.html" title="Metafunction returning the tag associated to T. ">tag_of</a></code> in the <code><a class="el" href="namespaceboost_1_1hana.html" title="Namespace containing everything in the library. ">boost::hana</a></code> namespace: </p><div class="fragment"><div class="line"><span class="keyword">struct </span>i_cant_modify_this;</div><div class="line"></div><div class="line"><span class="keyword">namespace </span><a class="code" href="namespaceboost.html">boost</a> { <span class="keyword">namespace </span>hana {</div><div class="line">    <span class="keyword">template</span> &lt;&gt;</div><div class="line">    <span class="keyword">struct </span>tag_of&lt;i_cant_modify_this&gt; {</div><div class="line">        <span class="keyword">using</span> type = MyTag;</div><div class="line">    };</div><div class="line">}}</div></div><!-- fragment --><p><code><a class="el" href="structboost_1_1hana_1_1tag__of.html" title="Metafunction returning the tag associated to T. ">tag_of</a></code> can also be specialized for all C++ types satisfying some boolean condition using <code>when</code>. <code>when</code> accepts a single compile-time boolean and enables the specialization of <code><a class="el" href="structboost_1_1hana_1_1tag__of.html" title="Metafunction returning the tag associated to T. ">tag_of</a></code> if and only if that boolean is <code>true</code>. This is similar to the well known C++ idiom of using a dummy template parameter with <code>std::enable_if</code> and relying on SFINAE. For example, we could specify the tag of all <code><a class="el" href="structboost_1_1fusion_1_1vector.html" title="Adapter for Boost.Fusion vectors. ">fusion::vector</a></code>s by doing: </p><div class="fragment"><div class="line"><span class="keyword">struct </span>BoostFusionVector;</div><div class="line"></div><div class="line"><span class="keyword">namespace </span><a class="code" href="namespaceboost.html">boost</a> { <span class="keyword">namespace </span>hana {</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>tag_of&lt;T, when&lt;</div><div class="line">        <a class="code" href="namespacestd.html">std</a>::is_same&lt;</div><div class="line">            typename fusion::traits::tag_of&lt;T&gt;::type,</div><div class="line">            fusion::traits::tag_of&lt;fusion::vector&lt;&gt;&gt;::type</div><div class="line">        &gt;{}</div><div class="line">    &gt;&gt; {</div><div class="line">        <span class="keyword">using</span> type = BoostFusionVector;</div><div class="line">    };</div><div class="line">}}</div></div><!-- fragment --><p>Also, when it is not specialized and when the given C++ type does not have a nested <code>hana_tag</code> alias, <code><a class="el" href="structboost_1_1hana_1_1tag__of.html" title="Metafunction returning the tag associated to T. ">tag_of</a>&lt;T&gt;</code> returns <code>T</code> itself. This makes tags a simple extension of normal C++ types. This is <em>super</em> useful, mainly for two reasons. First, this allows Hana to adopt a reasonable default behavior for some operations involving types that have no notion of tags. For example, Hana allows comparing with <code>equal</code> any two objects for which a valid <code>operator==</code> is defined, and that without any work on the user side. Second, it also means that you can ignore tags completely if you don't need their functionality; just use the normal C++ type of your objects and everything will "just work".</p>
 134 <p>Finally, also note that <code><a class="el" href="structboost_1_1hana_1_1tag__of.html" title="Metafunction returning the tag associated to T. ">tag_of</a>&lt;T&gt;</code> is always equivalent to <code><a class="el" href="structboost_1_1hana_1_1tag__of.html" title="Metafunction returning the tag associated to T. ">tag_of</a>&lt;U&gt;</code>, where <code>U</code> is the type <code>T</code> after being stripped of all references and cv-qualifiers. This makes it unnecessary to specialize <code><a class="el" href="structboost_1_1hana_1_1tag__of.html" title="Metafunction returning the tag associated to T. ">tag_of</a></code> for all reference and cv combinations, which would be a real pain. Also, <code><a class="el" href="structboost_1_1hana_1_1tag__of.html" title="Metafunction returning the tag associated to T. ">tag_of</a></code> is required to be idempotent. In other words, it must always be the case that <code><a class="el" href="structboost_1_1hana_1_1tag__of.html" title="Metafunction returning the tag associated to T. ">tag_of</a>&lt;<a class="el" href="structboost_1_1hana_1_1tag__of.html" title="Metafunction returning the tag associated to T. ">tag_of</a>&lt;T&gt;::type&gt;::type</code> is equivalent to <code><a class="el" href="structboost_1_1hana_1_1tag__of.html" title="Metafunction returning the tag associated to T. ">tag_of</a>&lt;T&gt;::type</code>.</p>
 135 <blockquote class="doxtable">
 136 <p><b>Tip 1</b><br />
 137 If compile-time performance is a serious concern, consider specializing the <code><a class="el" href="structboost_1_1hana_1_1tag__of.html" title="Metafunction returning the tag associated to T. ">tag_of</a></code> metafunction in Hana's namespace. When unspecialized, the metafunction has to use SFINAE, which tends to incur a larger compile-time overhead. For heavily used templated types, this can potentially make a difference. </p>
 138 </blockquote>
 139 <blockquote class="doxtable">
 140 <p><b>Tip 2</b><br />
 141 Consider using <code>tag_of_t</code> alias instead of <code><a class="el" href="structboost_1_1hana_1_1tag__of.html" title="Metafunction returning the tag associated to T. ">tag_of</a></code>, which reduces the amount of typing in dependent contexts. </p>
 142 </blockquote>
 143 <h2>Example </h2>
 144 <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_2tag__of_8hpp.html">boost/hana/core/tag_of.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">static_assert(std::is_same&lt;hana::tag_of&lt;int&gt;::type, <span class="keywordtype">int</span>&gt;{}, <span class="stringliteral">&quot;&quot;</span>);</div><div class="line">static_assert(std::is_same&lt;hana::tag_of&lt;int&amp;&gt;::type, <span class="keywordtype">int</span>&gt;{}, <span class="stringliteral">&quot;&quot;</span>);</div><div class="line">static_assert(std::is_same&lt;hana::tag_of&lt;int const&amp;&gt;::type, <span class="keywordtype">int</span>&gt;{}, <span class="stringliteral">&quot;&quot;</span>);</div><div class="line"></div><div class="line"><span class="keyword">struct </span>PersonTag;</div><div class="line"><span class="keyword">struct </span>Person { <span class="keyword">using</span> hana_tag = PersonTag; };</div><div class="line">static_assert(std::is_same&lt;hana::tag_of&lt;Person&gt;::type, PersonTag&gt;{}, <span class="stringliteral">&quot;&quot;</span>);</div><div class="line">static_assert(std::is_same&lt;hana::tag_of&lt;Person volatile&amp;&amp;&gt;::type, PersonTag&gt;{}, <span class="stringliteral">&quot;&quot;</span>);</div><div class="line"></div><div class="line"><span class="keywordtype">int</span> main() { }</div></div><!-- fragment --> </div></div><!-- contents -->
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