123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158159160161162163164165166 |
- <!--
- Copyright Louis Dionne 2013-2017
- Distributed under the Boost Software License, Version 1.0.
- (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
- -->
- <!-- boost-no-inspect -->
- <!-- HTML header for doxygen 1.8.9.1-->
- <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
- <html xmlns="http://www.w3.org/1999/xhtml">
- <head>
- <meta http-equiv="Content-Type" content="text/xhtml;charset=UTF-8"/>
- <meta http-equiv="X-UA-Compatible" content="IE=9"/>
- <meta name="generator" content="Doxygen 1.8.15"/>
- <title>Boost.Hana: boost::hana::common< T, U, enabler > Struct Template Reference</title>
- <link href="tabs.css" rel="stylesheet" type="text/css"/>
- <script type="text/javascript" src="jquery.js"></script>
- <script type="text/javascript" src="dynsections.js"></script>
- <link href="navtree.css" rel="stylesheet" type="text/css"/>
- <script type="text/javascript" src="resize.js"></script>
- <script type="text/javascript" src="navtreedata.js"></script>
- <script type="text/javascript" src="navtree.js"></script>
- <script type="text/javascript">
- /* @license magnet:?xt=urn:btih:cf05388f2679ee054f2beb29a391d25f4e673ac3&dn=gpl-2.0.txt GPL-v2 */
- $(document).ready(initResizable);
- /* @license-end */</script>
- <link href="search/search.css" rel="stylesheet" type="text/css"/>
- <script type="text/javascript" src="search/searchdata.js"></script>
- <script type="text/javascript" src="search/search.js"></script>
- <script type="text/javascript">
- /* @license magnet:?xt=urn:btih:cf05388f2679ee054f2beb29a391d25f4e673ac3&dn=gpl-2.0.txt GPL-v2 */
- $(document).ready(function() { init_search(); });
- /* @license-end */
- </script>
- <script type="text/x-mathjax-config">
- MathJax.Hub.Config({
- extensions: ["tex2jax.js"],
- jax: ["input/TeX","output/HTML-CSS"],
- });
- // Copyright Louis Dionne 2013-2017
- // Distributed under the Boost Software License, Version 1.0.
- // (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
- MathJax.Hub.Config({
- "HTML-CSS": {
- linebreaks: {
- automatic: true,
- width: "75% container"
- }
- }
- });
- </script><script type="text/javascript" async="async" src="https://cdn.mathjax.org/mathjax/latest/MathJax.js"></script>
- <link href="doxygen.css" rel="stylesheet" type="text/css" />
- <!-- Additional javascript for drawing charts. -->
- <script type="text/javascript" src="highcharts.js"></script>
- <script type="text/javascript" src="highcharts-data.js"></script>
- <script type="text/javascript" src="highcharts-exporting.js"></script>
- <script type="text/javascript" src="chart.js"></script>
- <script type="text/javascript" src="hana.js"></script>
- </head>
- <body>
- <div id="top"><!-- do not remove this div, it is closed by doxygen! -->
- <div id="titlearea">
- <table cellspacing="0" cellpadding="0">
- <tbody>
- <tr style="height: 56px;">
- <td id="projectlogo"><img alt="Logo" src="Boost.png"/></td>
- <td style="padding-left: 0.5em;">
- <div id="projectname">Boost.Hana
-  <span id="projectnumber">1.6.0</span>
- </div>
- <div id="projectbrief">Your standard library for metaprogramming</div>
- </td>
- <td> <div id="MSearchBox" class="MSearchBoxInactive">
- <span class="left">
- <img id="MSearchSelect" src="search/mag_sel.png"
- onmouseover="return searchBox.OnSearchSelectShow()"
- onmouseout="return searchBox.OnSearchSelectHide()"
- alt=""/>
- <input type="text" id="MSearchField" value="Search" accesskey="S"
- onfocus="searchBox.OnSearchFieldFocus(true)"
- onblur="searchBox.OnSearchFieldFocus(false)"
- onkeyup="searchBox.OnSearchFieldChange(event)"/>
- </span><span class="right">
- <a id="MSearchClose" href="javascript:searchBox.CloseResultsWindow()"><img id="MSearchCloseImg" border="0" src="search/close.png" alt=""/></a>
- </span>
- </div>
- </td>
- </tr>
- </tbody>
- </table>
- </div>
- <!-- end header part -->
- <!-- Generated by Doxygen 1.8.15 -->
- <script type="text/javascript">
- /* @license magnet:?xt=urn:btih:cf05388f2679ee054f2beb29a391d25f4e673ac3&dn=gpl-2.0.txt GPL-v2 */
- var searchBox = new SearchBox("searchBox", "search",false,'Search');
- /* @license-end */
- </script>
- </div><!-- top -->
- <div id="side-nav" class="ui-resizable side-nav-resizable">
- <div id="nav-tree">
- <div id="nav-tree-contents">
- <div id="nav-sync" class="sync"></div>
- </div>
- </div>
- <div id="splitbar" style="-moz-user-select:none;"
- class="ui-resizable-handle">
- </div>
- </div>
- <script type="text/javascript">
- /* @license magnet:?xt=urn:btih:cf05388f2679ee054f2beb29a391d25f4e673ac3&dn=gpl-2.0.txt GPL-v2 */
- $(document).ready(function(){initNavTree('structboost_1_1hana_1_1common.html','');});
- /* @license-end */
- </script>
- <div id="doc-content">
- <!-- window showing the filter options -->
- <div id="MSearchSelectWindow"
- onmouseover="return searchBox.OnSearchSelectShow()"
- onmouseout="return searchBox.OnSearchSelectHide()"
- onkeydown="return searchBox.OnSearchSelectKey(event)">
- </div>
- <!-- iframe showing the search results (closed by default) -->
- <div id="MSearchResultsWindow">
- <iframe src="javascript:void(0)" frameborder="0"
- name="MSearchResults" id="MSearchResults">
- </iframe>
- </div>
- <div class="header">
- <div class="headertitle">
- <div class="title">boost::hana::common< T, U, enabler > Struct Template Reference<div class="ingroups"><a class="el" href="group__group-core.html">Core</a></div></div> </div>
- </div><!--header-->
- <div class="contents">
- <a name="details" id="details"></a><h2 class="groupheader">Description</h2>
- <div class="textblock"><h3>template<typename T, typename U, optional when-based enabler><br />
- struct boost::hana::common< T, U, enabler ></h3>
- <p>Metafunction returning the common data type between two data types. </p>
- <p><code>common</code> is a natural extension of the <code>std::common_type</code> metafunction to data types. Given two data types <code>T</code> and <code>U</code>, we say that they share a common type <code>C</code> if both objects of data type <code>T</code> and objects of data type <code>U</code> may be converted (using <code>to</code>) to an object of data type <code>C</code>, and if that conversion is equality preserving. In other words, this means that for any objects <code>t1, t2</code> of data type <code>T</code> and <code>u1, u2</code> of data type <code>U</code>, the following law is satisfied: </p><div class="fragment"><div class="line">to<C>(t1) == to<C>(t2) <span class="keywordflow">if</span> and only <span class="keywordflow">if</span> t1 == t2</div><div class="line">to<C>(u1) == to<C>(u2) <span class="keywordflow">if</span> and only <span class="keywordflow">if</span> u1 == u2</div></div><!-- fragment --><p>The role of <code>common</code> is to provide an alias to such a <code>C</code> if it exists. In other words, if <code>T</code> and <code>U</code> have a common data type <code>C</code>, <code>common<T, U>::type</code> is an alias to <code>C</code>. Otherwise, <code>common<T, U></code> has no nested <code>type</code> and can be used in dependent contexts to exploit SFINAE. By default, the exact steps followed by <code>common</code> to determine the common type <code>C</code> of <code>T</code> and <code>U</code> are</p><ol type="1">
- <li>If <code>T</code> and <code>U</code> are the same, then <code>C</code> is <code>T</code>.</li>
- <li>Otherwise, if <code>true ? std::declval<T>() : std::declval<U>()</code> is well-formed, then <code>C</code> is the type of this expression after using <code>std::decay</code> on it. This is exactly the type that would have been returned by <code>std::common_type</code>, except that custom specializations of <code>std::common_type</code> are not taken into account.</li>
- <li>Otherwise, no common data type is detected and <code>common<T, U></code> does not have a nested <code>type</code> alias, unless it is specialized explicitly.</li>
- </ol>
- <p>As point 3 suggests, it is also possible (and sometimes necessary) to specialize <code>common</code> in the <code><a class="el" href="namespaceboost_1_1hana.html" title="Namespace containing everything in the library.">boost::hana</a></code> namespace for pairs of custom data types when the default behavior of <code>common</code> is not sufficient. Note that <code>when</code>-based specialization is supported when specializing <code>common</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>
- <blockquote class="doxtable">
- <h4>Rationale for requiring the conversion to be equality-preserving</h4>
- <p>This decision is aligned with a proposed concept design for the standard library (<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3351.pdf">N3351</a>). Also, if we did not require this, then all data types would trivially share the common data type <code>void</code>, since all objects can be converted to it. </p>
- </blockquote>
- <h2>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 <<a class="code" href="core_2common_8hpp.html">boost/hana/core/common.hpp</a>></span></div><div class="line"></div><div class="line"><span class="preprocessor">#include <type_traits></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">struct </span>Person { };</div><div class="line"><span class="keyword">struct </span>Employee : Person { };</div><div class="line"></div><div class="line">static_assert(std::is_same<hana::common<int, float>::type, <span class="keywordtype">float</span>>{}, <span class="stringliteral">""</span>);</div><div class="line">static_assert(std::is_same<hana::common<Person, Employee>::type, Person>{}, <span class="stringliteral">""</span>);</div><div class="line"></div><div class="line"><span class="keywordtype">int</span> main() { }</div></div><!-- fragment --> </div></div><!-- contents -->
- </div><!-- doc-content -->
- <!--
- Copyright Louis Dionne 2013-2017
- Distributed under the Boost Software License, Version 1.0.
- (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
- -->
- <!-- boost-no-inspect -->
- <!-- HTML footer for doxygen 1.8.9.1-->
- <!-- start footer part -->
- <div id="nav-path" class="navpath"><!-- id is needed for treeview function! -->
- <ul>
- <li class="navelem"><b>boost</b></li><li class="navelem"><a class="el" href="namespaceboost_1_1hana.html">hana</a></li><li class="navelem"><a class="el" href="structboost_1_1hana_1_1common.html">common</a></li>
- </ul>
- </div>
- </body>
- </html>
|