1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html> 3<head> 4<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 5<title>Class template make_recursive_variant</title> 6<link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> 7<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 8<link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> 9<link rel="up" href="../variant/reference.html#header.boost.variant.recursive_variant_hpp" title="Header <boost/variant/recursive_variant.hpp>"> 10<link rel="prev" href="make_variant_over.html" title="Class template make_variant_over"> 11<link rel="next" href="make_recurs_1_3_46_5_5_1_3.html" title="Class template make_recursive_variant_over"> 12</head> 13<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 14<table cellpadding="2" width="100%"><tr> 15<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> 16<td align="center"><a href="../../../index.html">Home</a></td> 17<td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> 18<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 19<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 20<td align="center"><a href="../../../more/index.htm">More</a></td> 21</tr></table> 22<hr> 23<div class="spirit-nav"> 24<a accesskey="p" href="make_variant_over.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../variant/reference.html#header.boost.variant.recursive_variant_hpp"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="make_recurs_1_3_46_5_5_1_3.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> 25</div> 26<div class="refentry"> 27<a name="boost.make_recursive_variant"></a><div class="titlepage"></div> 28<div class="refnamediv"> 29<h2><span class="refentrytitle">Class template make_recursive_variant</span></h2> 30<p>boost::make_recursive_variant — Simplifies declaration of recursive <code class="computeroutput">variant</code> types.</p> 31</div> 32<h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2> 33<div xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv"><pre class="synopsis"><span class="comment">// In header: <<a class="link" href="../variant/reference.html#header.boost.variant.recursive_variant_hpp" title="Header <boost/variant/recursive_variant.hpp>">boost/variant/recursive_variant.hpp</a>> 34 35</span><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> T1<span class="special">,</span> <span class="keyword">typename</span> T2 <span class="special">=</span> <span class="emphasis"><em><span class="identifier">unspecified</span></em></span><span class="special">,</span> <span class="special">...</span><span class="special">,</span> 36 <span class="keyword">typename</span> TN <span class="special">=</span> <span class="emphasis"><em><span class="identifier">unspecified</span></em></span><span class="special">></span> 37<span class="keyword">class</span> <a class="link" href="make_recursive_variant.html" title="Class template make_recursive_variant">make_recursive_variant</a> <span class="special">{</span> 38<span class="keyword">public</span><span class="special">:</span> 39 <span class="comment">// types</span> 40 <span class="keyword">typedef</span> <a class="link" href="variant.html" title="Class template variant">boost::variant</a><span class="special"><</span> <span class="emphasis"><em><span class="identifier">unspecified</span></em></span> <span class="special">></span> <a name="boost.make_recursive_variant.type"></a><span class="identifier">type</span><span class="special">;</span> 41<span class="special">}</span><span class="special">;</span></pre></div> 42<div class="refsect1"> 43<a name="id-1.3.46.5.6.3.4"></a><h2>Description</h2> 44<p><code class="computeroutput">type</code> has behavior equivalent in every respect to 45 some <code class="computeroutput">variant< U1, U2, ..., UN ></code>, where each type 46 <code class="computeroutput">U<span class="emphasis"><em>i</em></span></code> is the result of the 47 corresponding type <code class="computeroutput">T<span class="emphasis"><em>i</em></span></code> undergone a 48 transformation function. The following pseudo-code specifies the 49 behavior of this transformation (call it <code class="computeroutput">substitute</code>): 50 51 </p> 52<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 53<li class="listitem">If <code class="computeroutput">T<span class="emphasis"><em>i</em></span></code> is 54 <code class="computeroutput">boost::recursive_variant_</code> then: 55 <code class="computeroutput">variant< U1, U2, ..., UN ></code>;</li> 56<li class="listitem">Else if <code class="computeroutput">T<span class="emphasis"><em>i</em></span></code> is of the 57 form <code class="computeroutput">X *</code> then: 58 <code class="computeroutput">substitute(X) *</code>;</li> 59<li class="listitem">Else if <code class="computeroutput">T<span class="emphasis"><em>i</em></span></code> is of the 60 form <code class="computeroutput">X &</code> then: 61 <code class="computeroutput">substitute(X) &</code>;</li> 62<li class="listitem">Else if <code class="computeroutput">T<span class="emphasis"><em>i</em></span></code> is of the 63 form <code class="computeroutput">R (*)( X1, X2, ..., XN )</code> then: 64 <code class="computeroutput">substitute(R) (*)( substitute(X1), substitute(X2), ..., substitute(XN) )</code>;</li> 65<li class="listitem">Else if <code class="computeroutput">T<span class="emphasis"><em>i</em></span></code> is of the 66 form <code class="computeroutput">F < X1, X2, ..., XN ></code> then: 67 <code class="computeroutput">F< substitute(X1), substitute(X2), ..., substitute(XN) ></code>;</li> 68<li class="listitem">Else: <code class="computeroutput">T<span class="emphasis"><em>i</em></span></code>.</li> 69</ul></div> 70<p> 71 </p> 72<p>Note that cv-qualifiers are preserved and that the actual 73 process is generally a bit more complicated. However, the above does 74 convey the essential idea as well as describe the extent of the 75 substititions.</p> 76<p>Use of <code class="computeroutput">make_recursive_variant</code> is demonstrated in 77 <a class="xref" href="../variant/tutorial.html#variant.tutorial.recursive.recursive-variant" title="Recursive types with make_recursive_variant">the section called “Recursive types with <code class="computeroutput">make_recursive_variant</code>”</a>.</p> 78<p><span class="bold"><strong>Portability</strong></span>: Due to standard 79 conformance issues in several compilers, 80 <code class="computeroutput">make_recursive_variant</code> is not universally supported. On 81 these compilers the library indicates its lack of support via the 82 definition of the preprocessor symbol 83 <code class="computeroutput"><a class="link" href="../BOOST_VARIANT_1_3_46_5_3_9.html" title="Macro BOOST_VARIANT_NO_FULL_RECURSIVE_VARIANT_SUPPORT">BOOST_VARIANT_NO_FULL_RECURSIVE_VARIANT_SUPPORT</a></code>.</p> 84</div> 85</div> 86<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 87<td align="left"></td> 88<td align="right"><div class="copyright-footer">Copyright © 2002, 2003 Eric Friedman, Itay Maman<br>Copyright © 2014-2020 Antony Polukhin<p>Distributed under the Boost Software License, Version 1.0. 89 (See accompanying file <code class="filename">LICENSE_1_0.txt</code> or copy at 90 <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) 91 </p> 92</div></td> 93</tr></table> 94<hr> 95<div class="spirit-nav"> 96<a accesskey="p" href="make_variant_over.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../variant/reference.html#header.boost.variant.recursive_variant_hpp"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="make_recurs_1_3_46_5_5_1_3.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> 97</div> 98</body> 99</html> 100
