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<title>Class template make_recursive_variant</title>
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<a name="boost.make_recursive_variant"></a><div class="titlepage"></div>
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<h2><span class="refentrytitle">Class template make_recursive_variant</span></h2>
<p>boost::make_recursive_variant — Simplifies declaration of recursive <code class="computeroutput">variant</code> types.</p>
</div>
<h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2>
<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>>
</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>
<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>
<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>
<span class="keyword">public</span><span class="special">:</span>
<span class="comment">// types</span>
<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>
<span class="special">}</span><span class="special">;</span></pre></div>
<div class="refsect1">
<a name="idm45555032491824"></a><h2>Description</h2>
<p><code class="computeroutput">type</code> has behavior equivalent in every respect to
some <code class="computeroutput">variant< U1, U2, ..., UN ></code>, where each type
<code class="computeroutput">U<span class="emphasis"><em>i</em></span></code> is the result of the
corresponding type <code class="computeroutput">T<span class="emphasis"><em>i</em></span></code> undergone a
transformation function. The following pseudo-code specifies the
behavior of this transformation (call it <code class="computeroutput">substitute</code>):
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">If <code class="computeroutput">T<span class="emphasis"><em>i</em></span></code> is
<code class="computeroutput">boost::recursive_variant_</code> then:
<code class="computeroutput">variant< U1, U2, ..., UN ></code>;</li>
<li class="listitem">Else if <code class="computeroutput">T<span class="emphasis"><em>i</em></span></code> is of the
form <code class="computeroutput">X *</code> then:
<code class="computeroutput">substitute(X) *</code>;</li>
<li class="listitem">Else if <code class="computeroutput">T<span class="emphasis"><em>i</em></span></code> is of the
form <code class="computeroutput">X &</code> then:
<code class="computeroutput">substitute(X) &</code>;</li>
<li class="listitem">Else if <code class="computeroutput">T<span class="emphasis"><em>i</em></span></code> is of the
form <code class="computeroutput">R (*)( X1, X2, ..., XN )</code> then:
<code class="computeroutput">substitute(R) (*)( substitute(X1), substitute(X2), ..., substitute(XN) )</code>;</li>
<li class="listitem">Else if <code class="computeroutput">T<span class="emphasis"><em>i</em></span></code> is of the
form <code class="computeroutput">F < X1, X2, ..., XN ></code> then:
<code class="computeroutput">F< substitute(X1), substitute(X2), ..., substitute(XN) ></code>;</li>
<li class="listitem">Else: <code class="computeroutput">T<span class="emphasis"><em>i</em></span></code>.</li>
</ul></div>
<p>
</p>
<p>Note that cv-qualifiers are preserved and that the actual
process is generally a bit more complicated. However, the above does
convey the essential idea as well as describe the extent of the
substititions.</p>
<p>Use of <code class="computeroutput">make_recursive_variant</code> is demonstrated in
<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>
<p><span class="bold"><strong>Portability</strong></span>: Due to standard
conformance issues in several compilers,
<code class="computeroutput">make_recursive_variant</code> is not universally supported. On
these compilers the library indicates its lack of support via the
definition of the preprocessor symbol
<code class="computeroutput"><a class="link" href="../BOOST_VARIANT_NO_FULL_RECURSIVE_VARIANT_SUPPORT.html" title="Macro BOOST_VARIANT_NO_FULL_RECURSIVE_VARIANT_SUPPORT">BOOST_VARIANT_NO_FULL_RECURSIVE_VARIANT_SUPPORT</a></code>.</p>
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<td align="right"><div class="copyright-footer">Copyright © 2002, 2003 Eric Friedman, Itay Maman<p>Distributed under the Boost Software License, Version 1.0.
(See accompanying file <code class="filename">LICENSE_1_0.txt</code> or copy at
<a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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