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28<h2><span class="refentrytitle">Class template make_recursive_variant</span></h2>
29<p>boost::make_recursive_variant &#8212; Simplifies declaration of recursive <code class="computeroutput">variant</code> types.</p>
30</div>
31<h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2>
32<div xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv"><pre class="synopsis"><span class="bold"><strong>template</strong></span>&lt;<span class="bold"><strong>typename</strong></span> T1, <span class="bold"><strong>typename</strong></span> T2 = <span class="emphasis"><em>unspecified</em></span>, ...,
33         <span class="bold"><strong>typename</strong></span> TN = <span class="emphasis"><em>unspecified</em></span>&gt; 
34<span class="bold"><strong>class</strong></span> make_recursive_variant {
35<span class="bold"><strong>public</strong></span>:
36  <span class="emphasis"><em>// types</em></span>
37  <span class="bold"><strong>typedef</strong></span> <a href="boost/variant.html" title="Class template variant">boost::variant</a>&lt; <span class="emphasis"><em>unspecified</em></span> &gt; type;
38};</pre></div>
39<div class="refsect1" lang="en">
40<a name="id2836116"></a><h2>Description</h2>
41<p><code class="computeroutput">type</code> has behavior equivalent in every respect to
42          some <code class="computeroutput">variant&lt; U1, U2, ..., UN &gt;</code>, where each type
43          <code class="computeroutput">U<span class="emphasis"><em>i</em></span></code> is the result of the
44          corresponding type <code class="computeroutput">T<span class="emphasis"><em>i</em></span></code> undergone a
45          transformation function. The following pseudo-code specifies the
46          behavior of this transformation (call it <code class="computeroutput">substitute</code>):
47
48          </p>
49<div class="itemizedlist"><ul type="disc">
50<li>If <code class="computeroutput">T<span class="emphasis"><em>i</em></span></code> is
51              <code class="computeroutput">boost::recursive_variant_</code> then:
52              <code class="computeroutput">variant&lt; U1, U2, ..., UN &gt;</code>;</li>
53<li>Else if <code class="computeroutput">T<span class="emphasis"><em>i</em></span></code> is of the
54              form <code class="computeroutput">X *</code> then:
55              <code class="computeroutput">substitute(X) *</code>;</li>
56<li>Else if <code class="computeroutput">T<span class="emphasis"><em>i</em></span></code> is of the
57              form <code class="computeroutput">X &amp;</code> then:
58              <code class="computeroutput">substitute(X) &amp;</code>;</li>
59<li>Else if <code class="computeroutput">T<span class="emphasis"><em>i</em></span></code> is of the
60              form <code class="computeroutput">R (*)( X1, X2, ..., XN )</code> then:
61              <code class="computeroutput">substitute(R) (*)( substitute(X1), substitute(X2), ..., substitute(XN) )</code>;</li>
62<li>Else if <code class="computeroutput">T<span class="emphasis"><em>i</em></span></code> is of the
63              form <code class="computeroutput">F &lt; X1, X2, ..., XN &gt;</code> then:
64              <code class="computeroutput">F&lt; substitute(X1), substitute(X2), ..., substitute(XN) &gt;</code>;</li>
65<li>Else: <code class="computeroutput">T<span class="emphasis"><em>i</em></span></code>.</li>
66</ul></div>
67<p>Note that cv-qualifiers are preserved and that the actual
68          process is generally a bit more complicated. However, the above does
69          convey the essential idea as well as describe the extent of the
70          substititions.</p>
71<p>Use of <code class="computeroutput">make_recursive_variant</code> is demonstrated in
72          <a href="variant/tutorial.html#variant.tutorial.recursive.recursive-variant" title="Recursive types with make_recursive_variant">the section called &#8220;Recursive types with <code class="computeroutput">make_recursive_variant</code>&#8221;</a>.</p>
73<p><span class="bold"><strong>Portability</strong></span>: Due to standard
74          conformance issues in several compilers,
75          <code class="computeroutput">make_recursive_variant</code> is not universally supported. On
76          these compilers the library indicates its lack of support via the
77          definition of the preprocessor symbol
78          <code class="computeroutput"><a href="id2404411.html" title="Macro BOOST_VARIANT_NO_FULL_RECURSIVE_VARIANT_SUPPORT">BOOST_VARIANT_NO_FULL_RECURSIVE_VARIANT_SUPPORT</a></code>.</p>
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