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<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE header PUBLIC "-//Boost//DTD BoostBook XML V1.0//EN"
"http://www.boost.org/tools/boostbook/dtd/boostbook.dtd">
<header name="boost/variant/recursive_variant.hpp">
<namespace name="boost">
<typedef name="recursive_variant_">
<type><emphasis>unspecified</emphasis></type>
</typedef>
<class name="make_recursive_variant">
<purpose>Simplifies declaration of recursive <code>variant</code> types.</purpose>
<template>
<template-type-parameter name="T1"/>
<template-type-parameter name="T2">
<default><emphasis>unspecified</emphasis></default>
</template-type-parameter>
<template-varargs/>
<template-type-parameter name="TN">
<default><emphasis>unspecified</emphasis></default>
</template-type-parameter>
</template>
<description>
<para><code>type</code> has behavior equivalent in every respect to
some <code>variant< U1, U2, ..., UN ></code>, where each type
<code>U<emphasis>i</emphasis></code> is the result of the
corresponding type <code>T<emphasis>i</emphasis></code> undergone a
transformation function. The following pseudo-code specifies the
behavior of this transformation (call it <code>substitute</code>):
<itemizedlist>
<listitem>If <code>T<emphasis>i</emphasis></code> is
<code>boost::recursive_variant_</code> then:
<code>variant< U1, U2, ..., UN ></code>;</listitem>
<listitem>Else if <code>T<emphasis>i</emphasis></code> is of the
form <code>X *</code> then:
<code>substitute(X) *</code>;</listitem>
<listitem>Else if <code>T<emphasis>i</emphasis></code> is of the
form <code>X &</code> then:
<code>substitute(X) &</code>;</listitem>
<listitem>Else if <code>T<emphasis>i</emphasis></code> is of the
form <code>R (*)( X1, X2, ..., XN )</code> then:
<code>substitute(R) (*)( substitute(X1), substitute(X2), ..., substitute(XN) )</code>;</listitem>
<listitem>Else if <code>T<emphasis>i</emphasis></code> is of the
form <code>F < X1, X2, ..., XN ></code> then:
<code>F< substitute(X1), substitute(X2), ..., substitute(XN) ></code>;</listitem>
<listitem>Else: <code>T<emphasis>i</emphasis></code>.</listitem>
</itemizedlist>
</para>
<para>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.</para>
<para>Use of <code>make_recursive_variant</code> is demonstrated in
<xref linkend="variant.tutorial.recursive.recursive-variant"/>.</para>
<para><emphasis role="bold">Portability</emphasis>: Due to standard
conformance issues in several compilers,
<code>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><macroname>BOOST_VARIANT_NO_FULL_RECURSIVE_VARIANT_SUPPORT</macroname></code>.</para>
</description>
<typedef name="type">
<type><classname>boost::variant</classname>< <emphasis>unspecified</emphasis> ></type>
</typedef>
</class>
<class name="make_recursive_variant_over">
<purpose>
<simpara>Exposes a recursive <code>variant</code> whose bounded types
are the elements of the given type sequence.</simpara>
</purpose>
<template>
<template-type-parameter name="Sequence"/>
</template>
<typedef name="type">
<type>variant< <emphasis>unspecified</emphasis> ></type>
</typedef>
<description>
<simpara><code>type</code> has behavior equivalent in every respect to
<code><classname>make_recursive_variant</classname>< Sequence[0], Sequence[1], ... >::type</code>
(where <code>Sequence[<emphasis>i</emphasis>]</code> denotes the
<emphasis>i</emphasis>-th element of <code>Sequence</code>), except
that no upper limit is imposed on the number of types.</simpara>
<simpara><emphasis role="bold">Notes</emphasis>:</simpara>
<itemizedlist>
<listitem><code>Sequence</code> must meet the requirements of
<libraryname>MPL</libraryname>'s <emphasis>Sequence</emphasis>
concept.</listitem>
<listitem>Due to standard conformance problems in several compilers,
<code>make_recursive_variant_over</code> may not be supported on
your compiler. See
<code><macroname>BOOST_VARIANT_NO_TYPE_SEQUENCE_SUPPORT</macroname></code>
for more information.</listitem>
</itemizedlist>
</description>
</class>
</namespace>
</header>
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