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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">
<!--
Copyright 2003, Eric Friedman, Itay Maman.
Copyright 2016-2025 Antony Polukhin.
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
-->
<header name="boost/variant/variant_fwd.hpp">
<using-namespace name="boost"/>
<para>Provides forward declarations of the
<code><classname>boost::variant</classname></code>,
<code><classname>boost::make_variant_over</classname></code>,
<code><classname>boost::make_recursive_variant</classname></code>, and
<code><classname>boost::make_recursive_variant_over</classname></code>
class templates and the <code>boost::recursive_variant_</code> tag type.
Also defines several preprocessor symbols, as described below.</para>
<macro name="BOOST_VARIANT_ENUM_PARAMS" kind="functionlike">
<macro-parameter name="param"/>
<purpose>
<simpara>Enumerate parameters for use with
<code><classname>variant</classname></code>.</simpara>
</purpose>
<description>
<para> Expands to variadic template list in the following manner:
<code><programlisting>
BOOST_VARIANT_ENUM_PARAMS(T) => T0, TN...
BOOST_VARIANT_ENUM_PARAMS(class T) => class T0, class... TN
BOOST_VARIANT_ENUM_PARAMS(class Something) => class Something0, class... SomethingN
BOOST_VARIANT_ENUM_PARAMS(typename Something) => typename Something0, typename... SomethingN
BOOST_VARIANT_ENUM_PARAMS(Something) => Something0, SomethingN...
BOOST_VARIANT_ENUM_PARAMS(Something) => Something0, SomethingN...
</programlisting></code>
</para>
<para>
Otherwise expands to a comma-separated sequence of length
<code><macroname>BOOST_VARIANT_LIMIT_TYPES</macroname></code>, where
each element in the sequence consists of the concatenation of
<emphasis>param</emphasis> with its zero-based index into the
sequence. That is,
<code>param ## 0, param ## 1, ..., param ## BOOST_VARIANT_LIMIT_TYPES - 1</code>.</para>
<para><emphasis role="bold">Rationale</emphasis>: This macro greatly
simplifies for the user the process of declaring
<code><classname>variant</classname></code> types
in function templates or explicit partial specializations of class
templates, as shown in the
<link linkend="variant.tutorial.preprocessor">tutorial</link>.</para>
</description>
</macro>
<macro name="BOOST_VARIANT_ENUM_SHIFTED_PARAMS" kind="functionlike">
<macro-parameter name="param"/>
<purpose>
<simpara>Enumerate all but the first parameter for use with
<code><classname>variant</classname></code>.</simpara>
</purpose>
<description>
<para> Expands to variadic template list in the following manner:
<code><programlisting>
BOOST_VARIANT_ENUM_SHIFTED_PARAMS(T) => TN...
BOOST_VARIANT_ENUM_SHIFTED_PARAMS(class T) => class... TN
BOOST_VARIANT_ENUM_SHIFTED_PARAMS(class Something) => class... SomethingN
BOOST_VARIANT_ENUM_SHIFTED_PARAMS(typename Something) => typename... SomethingN
BOOST_VARIANT_ENUM_SHIFTED_PARAMS(Something) => SomethingN...
BOOST_VARIANT_ENUM_SHIFTED_PARAMS(Something) => SomethingN...
</programlisting></code>
</para>
<para>
Otherwise expands to a comma-separated sequence of length
<code><macroname>BOOST_VARIANT_LIMIT_TYPES</macroname> - 1</code>,
where each element in the sequence consists of the concatenation of
<emphasis>param</emphasis> with its one-based index into the sequence.
That is,
<code>param ## 1, ..., param ## BOOST_VARIANT_LIMIT_TYPES - 1</code>.</para>
<para><emphasis role="bold">Note</emphasis>: This macro results in the
same expansion as
<code><macroname>BOOST_VARIANT_ENUM_PARAMS</macroname></code> -- but
without the first term.</para>
</description>
</macro>
<macro name="BOOST_VARIANT_DO_NOT_SPECIALIZE_STD_HASH">
<purpose>
<simpara>Define this macro if you do not wish to have a <code><classname>std::hash</classname></code> specialization for
<code><classname>boost::variant</classname></code>.</simpara>
</purpose>
</macro>
<macro name="BOOST_VARIANT_NO_FULL_RECURSIVE_VARIANT_SUPPORT">
<purpose>
<simpara>Indicates
<code><classname>make_recursive_variant</classname></code> operates in
an implementation-defined manner.</simpara>
</purpose>
<description>
<para>Defined only if
<code><classname>make_recursive_variant</classname></code> does not
operate as documented on the target compiler, but rather in an
implementation-defined manner.</para>
<para><emphasis role="bold">Implementation Note</emphasis>: If
<code>BOOST_VARIANT_NO_FULL_RECURSIVE_VARIANT_SUPPORT</code> is
defined for the target compiler, the current implementation uses the
<libraryname>MPL</libraryname> lambda mechanism to approximate the
desired behavior. (In most cases, however, such compilers do not have
full lambda support either.)</para>
</description>
</macro>
</header>
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