172 lines
7.5 KiB
XML
172 lines
7.5 KiB
XML
<?xml version="1.0" encoding="utf-8" ?>
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<!DOCTYPE header PUBLIC "-//Boost//DTD BoostBook XML V1.0//EN"
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"http://www.boost.org/tools/boostbook/dtd/boostbook.dtd">
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<!--
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Copyright 2003, Eric Friedman, Itay Maman.
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Copyright 2016-2021 Antony Polukhin.
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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-->
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<header name="boost/variant/variant_fwd.hpp">
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<using-namespace name="boost"/>
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<para>Provides forward declarations of the
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<code><classname>boost::variant</classname></code>,
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<code><classname>boost::make_variant_over</classname></code>,
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<code><classname>boost::make_recursive_variant</classname></code>, and
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<code><classname>boost::make_recursive_variant_over</classname></code>
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class templates and the <code>boost::recursive_variant_</code> tag type.
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Also defines several preprocessor symbols, as described below.</para>
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<macro name="BOOST_VARIANT_LIMIT_TYPES">
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<purpose>
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<simpara>Expands to the length of the
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template parameter list for
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<code><classname>variant</classname></code>. Not used if
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<code><macroname>BOOST_VARIANT_DO_NOT_USE_VARIADIC_TEMPLATES</macroname></code>
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is not defined.
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</simpara>
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</purpose>
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<description>
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<para><emphasis role="bold">Note</emphasis>: Conforming
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implementations of <code>variant</code> must allow at least ten
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template arguments. That is, BOOST_VARIANT_LIMIT_TYPES must be greater
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or equal to <code>10</code>.</para>
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</description>
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</macro>
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<macro name="BOOST_VARIANT_DO_NOT_USE_VARIADIC_TEMPLATES">
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<purpose>
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<simpara>If this macro is defined, then library won't use C++11 variadic templates.
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Users may define this macro to make
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<code><macroname>BOOST_VARIANT_ENUM_PARAMS</macroname></code>
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and <code><macroname>BOOST_VARIANT_ENUM_SHIFTED_PARAMS</macroname></code> expand
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to a comma-separated sequence instead of variadic templates. Define this macro if
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your compiler has problems with compilation of variadic templates.
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</simpara>
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</purpose>
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</macro>
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<macro name="BOOST_VARIANT_ENUM_PARAMS" kind="functionlike">
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<macro-parameter name="param"/>
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<purpose>
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<simpara>Enumerate parameters for use with
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<code><classname>variant</classname></code>.</simpara>
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</purpose>
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<description>
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<para> When variadic templates are available and
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<code><macroname>BOOST_VARIANT_DO_NOT_USE_VARIADIC_TEMPLATES</macroname></code>
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is not defined, expands to variadic template list in the following manner:
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<code><programlisting>
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BOOST_VARIANT_ENUM_PARAMS(T) => T0, TN...
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BOOST_VARIANT_ENUM_PARAMS(class T) => class T0, class... TN
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BOOST_VARIANT_ENUM_PARAMS(class Something) => class Something0, class... SomethingN
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BOOST_VARIANT_ENUM_PARAMS(typename Something) => typename Something0, typename... SomethingN
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BOOST_VARIANT_ENUM_PARAMS(Something) => Something0, SomethingN...
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BOOST_VARIANT_ENUM_PARAMS(Something) => Something0, SomethingN...
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</programlisting></code>
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</para>
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<para>
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Otherwise expands to a comma-separated sequence of length
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<code><macroname>BOOST_VARIANT_LIMIT_TYPES</macroname></code>, where
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each element in the sequence consists of the concatenation of
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<emphasis>param</emphasis> with its zero-based index into the
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sequence. That is,
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<code>param ## 0, param ## 1, ..., param ## BOOST_VARIANT_LIMIT_TYPES - 1</code>.</para>
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<para><emphasis role="bold">Rationale</emphasis>: This macro greatly
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simplifies for the user the process of declaring
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<code><classname>variant</classname></code> types
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in function templates or explicit partial specializations of class
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templates, as shown in the
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<link linkend="variant.tutorial.preprocessor">tutorial</link>.</para>
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</description>
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</macro>
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<macro name="BOOST_VARIANT_ENUM_SHIFTED_PARAMS" kind="functionlike">
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<macro-parameter name="param"/>
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<purpose>
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<simpara>Enumerate all but the first parameter for use with
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<code><classname>variant</classname></code>.</simpara>
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</purpose>
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<description>
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<para> When variadic templates are available and
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<code><macroname>BOOST_VARIANT_DO_NOT_USE_VARIADIC_TEMPLATES</macroname></code>
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is not defined, expands to variadic template list in the following manner:
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<code><programlisting>
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BOOST_VARIANT_ENUM_SHIFTED_PARAMS(T) => TN...
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BOOST_VARIANT_ENUM_SHIFTED_PARAMS(class T) => class... TN
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BOOST_VARIANT_ENUM_SHIFTED_PARAMS(class Something) => class... SomethingN
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BOOST_VARIANT_ENUM_SHIFTED_PARAMS(typename Something) => typename... SomethingN
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BOOST_VARIANT_ENUM_SHIFTED_PARAMS(Something) => SomethingN...
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BOOST_VARIANT_ENUM_SHIFTED_PARAMS(Something) => SomethingN...
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</programlisting></code>
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</para>
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<para>
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Otherwise expands to a comma-separated sequence of length
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<code><macroname>BOOST_VARIANT_LIMIT_TYPES</macroname> - 1</code>,
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where each element in the sequence consists of the concatenation of
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<emphasis>param</emphasis> with its one-based index into the sequence.
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That is,
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<code>param ## 1, ..., param ## BOOST_VARIANT_LIMIT_TYPES - 1</code>.</para>
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<para><emphasis role="bold">Note</emphasis>: This macro results in the
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same expansion as
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<code><macroname>BOOST_VARIANT_ENUM_PARAMS</macroname></code> -- but
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without the first term.</para>
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</description>
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</macro>
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<macro name="BOOST_VARIANT_NO_TYPE_SEQUENCE_SUPPORT">
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<purpose>
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<simpara>Indicates absence of support for specifying the bounded types
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of a <code><classname>variant</classname></code> by the elements of a
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type sequence.</simpara>
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</purpose>
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<description>
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<para>Defined only if
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<code><classname>make_variant_over</classname></code> and
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<code><classname>make_recursive_variant_over</classname></code>
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are not supported for some reason on the target compiler.</para>
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</description>
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</macro>
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<macro name="BOOST_VARIANT_DO_NOT_SPECIALIZE_STD_HASH">
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<purpose>
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<simpara>Define this macro if you do not wish to have a <code><classname>std::hash</classname></code> specialization for
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<code><classname>boost::variant</classname></code>.</simpara>
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</purpose>
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</macro>
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<macro name="BOOST_VARIANT_NO_FULL_RECURSIVE_VARIANT_SUPPORT">
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<purpose>
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<simpara>Indicates
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<code><classname>make_recursive_variant</classname></code> operates in
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an implementation-defined manner.</simpara>
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</purpose>
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<description>
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<para>Defined only if
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<code><classname>make_recursive_variant</classname></code> does not
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operate as documented on the target compiler, but rather in an
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implementation-defined manner.</para>
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<para><emphasis role="bold">Implementation Note</emphasis>: If
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<code>BOOST_VARIANT_NO_FULL_RECURSIVE_VARIANT_SUPPORT</code> is
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defined for the target compiler, the current implementation uses the
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<libraryname>MPL</libraryname> lambda mechanism to approximate the
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desired behavior. (In most cases, however, such compilers do not have
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full lambda support either.)</para>
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</description>
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</macro>
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</header>
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