boost-react-native-bundle/boost_1_57_0/libs/variant/doc/reference/polymorphic_get.xml

Boost library as in https://sourceforge.net/projects/boost/files/boost/1.57.0/

<?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 2013-20014, 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/polymorphic_get.hpp">
  <namespace name="boost">

    <class name="bad_polymorphic_get">
      <inherit access="public">
        <classname>boost::bad_get</classname>
      </inherit>

      <purpose>
        <simpara>The exception thrown in the event of a failed application of
        <code><functionname>boost::polymorphic_get</functionname></code> on	the given
        operand value.</simpara>
      </purpose>

      <method	name="what"	specifiers="virtual" cv="const">
        <type>const	char *</type>
      </method>
    </class>

    <overloaded-function name="polymorphic_get">
      <signature>
        <template>
          <template-type-parameter name="U"/>
          <template-type-parameter name="T1"/>
          <template-type-parameter name="T2"/>
          <template-varargs/>
          <template-type-parameter name="TN"/>
        </template>

        <type>U *</type>

        <parameter name="operand">
          <paramtype><classname>variant</classname>&lt;T1, T2, ..., TN&gt; *</paramtype>
        </parameter>
      </signature>

      <signature>
        <template>
          <template-type-parameter name="U"/>
          <template-type-parameter name="T1"/>
          <template-type-parameter name="T2"/>
          <template-varargs/>
          <template-type-parameter name="TN"/>
        </template>

        <type>const U *</type>

        <parameter name="operand">
          <paramtype>const <classname>variant</classname>&lt;T1, T2, ..., TN&gt; *</paramtype>
        </parameter>
      </signature>
      
      <signature>
        <template>
          <template-type-parameter name="U"/>
          <template-type-parameter name="T1"/>
          <template-type-parameter name="T2"/>
          <template-varargs/>
          <template-type-parameter name="TN"/>
        </template>

        <type>U &amp;</type>

        <parameter name="operand">
          <paramtype><classname>variant</classname>&lt;T1, T2, ..., TN&gt; &amp;</paramtype>
        </parameter>
      </signature>

      <signature>
        <template>
          <template-type-parameter name="U"/>
          <template-type-parameter name="T1"/>
          <template-type-parameter name="T2"/>
          <template-varargs/>
          <template-type-parameter name="TN"/>
        </template>

        <type>const U &amp;</type>

        <parameter name="operand">
          <paramtype>const <classname>variant</classname>&lt;T1, T2, ..., TN&gt; &amp;</paramtype>
        </parameter>
      </signature>

      <purpose>
        <simpara>Retrieves a value of a specified type from a given
          <code><classname>variant</classname></code>.</simpara>
      </purpose>

      <description>
        <simpara>The <code>polymorphic_get</code> function allows run-time checked,
          type-safe retrieval of the content of the given
          <code><classname>variant</classname></code>. The function succeeds
          only if the content is of the specified type <code>U</code> or of type
          derived from type <code>U</code>, with
          failure indicated as described below.</simpara>
        <simpara><emphasis role="bold">Warning</emphasis>: After either
          <code>operand</code> or its content is destroyed (e.g., when the
          given <code><classname>variant</classname></code> is assigned a
          value of different type), the returned reference is invalidated.
          Thus, significant care and caution must be extended when handling
          the returned reference.</simpara>
      </description>

      <notes>
        <simpara>As part of its guarantee of type-safety, <code>polymorphic_get</code>
        enforces <code>const</code>-correctness. Thus, the specified type
        <code>U</code> must be <code>const</code>-qualified whenever
        <code>operand</code> or its content is likewise
        <code>const</code>-qualified. The converse, however, is not required:
        that is, the specified type <code>U</code> may be
        <code>const</code>-qualified even when <code>operand</code> and its
        content are not.</simpara>
      </notes>

      <returns>
        <simpara>If passed a pointer, <code>polymorphic_get</code> returns a pointer to
        the value content if it is of the specified type <code>U</code> or of type
        derived from type <code>U</code>;
        otherwise, a null pointer is returned. If passed a reference,
        <code>polymorphic_get</code> returns a reference to the value content if it is of
        the specified type <code>U</code>  or of type
          derived from type <code>U</code>; otherwise, an exception is thrown
        (see below).</simpara>
      </returns>

      <throws>
        <simpara>Overloads taking a
        <code><classname>variant</classname></code> pointer will not
        throw; the overloads taking a
        <code><classname>variant</classname></code> reference throw
        <code><classname>bad_polymorphic_get</classname></code> if the content is not of
        the specified type <code>U</code>or of type
        derived from type <code>U</code>.</simpara>
      </throws>

      <rationale>
        <simpara>While visitation via
        <code><functionname>apply_visitor</functionname></code>
        is generally preferred due to its greater safety, <code>polymorphic_get</code> may
        may be more convenient in some cases due to its straightforward
        usage.</simpara>
      </rationale>
    </overloaded-function>

  </namespace>
</header>