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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 2013-2025 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/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_relaxed_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><T1, T2, ..., TN> *</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><T1, T2, ..., TN> *</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 &</type>
<parameter name="operand">
<paramtype><classname>variant</classname><T1, T2, ..., TN> &</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><T1, T2, ..., TN> &</paramtype>
</parameter>
</signature>
<purpose>
<simpara>Retrieves a value of a specified type from a given
<code><classname>variant</classname></code>.</simpara>
<simpara>Unlike <functionname>polymorphic_strict_get</functionname> does not assert at compile time
that type <code>U</code> is one of the types that can be stored in variant.</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">Recomendation</emphasis>: Use
<functionname>polymorphic_get</functionname> or <functionname>polymorphic_strict_get</functionname>
in new code.
<functionname>polymorphic_strict_get</functionname>
provides more compile time checks and its behavior is closer to <code>std::get</code>
from C++ Standard Library.</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>
<overloaded-function name="polymorphic_strict_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><T1, T2, ..., TN> *</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><T1, T2, ..., TN> *</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 &</type>
<parameter name="operand">
<paramtype><classname>variant</classname><T1, T2, ..., TN> &</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><T1, T2, ..., TN> &</paramtype>
</parameter>
</signature>
<purpose>
<simpara>Retrieves a value of a specified type from a given
<code><classname>variant</classname></code>.</simpara>
</purpose>
<description>
<simpara>Acts exactly like <functionname>polymorphic_relaxed_get</functionname> but does a compile time check
that type <code>U</code> is one of the types that can be stored in variant.</simpara>
</description>
</overloaded-function>
<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><T1, T2, ..., TN> *</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><T1, T2, ..., TN> *</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 &</type>
<parameter name="operand">
<paramtype><classname>variant</classname><T1, T2, ..., TN> &</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><T1, T2, ..., TN> &</paramtype>
</parameter>
</signature>
<purpose>
<simpara>Retrieves a value of a specified type from a given
<code><classname>variant</classname></code>.</simpara>
</purpose>
<description>
<simpara>Evaluates to <functionname>polymorphic_strict_get</functionname>
if <code>BOOST_VARIANT_USE_RELAXED_GET_BY_DEFAULT</code>
is not defined. If <code>BOOST_VARIANT_USE_RELAXED_GET_BY_DEFAULT</code>
is defined then evaluates to <functionname>polymorphic_relaxed_get</functionname>. </simpara>
<simpara><emphasis role="bold">Recomendation</emphasis>: Use
<functionname>polymorphic_get</functionname> in new code without defining
<code>BOOST_VARIANT_USE_RELAXED_GET_BY_DEFAULT</code>. In that way
<functionname>polymorphic_get</functionname>
provides more compile time checks and its behavior is closer to <code>std::get</code>
from C++ Standard Library.</simpara>
</description>
</overloaded-function>
</namespace>
</header>
|
