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<title>Function polymorphic_relaxed_get</title>
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<a name="boost.polymorphic_relaxed_get"></a><div class="titlepage"></div>
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<h2><span class="refentrytitle">Function polymorphic_relaxed_get</span></h2>
<p>boost::polymorphic_relaxed_get — Retrieves a value of a specified type from a given
<code class="computeroutput"><a class="link" href="variant.html" title="Class template variant">variant</a></code>.Unlike <a class="link" href="polymorphic_strict_get.html" title="Function polymorphic_strict_get">polymorphic_strict_get</a> does not assert at compile time
that type <code class="computeroutput">U</code> is one of the types that can be stored in variant.</p>
</div>
<h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2>
<div xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv"><pre class="synopsis"><span class="comment">// In header: <<a class="link" href="../variant/reference.html#header.boost.variant.polymorphic_get_hpp" title="Header <boost/variant/polymorphic_get.hpp>">boost/variant/polymorphic_get.hpp</a>>
</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> U<span class="special">,</span> <span class="keyword">typename</span> T1<span class="special">,</span> <span class="keyword">typename</span> T2<span class="special">,</span> <span class="special">...</span><span class="special">,</span> <span class="keyword">typename</span> TN<span class="special">></span>
<span class="identifier">U</span> <span class="special">*</span> <span class="identifier">polymorphic_relaxed_get</span><span class="special">(</span><a class="link" href="variant.html" title="Class template variant">variant</a><span class="special"><</span><span class="identifier">T1</span><span class="special">,</span> <span class="identifier">T2</span><span class="special">,</span> <span class="special">...</span><span class="special">,</span> <span class="identifier">TN</span><span class="special">></span> <span class="special">*</span> operand<span class="special">)</span><span class="special">;</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> U<span class="special">,</span> <span class="keyword">typename</span> T1<span class="special">,</span> <span class="keyword">typename</span> T2<span class="special">,</span> <span class="special">...</span><span class="special">,</span> <span class="keyword">typename</span> TN<span class="special">></span>
<span class="keyword">const</span> <span class="identifier">U</span> <span class="special">*</span> <span class="identifier">polymorphic_relaxed_get</span><span class="special">(</span><span class="keyword">const</span> <a class="link" href="variant.html" title="Class template variant">variant</a><span class="special"><</span><span class="identifier">T1</span><span class="special">,</span> <span class="identifier">T2</span><span class="special">,</span> <span class="special">...</span><span class="special">,</span> <span class="identifier">TN</span><span class="special">></span> <span class="special">*</span> operand<span class="special">)</span><span class="special">;</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> U<span class="special">,</span> <span class="keyword">typename</span> T1<span class="special">,</span> <span class="keyword">typename</span> T2<span class="special">,</span> <span class="special">...</span><span class="special">,</span> <span class="keyword">typename</span> TN<span class="special">></span>
<span class="identifier">U</span> <span class="special">&</span> <span class="identifier">polymorphic_relaxed_get</span><span class="special">(</span><a class="link" href="variant.html" title="Class template variant">variant</a><span class="special"><</span><span class="identifier">T1</span><span class="special">,</span> <span class="identifier">T2</span><span class="special">,</span> <span class="special">...</span><span class="special">,</span> <span class="identifier">TN</span><span class="special">></span> <span class="special">&</span> operand<span class="special">)</span><span class="special">;</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> U<span class="special">,</span> <span class="keyword">typename</span> T1<span class="special">,</span> <span class="keyword">typename</span> T2<span class="special">,</span> <span class="special">...</span><span class="special">,</span> <span class="keyword">typename</span> TN<span class="special">></span>
<span class="keyword">const</span> <span class="identifier">U</span> <span class="special">&</span> <span class="identifier">polymorphic_relaxed_get</span><span class="special">(</span><span class="keyword">const</span> <a class="link" href="variant.html" title="Class template variant">variant</a><span class="special"><</span><span class="identifier">T1</span><span class="special">,</span> <span class="identifier">T2</span><span class="special">,</span> <span class="special">...</span><span class="special">,</span> <span class="identifier">TN</span><span class="special">></span> <span class="special">&</span> operand<span class="special">)</span><span class="special">;</span></pre></div>
<div class="refsect1">
<a name="idm45555030698720"></a><h2>Description</h2>
<p>The <code class="computeroutput">polymorphic_get</code> function allows run-time checked,
type-safe retrieval of the content of the given
<code class="computeroutput"><a class="link" href="variant.html" title="Class template variant">variant</a></code>. The function succeeds
only if the content is of the specified type <code class="computeroutput">U</code> or of type
derived from type <code class="computeroutput">U</code>, with
failure indicated as described below.</p>
<p><span class="bold"><strong>Recomendation</strong></span>: Use
<code class="computeroutput"><a class="link" href="polymorphic_get.html" title="Function polymorphic_get">polymorphic_get</a></code> or <code class="computeroutput"><a class="link" href="polymorphic_strict_get.html" title="Function polymorphic_strict_get">polymorphic_strict_get</a></code>
in new code.
<code class="computeroutput"><a class="link" href="polymorphic_strict_get.html" title="Function polymorphic_strict_get">polymorphic_strict_get</a></code>
provides more compile time checks and it's behavior is closer to <code class="computeroutput">std::get</code>
from C++ Standard Library.</p>
<p><span class="bold"><strong>Warning</strong></span>: After either
<code class="computeroutput">operand</code> or its content is destroyed (e.g., when the
given <code class="computeroutput"><a class="link" href="variant.html" title="Class template variant">variant</a></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.</p>
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<td><p><span class="term">Notes:</span></p></td>
<td>As part of its guarantee of type-safety, <code class="computeroutput">polymorphic_get</code>
enforces <code class="computeroutput">const</code>-correctness. Thus, the specified type
<code class="computeroutput">U</code> must be <code class="computeroutput">const</code>-qualified whenever
<code class="computeroutput">operand</code> or its content is likewise
<code class="computeroutput">const</code>-qualified. The converse, however, is not required:
that is, the specified type <code class="computeroutput">U</code> may be
<code class="computeroutput">const</code>-qualified even when <code class="computeroutput">operand</code> and its
content are not.</td>
</tr>
<tr>
<td><p><span class="term">Returns:</span></p></td>
<td>If passed a pointer, <code class="computeroutput">polymorphic_get</code> returns a pointer to
the value content if it is of the specified type <code class="computeroutput">U</code> or of type
derived from type <code class="computeroutput">U</code>;
otherwise, a null pointer is returned. If passed a reference,
<code class="computeroutput">polymorphic_get</code> returns a reference to the value content if it is of
the specified type <code class="computeroutput">U</code> or of type
derived from type <code class="computeroutput">U</code>; otherwise, an exception is thrown
(see below).</td>
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<tr>
<td><p><span class="term">Throws:</span></p></td>
<td>Overloads taking a
<code class="computeroutput"><a class="link" href="variant.html" title="Class template variant">variant</a></code> pointer will not
throw; the overloads taking a
<code class="computeroutput"><a class="link" href="variant.html" title="Class template variant">variant</a></code> reference throw
<code class="computeroutput"><a class="link" href="bad_polymorphic_get.html" title="Class bad_polymorphic_get">bad_polymorphic_get</a></code> if the content is not of
the specified type <code class="computeroutput">U</code>or of type
derived from type <code class="computeroutput">U</code>.</td>
</tr>
<tr>
<td><p><span class="term">Rationale:</span></p></td>
<td>While visitation via
<code class="computeroutput"><a class="link" href="apply_visitor.html" title="Function apply_visitor">apply_visitor</a></code>
is generally preferred due to its greater safety, <code class="computeroutput">polymorphic_get</code> may
may be more convenient in some cases due to its straightforward
usage.</td>
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<td align="right"><div class="copyright-footer">Copyright © 2002, 2003 Eric Friedman, Itay Maman<p>Distributed under the Boost Software License, Version 1.0.
(See accompanying file <code class="filename">LICENSE_1_0.txt</code> or copy at
<a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
</p>
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