<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Function polymorphic_relaxed_get</title> <link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.78.1"> <link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> <link rel="up" href="../variant/reference.html#header.boost.variant.polymorphic_get_hpp" title="Header <boost/variant/polymorphic_get.hpp>"> <link rel="prev" href="bad_polymorphic_get.html" title="Class bad_polymorphic_get"> <link rel="next" href="polymorphic_strict_get.html" title="Function polymorphic_strict_get"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table cellpadding="2" width="100%"><tr> <td valign="top"><img alt="Boost C++ Libraries" width="277" 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class="titlepage"></div> <div class="refnamediv"> <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> <div class="variablelist"><table border="0" class="variablelist compact"> <colgroup> <col align="left" valign="top"> <col> </colgroup> <tbody> <tr> <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> </tr> <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> </tr> </tbody> </table></div> </div> </div> <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> <td align="left"></td> <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> </div></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="bad_polymorphic_get.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../variant/reference.html#header.boost.variant.polymorphic_get_hpp"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="polymorphic_strict_get.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>