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<title>Chapter 4. Boost.Any</title>
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<div><h2 class="title">
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<a name="any"></a>Chapter 4. Boost.Any</h2></div>
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<div><div class="author"><h3 class="author">
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<span class="firstname">Kevlin</span> <span class="surname">Henney</span>
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</h3></div></div>
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<div><p class="copyright">Copyright © 2001 Kevlin Henney</p></div>
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<div><p class="copyright">Copyright © 2013-2021 Antony Polukhin</p></div>
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<div><div class="legalnotice">
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<a name="id-1.3.5.1.4"></a><p>Distributed under the Boost Software License, Version 1.0.
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(See accompanying file <code class="filename">LICENSE_1_0.txt</code> or copy at
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<a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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</p>
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</div></div>
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<div class="toc">
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<p><b>Table of Contents</b></p>
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<dl class="toc">
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<dt><span class="section"><a href="any.html#id-1.3.5.3">Introduction</a></span></dt>
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<dt><span class="section"><a href="any/s02.html">Examples</a></span></dt>
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<dt><span class="section"><a href="any/reference.html">Reference</a></span></dt>
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<dd><dl>
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<dt><span class="section"><a href="any/reference.html#any.ValueType"><span class="emphasis"><em>ValueType</em></span> requirements</a></span></dt>
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<dt><span class="section"><a href="any/reference.html#header.boost.any_hpp">Header <boost/any.hpp></a></span></dt>
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</dl></dd>
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<dt><span class="section"><a href="any/s04.html">Acknowledgements</a></span></dt>
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</dl>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="id-1.3.5.3"></a>Introduction</h2></div></div></div>
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<p>There are times when a generic (in the sense of
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<span class="emphasis"><em>general</em></span> as opposed to
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<span class="emphasis"><em>template-based programming</em></span>) type is needed:
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variables that are truly variable, accommodating values of many
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other more specific types rather than C++'s normal strict and
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static types. We can distinguish three basic kinds of generic
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type:</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem"><p>Converting types that can hold one of a number of
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possible value types, e.g. <code class="computeroutput">int</code> and
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<code class="computeroutput">string</code>, and freely convert between them, for
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instance interpreting <code class="computeroutput">5</code> as <code class="computeroutput">"5"</code> or
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vice-versa. Such types are common in scripting and other
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interpreted
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languages.
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<code class="computeroutput">boost::lexical_cast</code>
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supports such conversion functionality.</p></li>
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<li class="listitem"><p>
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Discriminated types that contain values of different types but
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do not attempt conversion between them, i.e. <code class="computeroutput">5</code> is
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held strictly as an <code class="computeroutput">int</code> and is not implicitly
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convertible either to <code class="computeroutput">"5"</code> or to
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<code class="computeroutput">5.0</code>. Their indifference to interpretation but
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awareness of type effectively makes them safe, generic
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containers of single values, with no scope for surprises from
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ambiguous conversions.</p></li>
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<li class="listitem"><p>
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Indiscriminate types that can refer to anything but are
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oblivious to the actual underlying type, entrusting all forms
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of access and interpretation to the programmer. This niche is
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dominated by <code class="computeroutput">void *</code>, which offers plenty of scope
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for surprising, undefined behavior.</p></li>
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</ul></div>
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<p>The <code class="computeroutput"><a class="link" href="boost/any.html" title="Class any">boost::any</a></code> class
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(based on the class of the same name described in <a href="http://www.two-sdg.demon.co.uk/curbralan/papers/ValuedConversions.pdf" target="_top">"Valued
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Conversions"</a> by Kevlin Henney, <span class="emphasis"><em>C++
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Report</em></span> 12(7), July/August 2000) is a variant value type
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based on the second category. It supports copying of any value
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type and safe checked extraction of that value strictly against
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its type. A similar design, offering more appropriate operators,
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can be used for a generalized function adaptor,
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<code class="computeroutput">any_function</code>, a generalized iterator adaptor,
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<code class="computeroutput">any_iterator</code>, and other object types that need
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uniform runtime treatment but support only compile-time template
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parameter conformance.</p>
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</div>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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