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<div class="chapter">
<div class="titlepage"><div>
<div><h2 class="title">
<a name="any"></a>Chapter 4. Boost.Any</h2></div>
<div><div class="author"><h3 class="author">
<span class="firstname">Kevlin</span> <span class="surname">Henney</span>
</h3></div></div>
<div><p class="copyright">Copyright © 2001 Kevlin Henney</p></div>
<div><div class="legalnotice">
<a name="idp135760192"></a><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>)
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<p><b>Table of Contents</b></p>
<dl class="toc">
<dt><span class="section"><a href="any.html#idp135762960">Introduction</a></span></dt>
<dt><span class="section"><a href="any/s02.html">Examples</a></span></dt>
<dt><span class="section"><a href="any/reference.html">Reference</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="any/reference.html#any.ValueType"><span class="emphasis"><em>ValueType</em></span> requirements</a></span></dt>
<dt><span class="section"><a href="any/reference.html#header.boost.any_hpp">Header <boost/any.hpp></a></span></dt>
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<dt><span class="section"><a href="any/s04.html">Acknowledgements</a></span></dt>
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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="idp135762960"></a>Introduction</h2></div></div></div>
<p>There are times when a generic (in the sense of
<span class="emphasis"><em>general</em></span> as opposed to
<span class="emphasis"><em>template-based programming</em></span>) type is needed:
variables that are truly variable, accommodating values of many
other more specific types rather than C++'s normal strict and
static types. We can distinguish three basic kinds of generic
type:</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>Converting types that can hold one of a number of
possible value types, e.g. <code class="computeroutput">int</code> and
<code class="computeroutput">string</code>, and freely convert between them, for
instance interpreting <code class="computeroutput">5</code> as <code class="computeroutput">"5"</code> or
vice-versa. Such types are common in scripting and other
interpreted
languages.
<code class="computeroutput">boost::lexical_cast</code>
supports such conversion functionality.</p></li>
<li class="listitem"><p>
Discriminated types that contain values of different types but
do not attempt conversion between them, i.e. <code class="computeroutput">5</code> is
held strictly as an <code class="computeroutput">int</code> and is not implicitly
convertible either to <code class="computeroutput">"5"</code> or to
<code class="computeroutput">5.0</code>. Their indifference to interpretation but
awareness of type effectively makes them safe, generic
containers of single values, with no scope for surprises from
ambiguous conversions.</p></li>
<li class="listitem"><p>
Indiscriminate types that can refer to anything but are
oblivious to the actual underlying type, entrusting all forms
of access and interpretation to the programmer. This niche is
dominated by <code class="computeroutput">void *</code>, which offers plenty of scope
for surprising, undefined behavior.</p></li>
</ul></div>
<p>The <code class="computeroutput"><a class="link" href="boost/any.html" title="Class any">boost::any</a></code> class
(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
Conversions"</a> by Kevlin Henney, <span class="emphasis"><em>C++
Report</em></span> 12(7), July/August 2000) is a variant value type
based on the second category. It supports copying of any value
type and safe checked extraction of that value strictly against
its type. A similar design, offering more appropriate operators,
can be used for a generalized function adaptor,
<code class="computeroutput">any_function</code>, a generalized iterator adaptor,
<code class="computeroutput">any_iterator</code>, and other object types that need
uniform runtime treatment but support only compile-time template
parameter conformance.</p>
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