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  <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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  <title>Chapter 4. Boost.Any</title>
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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><p class="copyright">Copyright © 2013-2022 Antony Polukhin</p></div>
  <div><div class="legalnotice">
  <a name="id-1.3.5.1.4"></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>)
        </p>
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  <div class="toc">
  <p><b>Table of Contents</b></p>
  <dl class="toc">
  <dt><span class="section"><a href="any.html#id-1.3.5.3">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 &lt;boost/any.hpp&gt;</a></span></dt>
  <dt><span class="section"><a href="any/reference.html#header.boost.any.bad_any_cast_hpp">Header &lt;boost/any/bad_any_cast.hpp&gt;</a></span></dt>
  <dt><span class="section"><a href="any/reference.html#header.boost.any.basic_any_hpp">Header &lt;boost/any/basic_any.hpp&gt;</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="id-1.3.5.3"></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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