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 <div><h2 class="title">
 <a name="any"></a>Chapter&#160;2.&#160;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 &#169; 2001 Kevlin Henney</p></div>
 <div><div class="legalnotice">
 <a name="id1010074"></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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 <div class="toc">
 <p><b>Table of Contents</b></p>
 <dl>
 <dt><span class="section"><a href="any.html#id1010098">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>
 </dl></dd>
 <dt><span class="section"><a href="any/s04.html">Acknowledgements</a></span></dt>
 </dl>
 </div>
 <div class="section">
 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
 <a name="id1010098"></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" 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>
 </div>
 </div>
 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 <td align="left"><p><small>Last revised: July 26, 2009 at 21:11:03 +0100</small></p></td>
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	<div><h2 class="title">
	<a name="any"></a>Chapter 2. 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="id1010074"></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>
	</div></div>
	</div></div>
	<div class="toc">
	<p><b>Table of Contents</b></p>
	<dl>
	<dt><span class="section"><a href="any.html#id1010098">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>
	</dl></dd>
	<dt><span class="section"><a href="any/s04.html">Acknowledgements</a></span></dt>
	</dl>
	</div>
	<div class="section">
	<div class="titlepage"><div><div><h2 class="title" style="clear: both">
	<a name="id1010098"></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" 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>
	</div>
	</div>
	<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
	<td align="left"><p><small>Last revised: July 26, 2009 at 21:11:03 +0100</small></p></td>
	<td align="right"><div class="copyright-footer"></div></td>
	</tr></table>
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