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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>) |
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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 <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> |
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