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| 26 | <div class="titlepage"><div> |
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| 27 | <div><h2 class="title"> |
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| 28 | <a name="any"></a>Chapter 1. Boost.Any</h2></div> |
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| 29 | <div><div class="author"><h3 class="author"> |
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| 30 | <span class="firstname">Kevlin</span> <span class="surname">Henney</span> |
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| 31 | </h3></div></div> |
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| 32 | <div><p class="copyright">Copyright © 2001 Kevlin Henney</p></div> |
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| 33 | </div></div> |
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| 34 | <div class="toc"> |
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| 35 | <p><b>Table of Contents</b></p> |
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| 36 | <dl> |
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| 37 | <dt><span class="section"><a href="any.html#id936676">Introduction</a></span></dt> |
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| 38 | <dt><span class="section"><a href="any/s02.html">Examples</a></span></dt> |
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| 39 | <dt><span class="section"><a href="any/reference.html">Reference</a></span></dt> |
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| 40 | <dd><dl> |
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| 41 | <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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| 42 | <dt><span class="section"><a href="any/reference.html#header.boost.any.hpp">Header <boost/any.hpp></a></span></dt> |
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| 43 | </dl></dd> |
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| 44 | <dt><span class="section"><a href="any/s04.html">Acknowledgements</a></span></dt> |
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| 45 | </dl> |
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| 46 | </div> |
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| 47 | <div class="section" lang="en"> |
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| 48 | <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
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| 49 | <a name="id936676"></a>Introduction</h2></div></div></div> |
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| 50 | <p>There are times when a generic (in the sense of |
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| 51 | <span class="emphasis"><em>general</em></span> as opposed to |
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| 52 | <span class="emphasis"><em>template-based programming</em></span>) type is needed: |
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| 53 | variables that are truly variable, accommodating values of many |
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| 54 | other more specific types rather than C++'s normal strict and |
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| 55 | static types. We can distinguish three basic kinds of generic |
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| 56 | type:</p> |
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| 57 | <div class="itemizedlist"><ul type="disc"> |
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| 58 | <li><p>Converting types that can hold one of a number of |
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| 59 | possible value types, e.g. <code class="computeroutput">int</code> and |
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| 60 | <code class="computeroutput">string</code>, and freely convert between them, for |
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| 61 | instance interpreting <code class="computeroutput">5</code> as <code class="computeroutput">"5"</code> or |
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| 62 | vice-versa. Such types are common in scripting and other |
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| 63 | interpreted |
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| 64 | languages. |
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| 65 | <code class="computeroutput">boost::lexical_cast</code> |
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| 66 | supports such conversion functionality.</p></li> |
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| 67 | <li><p> |
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| 68 | Discriminated types that contain values of different types but |
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| 69 | do not attempt conversion between them, i.e. <code class="computeroutput">5</code> is |
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| 70 | held strictly as an <code class="computeroutput">int</code> and is not implicitly |
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| 71 | convertible either to <code class="computeroutput">"5"</code> or to |
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| 72 | <code class="computeroutput">5.0</code>. Their indifference to interpretation but |
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| 73 | awareness of type effectively makes them safe, generic |
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| 74 | containers of single values, with no scope for surprises from |
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| 75 | ambiguous conversions.</p></li> |
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| 76 | <li><p> |
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| 77 | Indiscriminate types that can refer to anything but are |
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| 78 | oblivious to the actual underlying type, entrusting all forms |
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| 79 | of access and interpretation to the programmer. This niche is |
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| 80 | dominated by <code class="computeroutput">void *</code>, which offers plenty of scope |
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| 81 | for surprising, undefined behavior.</p></li> |
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| 82 | </ul></div> |
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| 83 | <p>The <code class="computeroutput"><a href="boost/any.html" title="Class any">boost::any</a></code> class |
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| 84 | (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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| 85 | Conversions"</a> by Kevlin Henney, <span class="emphasis"><em>C++ |
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| 86 | Report</em></span> 12(7), July/August 2000) is a variant value type |
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| 87 | based on the second category. It supports copying of any value |
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| 88 | type and safe checked extraction of that value strictly against |
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| 89 | its type. A similar design, offering more appropriate operators, |
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| 90 | can be used for a generalized function adaptor, |
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| 91 | <code class="computeroutput">any_function</code>, a generalized iterator adaptor, |
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| 92 | <code class="computeroutput">any_iterator</code>, and other object types that need |
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| 93 | uniform runtime treatment but support only compile-time template |
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| 94 | parameter conformance.</p> |
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| 95 | </div> |
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| 96 | </div> |
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| 97 | <table width="100%"><tr> |
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| 98 | <td align="left"><small><p>Last revised: November 04, 2005 at 09:34:36 GMT</p></small></td> |
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