1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
|
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
<title>Chapter 4. Boost.Any</title>
<link rel="stylesheet" href="../../doc/src/boostbook.css" type="text/css">
<meta name="generator" content="DocBook XSL Stylesheets V1.79.1">
<link rel="home" href="index.html" title="The Boost C++ Libraries BoostBook Documentation Subset">
<link rel="up" href="libraries.html" title="Part I. The Boost C++ Libraries (BoostBook Subset)">
<link rel="prev" href="align/history.html" title="History">
<link rel="next" href="any/s02.html" title="Examples">
</head>
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
<table cellpadding="2" width="100%"><tr>
<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../boost.png"></td>
<td align="center"><a href="../../index.html">Home</a></td>
<td align="center"><a href="../../libs/libraries.htm">Libraries</a></td>
<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
<td align="center"><a href="../../more/index.htm">More</a></td>
</tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="align/history.html"><img src="../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="libraries.html"><img src="../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="index.html"><img src="../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="any/s02.html"><img src="../../doc/src/images/next.png" alt="Next"></a>
</div>
<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="idp116934240"></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 class="toc">
<dt><span class="section"><a href="any.html#idp116937008">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="idp116937008"></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>
</div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer"></div></td>
</tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="align/history.html"><img src="../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="libraries.html"><img src="../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="index.html"><img src="../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="any/s02.html"><img src="../../doc/src/images/next.png" alt="Next"></a>
</div>
</body>
</html>
|
