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| author | Anas Nashif <anas.nashif@intel.com> | 2012-10-30 12:57:26 -0700 |
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| committer | Anas Nashif <anas.nashif@intel.com> | 2012-10-30 12:57:26 -0700 |
| commit | 1a78a62555be32868418fe52f8e330c9d0f95d5a (patch) | |
| tree | d3765a80e7d3b9640ec2e930743630cd6b9fce2b /doc/html/variant.html | |
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diff --git a/doc/html/variant.html b/doc/html/variant.html new file mode 100644 index 0000000000..a017ee0019 --- /dev/null +++ b/doc/html/variant.html @@ -0,0 +1,215 @@ +<html> +<head> +<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> +<title>Chapter 34. Boost.Variant</title> +<link rel="stylesheet" href="../../doc/src/boostbook.css" type="text/css"> +<meta name="generator" content="DocBook XSL Stylesheets V1.76.1"> +<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="unordered/bibliography.html" title="Bibliography"> +<link rel="next" href="variant/tutorial.html" title="Tutorial"> +</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="unordered/bibliography.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="variant/tutorial.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="variant"></a>Chapter 34. Boost.Variant</h2></div> +<div><div class="author"><h3 class="author"> +<span class="firstname">Eric</span> <span class="surname">Friedman</span> +</h3></div></div> +<div><div class="author"><h3 class="author"> +<span class="firstname">Itay</span> <span class="surname">Maman</span> +</h3></div></div> +<div><p class="copyright">Copyright © 2002, 2003 Eric Friedman, Itay Maman</p></div> +<div><div class="legalnotice"> +<a name="id3695447"></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="variant.html#variant.intro">Introduction</a></span></dt> +<dd><dl> +<dt><span class="section"><a href="variant.html#variant.abstract">Abstract</a></span></dt> +<dt><span class="section"><a href="variant.html#variant.motivation">Motivation</a></span></dt> +</dl></dd> +<dt><span class="section"><a href="variant/tutorial.html">Tutorial</a></span></dt> +<dd><dl> +<dt><span class="section"><a href="variant/tutorial.html#variant.tutorial.basic">Basic Usage</a></span></dt> +<dt><span class="section"><a href="variant/tutorial.html#variant.tutorial.advanced">Advanced Topics</a></span></dt> +</dl></dd> +<dt><span class="section"><a href="variant/reference.html">Reference</a></span></dt> +<dd><dl> +<dt><span class="section"><a href="variant/reference.html#variant.concepts">Concepts</a></span></dt> +<dt><span class="section"><a href="variant/reference.html#header.boost.variant_hpp">Header <boost/variant.hpp></a></span></dt> +<dt><span class="section"><a href="variant/reference.html#header.boost.variant.variant_fwd_hpp">Header <boost/variant/variant_fwd.hpp></a></span></dt> +<dt><span class="section"><a href="variant/reference.html#header.boost.variant.variant_hpp">Header <boost/variant/variant.hpp></a></span></dt> +<dt><span class="section"><a href="variant/reference.html#header.boost.variant.recursive_variant_hpp">Header <boost/variant/recursive_variant.hpp></a></span></dt> +<dt><span class="section"><a href="variant/reference.html#header.boost.variant.recursive_wrapper_hpp">Header <boost/variant/recursive_wrapper.hpp></a></span></dt> +<dt><span class="section"><a href="variant/reference.html#header.boost.variant.apply_visitor_hpp">Header <boost/variant/apply_visitor.hpp></a></span></dt> +<dt><span class="section"><a href="variant/reference.html#header.boost.variant.get_hpp">Header <boost/variant/get.hpp></a></span></dt> +<dt><span class="section"><a href="variant/reference.html#header.boost.variant.bad_visit_hpp">Header <boost/variant/bad_visit.hpp></a></span></dt> +<dt><span class="section"><a href="variant/reference.html#header.boost.variant.static_visitor_hpp">Header <boost/variant/static_visitor.hpp></a></span></dt> +<dt><span class="section"><a href="variant/reference.html#header.boost.variant.visitor_ptr_hpp">Header <boost/variant/visitor_ptr.hpp></a></span></dt> +</dl></dd> +<dt><span class="section"><a href="variant/design.html">Design Overview</a></span></dt> +<dd><dl><dt><span class="section"><a href="variant/design.html#variant.design.never-empty">"Never-Empty" Guarantee</a></span></dt></dl></dd> +<dt><span class="section"><a href="variant/misc.html">Miscellaneous Notes</a></span></dt> +<dd><dl> +<dt><span class="section"><a href="variant/misc.html#variant.versus-any">Boost.Variant vs. Boost.Any</a></span></dt> +<dt><span class="section"><a href="variant/misc.html#id3714040">Portability</a></span></dt> +<dt><span class="section"><a href="variant/misc.html#variant.troubleshooting">Troubleshooting</a></span></dt> +<dt><span class="section"><a href="variant/misc.html#variant.ack">Acknowledgments</a></span></dt> +</dl></dd> +<dt><span class="section"><a href="variant/refs.html">References</a></span></dt> +</dl> +</div> +<div class="section"> +<div class="titlepage"><div><div><h2 class="title" style="clear: both"> +<a name="variant.intro"></a>Introduction</h2></div></div></div> +<div class="toc"><dl> +<dt><span class="section"><a href="variant.html#variant.abstract">Abstract</a></span></dt> +<dt><span class="section"><a href="variant.html#variant.motivation">Motivation</a></span></dt> +</dl></div> +<div class="section"> +<div class="titlepage"><div><div><h3 class="title"> +<a name="variant.abstract"></a>Abstract</h3></div></div></div> +<p>The <code class="computeroutput">variant</code> class template is a safe, generic, stack-based +discriminated union container, offering a simple solution for manipulating an +object from a heterogeneous set of types in a uniform manner. Whereas +standard containers such as <code class="computeroutput">std::vector</code> may be thought of as +"<span class="bold"><strong>multi-value, single type</strong></span>," +<code class="computeroutput">variant</code> is "<span class="bold"><strong>multi-type, +single value</strong></span>."</p> +<p>Notable features of <code class="computeroutput"><a class="link" href="boost/variant.html" title="Class template variant">boost::variant</a></code> +include:</p> +<div class="itemizedlist"><ul class="itemizedlist" type="disc"> +<li class="listitem">Full value semantics, including adherence to standard + overload resolution rules for conversion operations.</li> +<li class="listitem">Compile-time type-safe value visitation via + <code class="computeroutput"><a class="link" href="boost/apply_visitor.html" title="Function apply_visitor">boost::apply_visitor</a></code>.</li> +<li class="listitem">Run-time checked explicit value retrieval via + <code class="computeroutput"><a class="link" href="boost/get_id1673528.html" title="Function get">boost::get</a></code>.</li> +<li class="listitem">Support for recursive variant types via both + <code class="computeroutput"><a class="link" href="boost/make_recursive_variant.html" title="Class template make_recursive_variant">boost::make_recursive_variant</a></code> and + <code class="computeroutput"><a class="link" href="boost/recursive_wrapper.html" title="Class template recursive_wrapper">boost::recursive_wrapper</a></code>.</li> +<li class="listitem">Efficient implementation -- stack-based when possible (see + <a class="xref" href="variant/design.html#variant.design.never-empty" title='"Never-Empty" Guarantee'>the section called “"Never-Empty" Guarantee”</a> for more details).</li> +</ul></div> +</div> +<div class="section"> +<div class="titlepage"><div><div><h3 class="title"> +<a name="variant.motivation"></a>Motivation</h3></div></div></div> +<div class="toc"><dl> +<dt><span class="section"><a href="variant.html#variant.motivation.problem">Problem</a></span></dt> +<dt><span class="section"><a href="variant.html#variant.motivation.solution">Solution: A Motivating Example</a></span></dt> +</dl></div> +<div class="section"> +<div class="titlepage"><div><div><h4 class="title"> +<a name="variant.motivation.problem"></a>Problem</h4></div></div></div> +<p>Many times, during the development of a C++ program, the +programmer finds himself in need of manipulating several distinct +types in a uniform manner. Indeed, C++ features direct language +support for such types through its <code class="computeroutput">union</code> +keyword:</p> +<pre class="programlisting">union { int i; double d; } u; +u.d = 3.14; +u.i = 3; // overwrites u.d (OK: u.d is a POD type)</pre> +<p>C++'s <code class="computeroutput">union</code> construct, however, is nearly +useless in an object-oriented environment. The construct entered +the language primarily as a means for preserving compatibility with +C, which supports only POD (Plain Old Data) types, and so does not +accept types exhibiting non-trivial construction or +destruction:</p> +<pre class="programlisting">union { + int i; + std::string s; // illegal: std::string is not a POD type! +} u;</pre> +<p>Clearly another approach is required. Typical solutions +feature the dynamic-allocation of objects, which are subsequently +manipulated through a common base type (often a virtual base class + [<a class="link" href="variant/refs.html#variant.refs.hen01">Hen01</a>] +or, more dangerously, a <code class="computeroutput">void*</code>). Objects of +concrete type may be then retrieved by way of a polymorphic downcast +construct (e.g., <code class="computeroutput">dynamic_cast</code>, +<code class="computeroutput"><a class="link" href="boost/any_cast.html" title="Function any_cast">boost::any_cast</a></code>, etc.).</p> +<p>However, solutions of this sort are highly error-prone, due +to the following:</p> +<div class="itemizedlist"><ul class="itemizedlist" type="disc"> +<li class="listitem"> +<span class="emphasis"><em>Downcast errors cannot be detected at + compile-time.</em></span> Thus, incorrect usage of downcast + constructs will lead to bugs detectable only at run-time.</li> +<li class="listitem"> +<span class="emphasis"><em>Addition of new concrete types may be + ignored.</em></span> If a new concrete type is added to the + hierarchy, existing downcast code will continue to work as-is, + wholly ignoring the new type. Consequently, the programmer must + manually locate and modify code at numerous locations, which often + results in run-time errors that are difficult to find.</li> +</ul></div> +<p>Furthermore, even when properly implemented, these solutions tend +to incur a relatively significant abstraction penalty due to the use of +the heap, virtual function calls, and polymorphic downcasts.</p> +</div> +<div class="section"> +<div class="titlepage"><div><div><h4 class="title"> +<a name="variant.motivation.solution"></a>Solution: A Motivating Example</h4></div></div></div> +<p>The <code class="computeroutput"><a class="link" href="boost/variant.html" title="Class template variant">boost::variant</a></code> class template +addresses these issues in a safe, straightforward, and efficient manner. The +following example demonstrates how the class can be used:</p> +<pre class="programlisting">#include "boost/variant.hpp" +#include <iostream> + +class my_visitor : public <code class="computeroutput"><a class="link" href="boost/static_visitor.html" title="Class template static_visitor">boost::static_visitor</a></code><int> +{ +public: + int operator()(int i) const + { + return i; + } + + int operator()(const <code class="computeroutput">std::string</code> & str) const + { + return str.length(); + } +}; + +int main() +{ + <code class="computeroutput"><a class="link" href="boost/variant.html" title="Class template variant">boost::variant</a></code>< int, std::string > u("hello world"); + std::cout << u; // output: hello world + + int result = <code class="computeroutput"><a class="link" href="boost/apply_visitor.html" title="Function apply_visitor">boost::apply_visitor</a></code>( my_visitor(), u ); + std::cout << result; // output: 11 (i.e., length of "hello world") +} +</pre> +</div> +</div> +</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="unordered/bibliography.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="variant/tutorial.html"><img src="../../doc/src/images/next.png" alt="Next"></a> +</div> +</body> +</html> |