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<title>Chapter 39. Boost.Variant</title>
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<div class="chapter">
<div class="titlepage"><div>
<div><h2 class="title">
<a name="variant"></a>Chapter 39. 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="idm45555034029472"></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>
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<div class="toc">
<p><b>Table of Contents</b></p>
<dl class="toc">
<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.multivisitors_hpp">Header <boost/variant/multivisitors.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.polymorphic_get_hpp">Header <boost/variant/polymorphic_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#idm45555030186032">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 class="toc">
<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" style="list-style-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_idm45507165650576.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 class="toc">
<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_idm45507162615824.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" style="list-style-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>
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