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- BoostBook element rationale"><link rel="refentry" href="Assignable.html" title="Concept Assignable"><link rel="refentry" href="InputIterator.html" title="Concept InputIterator"><link rel="refentry" href="OutputIterator.html" title="Concept OutputIterator"><link rel="refentry" href="ForwardIterator.html" title="Concept ForwardIterator"><link rel="refentry" href="BidirectionalIterator.html" title="Concept BidirectionalIterator"><link rel="refentry" href="RandomAccessIterator.html" title="Concept RandomAccessIterator"><link rel="refentry" href="DefaultConstructible.html" title="Concept DefaultConstructible"><link rel="refentry" href="CopyConstructible.html" title="Concept CopyConstructible"><link rel="refentry" href="EqualityComparable.html" title="Concept EqualityComparable"><link rel="refentry" href="LessThanComparable.html" title="Concept LessThanComparable"><link rel="refentry" href="SignedInteger.html" title="Concept SignedInteger"><link rel="section" href="variant.html#variant.intro" title="Introduction"><link rel="section" href="variant.tutorial.html" title="Tutorial"><link rel="section" href="variant.reference.html" title="Reference"><link rel="section" href="variant.design.html" title="Design Overview"><link rel="section" href="variant.misc.html" title="Miscellaneous Notes"><link rel="section" href="variant.refs.html" title="References"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" border="1" cellpadding="2" bgcolor="#007F7F"><tr><td bgcolor="#FFFFFF"><img src="../../c++boost.gif" alt="c++boost.gif (8819 bytes)" width="277" height="86"></td><td><a href="../../index.htm"><font color="#FFFFFF" size="4" face="Arial">Home</font></a></td><td><a href="libraries.html"><font color="#FFFFFF" size="4" face="Arial">Libraries</font></a></td><td><a href="../../people/people.htm"><font color="#FFFFFF" size="4" face="Arial">People</font></a></td><td><a href="../../more/faq.htm"><font color="#FFFFFF" size="4" face="Arial">FAQ</font></a></td><td><a href="../../more/index.htm"><font color="#FFFFFF" size="4" face="Arial">More</font></a></td></tr></table><div class="chapter" lang="en"><div class="titlepage"><div><div><h2 class="title"><a name="variant"></a>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"><p>Permission to copy, use, sell and distribute this software
- is granted provided this copyright notice appears in all copies.
- Permission to modify the code and to distribute modified code is
- granted provided this copyright notice appears in all copies, and
- a notice that the code was modified is included with the copyright
- notice.</p><p> This software is provided "as is" without express or
- implied warranty, and with no claim as to its suitability for any
- purpose.</p></div></div></div><div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><a href="variant.html#variant.intro">Introduction</a></dt><dd><dl><dt><a href="variant.html#variant.abstract">Abstract</a></dt><dt><a href="variant.html#variant.motivation">Motivation</a></dt></dl></dd><dt><a href="variant.tutorial.html">Tutorial</a></dt><dd><dl><dt><a href="variant.tutorial.html#variant.tutorial.basic">Basic Usage</a></dt><dt><a href="variant.tutorial.html#variant.tutorial.advanced">Advanced Topics</a></dt></dl></dd><dt><a href="variant.reference.html">Reference</a></dt><dd><dl><dt><a href="variant.reference.html#variant.concepts">Concepts</a></dt><dt><a href="variant.reference.html#header.boost.variant.hpp">Header <boost/variant.hpp></a></dt><dt><a href="variant.reference.html#header.boost.variant.variant_fwd.hpp">Header <boost/variant/variant_fwd.hpp></a></dt><dt><a href="variant.reference.html#header.boost.variant.variant.hpp">Header <boost/variant/variant.hpp></a></dt><dt><a href="variant.reference.html#header.boost.variant.recursive_variant.hpp">Header <boost/variant/recursive_variant.hpp></a></dt><dt><a href="variant.reference.html#header.boost.variant.recursive_wrapper.hpp">Header <boost/variant/recursive_wrapper.hpp></a></dt><dt><a href="variant.reference.html#header.boost.variant.apply_visitor.hpp">Header <boost/variant/apply_visitor.hpp></a></dt><dt><a href="variant.reference.html#header.boost.variant.get.hpp">Header <boost/variant/get.hpp></a></dt><dt><a href="variant.reference.html#header.boost.variant.bad_visit.hpp">Header <boost/variant/bad_visit.hpp></a></dt><dt><a href="variant.reference.html#header.boost.variant.static_visitor.hpp">Header <boost/variant/static_visitor.hpp></a></dt><dt><a href="variant.reference.html#header.boost.variant.visitor_ptr.hpp">Header <boost/variant/visitor_ptr.hpp></a></dt></dl></dd><dt><a href="variant.design.html">Design Overview</a></dt><dd><dl><dt><a href="variant.design.html#variant.design.never-empty">"Never-Empty" Guarantee</a></dt></dl></dd><dt><a href="variant.misc.html">Miscellaneous Notes</a></dt><dd><dl><dt><a href="variant.misc.html#variant.versus-any">Boost.Variant vs. Boost.Any</a></dt><dt><a href="variant.misc.html#id2539312">Portability</a></dt><dt><a href="variant.misc.html#variant.troubleshooting">Troubleshooting</a></dt><dt><a href="variant.misc.html#variant.ack">Acknowledgments</a></dt></dl></dd><dt><a href="variant.refs.html">References</a></dt></dl></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="variant.intro"></a>Introduction</h3></div></div><div></div></div><div class="toc"><dl><dt><a href="variant.html#variant.abstract">Abstract</a></dt><dt><a href="variant.html#variant.motivation">Motivation</a></dt></dl></div><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="variant.abstract"></a>Abstract</h4></div></div><div></div></div><p>The <tt class="computeroutput">variant</tt> 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 <tt class="computeroutput">std::vector</tt> may be thought of as
- "<span class="bold"><b>multi-value, single type</b></span>,"
- <tt class="computeroutput">variant</tt> is "<span class="bold"><b>multi-type,
- single value</b></span>."</p><p>Notable features of <tt class="computeroutput"><a href="class.boost.variant.html" title="Class template variant">boost::variant</a></tt>
- include:</p><div class="itemizedlist"><ul type="disc"><li>Full value semantics, including adherence to standard
- overload resolution rules for conversion operations.</li><li>Compile-time type-safe value visitation via
- <tt class="computeroutput"><a href="id2557051-bb.html" title="Function apply_visitor">boost::apply_visitor</a></tt>.</li><li>Run-time checked explicit value retrieval via
- <tt class="computeroutput"><a href="id2493238-bb.html" title="Function get">boost::get</a></tt>.</li><li>Support for recursive variant types via both
- <tt class="computeroutput"><a href="class.boost.make_recursive_variant.html" title="Class template make_recursive_variant">boost::make_recursive_variant</a></tt> and
- <tt class="computeroutput"><a href="class.boost.recursive_wrapper.html" title="Class template recursive_wrapper">boost::recursive_wrapper</a></tt>.</li><li>Efficient implementation -- stack-based when possible (see
- <a 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" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="variant.motivation"></a>Motivation</h4></div></div><div></div></div><div class="toc"><dl><dt><a href="variant.html#variant.motivation.problem">Problem</a></dt><dt><a href="variant.html#variant.motivation.solution">Solution: A Motivating Example</a></dt></dl></div><div class="section" lang="en"><div class="titlepage"><div><div><h5 class="title"><a name="variant.motivation.problem"></a>Problem</h5></div></div><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 <tt class="computeroutput">union</tt>
- 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 <tt class="computeroutput">union</tt> 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 href="variant.refs.html#variant.refs.hen01">Hen01</a>]
- or, more dangerously, a <tt class="computeroutput">void*</tt>). Objects of
- concrete type may be then retrieved by way of a polymorphic downcast
- construct (e.g., <tt class="computeroutput">dynamic_cast</tt>,
- <tt class="computeroutput"><a href="id2383300-bb.html" title="Function any_cast">boost::any_cast</a></tt>, etc.).</p><p>However, solutions of this sort are highly error-prone, due
- to the following:</p><div class="itemizedlist"><ul type="disc"><li><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><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" lang="en"><div class="titlepage"><div><div><h5 class="title"><a name="variant.motivation.solution"></a>Solution: A Motivating Example</h5></div></div><div></div></div><p>The <tt class="computeroutput"><a href="class.boost.variant.html" title="Class template variant">boost::variant</a></tt> 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 <a href="class.boost.static_visitor.html" title="Class template static_visitor">boost::static_visitor</a><int>
- {
- public:
- int operator()(int i) const
- {
- return i;
- }
-
- int operator()(const std::string & str) const
- {
- return str.length();
- }
- };
- int main()
- {
- <a href="class.boost.variant.html" title="Class template variant">boost::variant</a>< int, std::string > u("hello world");
- std::cout << u; // output: hello world
- int result = <a href="id2557051-bb.html" title="Function apply_visitor">boost::apply_visitor</a>( 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"><small>Last revised: , at GMT</small></td><td align="right"><small></small></td></tr></table><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="signals.tests.html"><img src="images/prev.png" alt="Prev"></a> </td><td width="20%" align="center"><a accesskey="u" href="index.html"><img src="images/up.png" alt="Up"></a></td><td width="40%" align="right"> <a accesskey="n" href="variant.tutorial.html"><img src="images/next.png" alt="Next"></a></td></tr><tr><td width="40%" align="left" valign="top">Testsuite </td><td width="20%" align="center"><a accesskey="h" href="index.html"><img src="images/home.png" alt="Home"></a></td><td width="40%" align="right" valign="top"> Tutorial</td></tr></table></div></body></html>
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