boost_1_45_0/libs/variant/doc/introduction.xml

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<!DOCTYPE section PUBLIC "-//Boost//DTD BoostBook XML V1.0//EN"
  "http://www.boost.org/tools/boostbook/dtd/boostbook.dtd">
<section id="variant.intro">
  <title>Introduction</title>

  <using-namespace name="boost"/>

<section id="variant.abstract">
  <title>Abstract</title>

<para>The <code>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>std::vector</code> may be thought of as
"<emphasis role="bold">multi-value, single type</emphasis>,"
<code>variant</code> is "<emphasis role="bold">multi-type,
single value</emphasis>."</para>

<para>Notable features of <code><classname>boost::variant</classname></code>
include:</para>

<itemizedlist>
  <listitem>Full value semantics, including adherence to standard
    overload resolution rules for conversion operations.</listitem>
  <listitem>Compile-time type-safe value visitation via
    <code><functionname>boost::apply_visitor</functionname></code>.</listitem>
  <listitem>Run-time checked explicit value retrieval via
    <code><functionname>boost::get</functionname></code>.</listitem>
  <listitem>Support for recursive variant types via both
    <code><classname>boost::make_recursive_variant</classname></code> and
    <code><classname>boost::recursive_wrapper</classname></code>.</listitem>
  <listitem>Efficient implementation -- stack-based when possible (see
    <xref linkend="variant.design.never-empty"/> for more details).</listitem>
</itemizedlist>

</section>

<section id="variant.motivation">
  <title>Motivation</title>

<section id="variant.motivation.problem">
  <title>Problem</title>

<para>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>union</code> 
keyword:</para>

<programlisting>union { int i; double d; } u;
u.d = 3.14;
u.i = 3; // overwrites u.d (OK: u.d is a POD type)</programlisting>

<para>C++'s <code>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:</para>

<programlisting>union {
  int i;
  std::string s; // illegal: std::string is not a POD type!
} u;</programlisting>

<para>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
    [<link linkend="variant.refs.hen01">Hen01</link>]
or, more dangerously, a <code>void*</code>). Objects of
concrete type may be then retrieved by way of a polymorphic downcast
construct (e.g., <code>dynamic_cast</code>,
<code><functionname>boost::any_cast</functionname></code>, etc.).</para>

<para>However, solutions of this sort are highly error-prone, due
to the following:</para>

<itemizedlist>
  <listitem><emphasis>Downcast errors cannot be detected at
    compile-time.</emphasis> Thus, incorrect usage of downcast
    constructs will lead to bugs detectable only at run-time.</listitem>
  <listitem><emphasis>Addition of new concrete types may be 
    ignored.</emphasis> 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.</listitem>
</itemizedlist>

<para>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.</para>

</section>

<section id="variant.motivation.solution">
  <title>Solution: A Motivating Example</title>

<para>The <code><classname>boost::variant</classname></code> class template
addresses these issues in a safe, straightforward, and efficient manner. The
following example demonstrates how the class can be used:</para>

<programlisting>#include "boost/variant.hpp"
#include &lt;iostream&gt;

class my_visitor : public <classname>boost::static_visitor</classname>&lt;int&gt;
{
public:
    int operator()(int i) const
    {
        return i;
    }
    
    int operator()(const <classname>std::string</classname> &amp; str) const
    {
        return str.length();
    }
};

int main()
{
    <classname>boost::variant</classname>&lt; int, std::string &gt; u("hello world");
    std::cout &lt;&lt; u; // output: hello world

    int result = <functionname>boost::apply_visitor</functionname>( my_visitor(), u );
    std::cout &lt;&lt; result; // output: 11 (i.e., length of "hello world")
}
</programlisting>

</section>

</section>

</section>