boost/libs/variant/doc/tutorial/basic.xml

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<section id="variant.tutorial.basic">
  <title>Basic Usage</title>

<using-namespace name="boost"/>
<using-class name="boost::variant"/>

<para>A discriminated union container on some set of types is defined by
  instantiating the <code><classname>boost::variant</classname></code> class
  template with the desired types. These types are called
  <emphasis role="bold">bounded types</emphasis> and are subject to the
  requirements of the
  <link linkend="variant.concepts.bounded-type"><emphasis>BoundedType</emphasis></link>
  concept. Any number of bounded types may be specified, up to some
  implementation-defined limit (see 
  <code><macroname>BOOST_VARIANT_LIMIT_TYPES</macroname></code>).</para>

<para>For example, the following declares a discriminated union container on
  <code>int</code> and <code>std::string</code>:

<programlisting><classname>boost::variant</classname>&lt; int, std::string &gt; v;</programlisting>

</para>

<para>By default, a <code>variant</code> default-constructs its first
  bounded type, so <code>v</code> initially contains <code>int(0)</code>. If
  this is not desired, or if the first bounded type is not
  default-constructible, a <code>variant</code> can be constructed
  directly from any value convertible to one of its bounded types. Similarly,
  a <code>variant</code> can be assigned any value convertible to one of its
  bounded types, as demonstrated in the following:

<programlisting>v = "hello";</programlisting>

</para>

<para>Now <code>v</code> contains a <code>std::string</code> equal to
  <code>"hello"</code>. We can demonstrate this by
  <emphasis role="bold">streaming</emphasis>&nbsp;<code>v</code> to standard
  output:

<programlisting>std::cout &lt;&lt; v &lt;&lt; std::endl;</programlisting>

</para>

<para>Usually though, we would like to do more with the content of a
  <code>variant</code> than streaming. Thus, we need some way to access the
  contained value. There are two ways to accomplish this:
  <code><functionname>apply_visitor</functionname></code>, which is safest
  and very powerful, and
  <code><functionname>get</functionname>&lt;T&gt;</code>, which is
  sometimes more convenient to use.</para>

<para>For instance, suppose we wanted to concatenate to the string contained
  in <code>v</code>. With <emphasis role="bold">value retrieval</emphasis>
  by <code><functionname>get</functionname></code>, this may be accomplished
  quite simply, as seen in the following:

<programlisting>std::string&amp; str = <functionname>boost::get</functionname>&lt;std::string&gt;(v);
str += " world! ";</programlisting>

</para>

<para>As desired, the <code>std::string</code> contained by <code>v</code> now
  is equal to <code>"hello world! "</code>. Again, we can demonstrate this by
  streaming <code>v</code> to standard output:

<programlisting>std::cout &lt;&lt; v &lt;&lt; std::endl;</programlisting>

</para>

<para>While use of <code>get</code> is perfectly acceptable in this trivial
  example, <code>get</code> generally suffers from several significant
  shortcomings. For instance, if we were to write a function accepting a
  <code>variant&lt;int, std::string&gt;</code>, we would not know whether
  the passed <code>variant</code> contained an <code>int</code> or a
  <code>std::string</code>. If we insisted upon continued use of
  <code>get</code>, we would need to query the <code>variant</code> for its
  contained type. The following function, which &quot;doubles&quot; the
  content of the given <code>variant</code>, demonstrates this approach:

<programlisting>void times_two( boost::variant&lt; int, std::string &gt; &amp; operand )
{
    if ( int* pi = <functionname>boost::get</functionname>&lt;int&gt;( &amp;operand ) )
        *pi *= 2;
    else if ( std::string* pstr = <functionname>boost::get</functionname>&lt;std::string&gt;( &amp;operand ) )
        *pstr += *pstr;
}</programlisting>

</para>

<para>However, such code is quite brittle, and without careful attention will
  likely lead to the introduction of subtle logical errors detectable only at
  runtime. For instance, consider if we wished to extend
  <code>times_two</code> to operate on a <code>variant</code> with additional
  bounded types. Specifically, let's add
  <code>std::complex&lt;double&gt;</code> to the set. Clearly, we would need
  to at least change the function declaration:

<programlisting>void times_two( boost::variant&lt; int, std::string, std::complex&lt;double&gt; &gt; &amp; operand )
{
    // as above...?
}</programlisting>

</para>

<para>Of course, additional changes are required, for currently if the passed
  <code>variant</code> in fact contained a <code>std::complex</code> value,
  <code>times_two</code> would silently return -- without any of the desired
  side-effects and without any error. In this case, the fix is obvious. But in
  more complicated programs, it could take considerable time to identify and
  locate the error in the first place.</para>

<para>Thus, real-world use of <code>variant</code> typically demands an access
  mechanism more robust than <code>get</code>. For this reason,
  <code>variant</code> supports compile-time checked
  <emphasis role="bold">visitation</emphasis> via
  <code><functionname>apply_visitor</functionname></code>. Visitation requires
  that the programmer explicitly handle (or ignore) each bounded type. Failure
  to do so results in a compile-time error.</para>

<para>Visitation of a <code>variant</code> requires a visitor object. The
  following demonstrates one such implementation of a visitor implementating
  behavior identical to <code>times_two</code>:

<programlisting>class times_two_visitor
    : public <classname>boost::static_visitor</classname>&lt;&gt;
{
public:

    void operator()(int &amp; i) const
    {
        i *= 2;
    }

    void operator()(std::string &amp; str) const
    {
        str += str;
    }

};</programlisting>

</para>

<para>With the implementation of the above visitor, we can then apply it to
  <code>v</code>, as seen in the following:

<programlisting><functionname>boost::apply_visitor</functionname>( times_two_visitor(), v );</programlisting>

</para>

<para>As expected, the content of <code>v</code> is now a
  <code>std::string</code> equal to <code>"hello world! hello world! "</code>.
  (We'll skip the verification this time.)</para>

<para>In addition to enhanced robustness, visitation provides another
  important advantage over <code>get</code>: the ability to write generic
  visitors. For instance, the following visitor will &quot;double&quot; the
  content of <emphasis>any</emphasis>&nbsp;<code>variant</code> (provided its
  bounded types each support operator+=):

<programlisting>class times_two_generic
    : public <classname>boost::static_visitor</classname>&lt;&gt;
{
public:

    template &lt;typename T&gt;
    void operator()( T &amp; operand ) const
    {
        operand += operand;
    }

};</programlisting>

</para>

<para>Again, <code>apply_visitor</code> sets the wheels in motion:

<programlisting><functionname>boost::apply_visitor</functionname>( times_two_generic(), v );</programlisting>

</para>

<para>While the initial setup costs of visitation may exceed that required for
  <code>get</code>, the benefits quickly become significant. Before concluding
  this section, we should explore one last benefit of visitation with
  <code>apply_visitor</code>:
  <emphasis role="bold">delayed visitation</emphasis>. Namely, a special form
  of <code>apply_visitor</code> is available that does not immediately apply
  the given visitor to any <code>variant</code> but rather returns a function
  object that operates on any <code>variant</code> given to it. This behavior
  is particularly useful when operating on sequences of <code>variant</code>
  type, as the following demonstrates:

<programlisting>std::vector&lt; <classname>boost::variant</classname>&lt;int, std::string&gt; &gt; vec;
vec.push_back( 21 );
vec.push_back( "hello " );

times_two_generic visitor;
std::for_each(
      vec.begin(), vec.end()
   , <functionname>boost::apply_visitor</functionname>(visitor)
   );</programlisting>

</para>

</section>