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<div class="refentry">
<a name="boost.variant"></a><div class="titlepage"></div>
<div class="refnamediv">
<h2><span class="refentrytitle">Class template variant</span></h2>
<p>boost::variant — Safe, generic, stack-based discriminated union container.</p>
</div>
<h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2>
<div xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv"><pre class="synopsis"><span class="comment">// In header: <<a class="link" href="../variant/reference.html#header.boost.variant.variant_hpp" title="Header <boost/variant/variant.hpp>">boost/variant/variant.hpp</a>>
</span><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> T1<span class="special">,</span> <span class="keyword">typename</span> T2 <span class="special">=</span> <span class="emphasis"><em><span class="identifier">unspecified</span></em></span><span class="special">,</span> <span class="special">...</span><span class="special">,</span>
<span class="keyword">typename</span> TN <span class="special">=</span> <span class="emphasis"><em><span class="identifier">unspecified</span></em></span><span class="special">></span>
<span class="keyword">class</span> <a class="link" href="variant.html" title="Class template variant">variant</a> <span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
<span class="comment">// types</span>
<span class="keyword">typedef</span> <span class="emphasis"><em><span class="identifier">unspecified</span></em></span> <a name="boost.variant.types"></a><span class="identifier">types</span><span class="special">;</span>
<span class="comment">// <a class="link" href="variant.html#boost.variantconstruct-copy-destruct">construct/copy/destruct</a></span>
<a class="link" href="variant.html#id1381472-bb"><span class="identifier">variant</span></a><span class="special">(</span><span class="special">)</span><span class="special">;</span>
<a class="link" href="variant.html#id621352-bb"><span class="identifier">variant</span></a><span class="special">(</span><span class="keyword">const</span> <span class="identifier">variant</span> <span class="special">&</span><span class="special">)</span><span class="special">;</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> T<span class="special">></span> <a class="link" href="variant.html#id651681-bb"><span class="identifier">variant</span></a><span class="special">(</span><span class="identifier">T</span> <span class="special">&</span><span class="special">)</span><span class="special">;</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> T<span class="special">></span> <a class="link" href="variant.html#id1489812-bb"><span class="identifier">variant</span></a><span class="special">(</span><span class="keyword">const</span> <span class="identifier">T</span> <span class="special">&</span><span class="special">)</span><span class="special">;</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> U1<span class="special">,</span> <span class="keyword">typename</span> U2<span class="special">,</span> <span class="special">...</span><span class="special">,</span> <span class="keyword">typename</span> UN<span class="special">></span>
<a class="link" href="variant.html#id1374589-bb"><span class="identifier">variant</span></a><span class="special">(</span><span class="identifier">variant</span><span class="special"><</span><span class="identifier">U1</span><span class="special">,</span> <span class="identifier">U2</span><span class="special">,</span> <span class="special">...</span><span class="special">,</span> <span class="identifier">UN</span><span class="special">></span> <span class="special">&</span><span class="special">)</span><span class="special">;</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> U1<span class="special">,</span> <span class="keyword">typename</span> U2<span class="special">,</span> <span class="special">...</span><span class="special">,</span> <span class="keyword">typename</span> UN<span class="special">></span>
<a class="link" href="variant.html#id1364798-bb"><span class="identifier">variant</span></a><span class="special">(</span><span class="keyword">const</span> <span class="identifier">variant</span><span class="special"><</span><span class="identifier">U1</span><span class="special">,</span> <span class="identifier">U2</span><span class="special">,</span> <span class="special">...</span><span class="special">,</span> <span class="identifier">UN</span><span class="special">></span> <span class="special">&</span><span class="special">)</span><span class="special">;</span>
<a class="link" href="variant.html#id641552-bb"><span class="special">~</span><span class="identifier">variant</span></a><span class="special">(</span><span class="special">)</span><span class="special">;</span>
<span class="comment">// <a class="link" href="variant.html#id1162116-bb">modifiers</a></span>
<span class="keyword">void</span> <a class="link" href="variant.html#id1353905-bb"><span class="identifier">swap</span></a><span class="special">(</span><span class="identifier">variant</span> <span class="special">&</span><span class="special">)</span><span class="special">;</span>
<span class="identifier">variant</span> <span class="special">&</span> <a class="link" href="variant.html#id1374533-bb"><span class="keyword">operator</span><span class="special">=</span></a><span class="special">(</span><span class="keyword">const</span> <span class="identifier">variant</span> <span class="special">&</span><span class="special">)</span><span class="special">;</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> T<span class="special">></span> <span class="identifier">variant</span> <span class="special">&</span> <a class="link" href="variant.html#id668119-bb"><span class="keyword">operator</span><span class="special">=</span></a><span class="special">(</span><span class="keyword">const</span> <span class="identifier">T</span> <span class="special">&</span><span class="special">)</span><span class="special">;</span>
<span class="comment">// <a class="link" href="variant.html#id1376600-bb">queries</a></span>
<span class="keyword">int</span> <a class="link" href="variant.html#id648404-bb"><span class="identifier">which</span></a><span class="special">(</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
<span class="keyword">bool</span> <a class="link" href="variant.html#id1138398-bb"><span class="identifier">empty</span></a><span class="special">(</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
<span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">type_info</span> <span class="special">&</span> <a class="link" href="variant.html#id1493085-bb"><span class="identifier">type</span></a><span class="special">(</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
<span class="comment">// <a class="link" href="variant.html#id1364802-bb">relational</a></span>
<span class="keyword">bool</span> <a class="link" href="variant.html#id623589-bb"><span class="keyword">operator</span><span class="special">==</span></a><span class="special">(</span><span class="keyword">const</span> <span class="identifier">variant</span> <span class="special">&</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> U<span class="special">></span> <span class="keyword">void</span> <a class="link" href="variant.html#id663596-bb"><span class="keyword">operator</span><span class="special">==</span></a><span class="special">(</span><span class="keyword">const</span> <span class="identifier">U</span> <span class="special">&</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
<span class="keyword">bool</span> <a class="link" href="variant.html#id1375023-bb"><span class="keyword">operator</span><span class="special"><</span></a><span class="special">(</span><span class="keyword">const</span> <span class="identifier">variant</span> <span class="special">&</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> U<span class="special">></span> <span class="keyword">void</span> <a class="link" href="variant.html#id621172-bb"><span class="keyword">operator</span><span class="special"><</span></a><span class="special">(</span><span class="keyword">const</span> <span class="identifier">U</span> <span class="special">&</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
<span class="special">}</span><span class="special">;</span></pre></div>
<div class="refsect1">
<a name="id3083084"></a><h2>Description</h2>
<p>The <code class="computeroutput">variant</code> class template (inspired by Andrei
Alexandrescu's class of the same name
[<a class="link" href="../variant/refs.html#variant.refs.ale01a">Ale01A</a>]) is an efficient,
<a class="link" href="../variant/tutorial.html#variant.tutorial.recursive" title="Recursive variant types">recursive-capable</a>,
bounded discriminated union value type capable of containing any value
type (either POD or non-POD). It supports construction from any type
convertible to one of its bounded types or from a source
<code class="computeroutput">variant</code> whose bounded types are each convertible to one
of the destination <code class="computeroutput">variant</code>'s bounded types. As well,
through <code class="computeroutput"><a class="link" href="apply_visitor.html" title="Function apply_visitor">apply_visitor</a></code>,
<code class="computeroutput">variant</code> supports compile-time checked, type-safe
visitation; and through <code class="computeroutput"><a class="link" href="get_id1478742.html" title="Function get">get</a></code>,
<code class="computeroutput">variant</code> supports run-time checked, type-safe value
retrieval.</p>
<p><span class="bold"><strong>Notes</strong></span>:</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">The bounded types of the <code class="computeroutput">variant</code> are exposed
via the nested typedef <code class="computeroutput">types</code>, which is an
<a href="../../../libs/mpl/index.html" target="_top">MPL</a>-compatible Sequence containing the
set of types that must be handled by any
<a class="link" href="../variant/reference.html#variant.concepts.static-visitor" title="StaticVisitor">visitor</a> to
the <code class="computeroutput">variant</code>.</li>
<li class="listitem">All members of <code class="computeroutput">variant</code> satisfy at least the
basic guarantee of exception-safety. That is, all operations on
a <code class="computeroutput">variant</code> remain defined even after previous
operations have failed.</li>
<li class="listitem">Each type specified as a template argument to
<code class="computeroutput">variant</code> must meet the requirements of the
<span class="emphasis"><em><a class="link" href="../variant/reference.html#variant.concepts.bounded-type" title="BoundedType">BoundedType</a></em></span>
concept.</li>
<li class="listitem">Each type specified as a template argument to
<code class="computeroutput">variant</code> must be distinct after removal of qualifiers.
Thus, for instance, both <code class="computeroutput">variant<int, int></code> and
<code class="computeroutput">variant<int, const int></code> have undefined
behavior.</li>
<li class="listitem">Conforming implementations of <code class="computeroutput">variant</code> must
allow at least ten types as template arguments. The exact number
of allowed arguments is exposed by the preprocessor macro
<code class="computeroutput"><a class="link" href="../BOOST_VARIANT_LIMIT_TYPES.html" title="Macro BOOST_VARIANT_LIMIT_TYPES">BOOST_VARIANT_LIMIT_TYPES</a></code>.
(See <code class="computeroutput"><a class="link" href="make_variant_over.html" title="Class template make_variant_over">make_variant_over</a></code> for a
means to specify the bounded types of a <code class="computeroutput">variant</code> by
the elements of an <a href="../../../libs/mpl/index.html" target="_top">MPL</a> or compatible
Sequence, thus overcoming this limitation.)</li>
</ul></div>
<div class="refsect2">
<a name="id3083328"></a><h3>
<a name="boost.variantconstruct-copy-destruct"></a><code class="computeroutput">variant</code>
public
construct/copy/destruct</h3>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
<pre class="literallayout"><a name="id1381472-bb"></a><span class="identifier">variant</span><span class="special">(</span><span class="special">)</span><span class="special">;</span></pre>
<div class="variablelist"><table border="0">
<col align="left" valign="top">
<tbody>
<tr>
<td><p><span class="term">Requires:</span></p></td>
<td>The first bounded type of the <code class="computeroutput">variant</code> (i.e.,
<code class="computeroutput">T1</code>) must fulfill the requirements of the
<span class="emphasis"><em>DefaultConstructible</em></span> [20.1.4]
concept.</td>
</tr>
<tr>
<td><p><span class="term">Postconditions:</span></p></td>
<td>Content of <code class="computeroutput">*this</code> is the default value of the
first bounded type (i.e, <code class="computeroutput">T1</code>).</td>
</tr>
<tr>
<td><p><span class="term">Throws:</span></p></td>
<td>May fail with any exceptions arising from the default
constructor of <code class="computeroutput">T1</code>.</td>
</tr>
</tbody>
</table></div>
</li>
<li class="listitem">
<pre class="literallayout"><a name="id621352-bb"></a><span class="identifier">variant</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">variant</span> <span class="special">&</span> other<span class="special">)</span><span class="special">;</span></pre>
<div class="variablelist"><table border="0">
<col align="left" valign="top">
<tbody>
<tr>
<td><p><span class="term">Postconditions:</span></p></td>
<td>Content of <code class="computeroutput">*this</code> is a copy of the content of
<code class="computeroutput">other</code>.</td>
</tr>
<tr>
<td><p><span class="term">Throws:</span></p></td>
<td>May fail with any exceptions arising from the
copy constructor of <code class="computeroutput">other</code>'s contained type.</td>
</tr>
</tbody>
</table></div>
</li>
<li class="listitem">
<pre class="literallayout"><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> T<span class="special">></span> <a name="id651681-bb"></a><span class="identifier">variant</span><span class="special">(</span><span class="identifier">T</span> <span class="special">&</span> operand<span class="special">)</span><span class="special">;</span></pre>
<div class="variablelist"><table border="0">
<col align="left" valign="top">
<tbody>
<tr>
<td><p><span class="term">Requires:</span></p></td>
<td>
<code class="computeroutput">T</code> must be unambiguously convertible to one of
the bounded types (i.e., <code class="computeroutput">T1</code>, <code class="computeroutput">T2</code>,
etc.).</td>
</tr>
<tr>
<td><p><span class="term">Postconditions:</span></p></td>
<td>Content of <code class="computeroutput">*this</code> is the best conversion of
<code class="computeroutput">operand</code> to one of the bounded types, as determined
by standard overload resolution rules.</td>
</tr>
<tr>
<td><p><span class="term">Throws:</span></p></td>
<td>May fail with any exceptions arising from the conversion of
<code class="computeroutput">operand</code> to one of the bounded types.</td>
</tr>
</tbody>
</table></div>
</li>
<li class="listitem">
<pre class="literallayout"><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> T<span class="special">></span> <a name="id1489812-bb"></a><span class="identifier">variant</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">T</span> <span class="special">&</span> operand<span class="special">)</span><span class="special">;</span></pre>
<div class="variablelist"><table border="0">
<col align="left" valign="top">
<tbody><tr>
<td><p><span class="term">Notes:</span></p></td>
<td>Same semantics as previous constructor, but allows
construction from temporaries.</td>
</tr></tbody>
</table></div>
</li>
<li class="listitem">
<pre class="literallayout"><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> U1<span class="special">,</span> <span class="keyword">typename</span> U2<span class="special">,</span> <span class="special">...</span><span class="special">,</span> <span class="keyword">typename</span> UN<span class="special">></span>
<a name="id1374589-bb"></a><span class="identifier">variant</span><span class="special">(</span><span class="identifier">variant</span><span class="special"><</span><span class="identifier">U1</span><span class="special">,</span> <span class="identifier">U2</span><span class="special">,</span> <span class="special">...</span><span class="special">,</span> <span class="identifier">UN</span><span class="special">></span> <span class="special">&</span> operand<span class="special">)</span><span class="special">;</span></pre>
<div class="variablelist"><table border="0">
<col align="left" valign="top">
<tbody>
<tr>
<td><p><span class="term">Requires:</span></p></td>
<td>
<span class="emphasis"><em>Every</em></span> one of <code class="computeroutput">U1</code>,
<code class="computeroutput">U2</code>, ..., <code class="computeroutput">UN</code> must have an unambiguous
conversion to one of the bounded types (i.e., <code class="computeroutput">T1</code>,
<code class="computeroutput">T2</code>, ..., <code class="computeroutput">TN</code>).</td>
</tr>
<tr>
<td><p><span class="term">Postconditions:</span></p></td>
<td>If <code class="computeroutput">variant<U1, U2, ..., UN></code> is itself
one of the bounded types, then content of <code class="computeroutput">*this</code> is a
copy of <code class="computeroutput">operand</code>. Otherwise, content of
<code class="computeroutput">*this</code> is the best conversion of the content of
<code class="computeroutput">operand</code> to one of the bounded types, as determined
by standard overload resolution rules.</td>
</tr>
<tr>
<td><p><span class="term">Throws:</span></p></td>
<td>If <code class="computeroutput">variant<U1, U2, ..., UN></code> is itself
one of the bounded types, then may fail with any exceptions arising
from the copy constructor of
<code class="computeroutput">variant<U1, U2, ..., UN></code>. Otherwise, may fail
with any exceptions arising from the conversion of the content of
<code class="computeroutput">operand</code> to one of the bounded types.</td>
</tr>
</tbody>
</table></div>
</li>
<li class="listitem">
<pre class="literallayout"><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> U1<span class="special">,</span> <span class="keyword">typename</span> U2<span class="special">,</span> <span class="special">...</span><span class="special">,</span> <span class="keyword">typename</span> UN<span class="special">></span>
<a name="id1364798-bb"></a><span class="identifier">variant</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">variant</span><span class="special"><</span><span class="identifier">U1</span><span class="special">,</span> <span class="identifier">U2</span><span class="special">,</span> <span class="special">...</span><span class="special">,</span> <span class="identifier">UN</span><span class="special">></span> <span class="special">&</span> operand<span class="special">)</span><span class="special">;</span></pre>
<div class="variablelist"><table border="0">
<col align="left" valign="top">
<tbody><tr>
<td><p><span class="term">Notes:</span></p></td>
<td>Same semantics as previous constructor, but allows
construction from temporaries.</td>
</tr></tbody>
</table></div>
</li>
<li class="listitem">
<pre class="literallayout"><a name="id641552-bb"></a><span class="special">~</span><span class="identifier">variant</span><span class="special">(</span><span class="special">)</span><span class="special">;</span></pre>
<div class="variablelist"><table border="0">
<col align="left" valign="top">
<tbody>
<tr>
<td><p><span class="term">Effects:</span></p></td>
<td>Destroys the content of <code class="computeroutput">*this</code>.</td>
</tr>
<tr>
<td><p><span class="term">Throws:</span></p></td>
<td>Will not throw.</td>
</tr>
</tbody>
</table></div>
</li>
</ol></div>
</div>
<div class="refsect2">
<a name="id3084193"></a><h3>
<a name="id1162116-bb"></a><code class="computeroutput">variant</code> modifiers</h3>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
<pre class="literallayout"><span class="keyword">void</span> <a name="id1353905-bb"></a><span class="identifier">swap</span><span class="special">(</span><span class="identifier">variant</span> <span class="special">&</span> other<span class="special">)</span><span class="special">;</span></pre>
<div class="variablelist"><table border="0">
<col align="left" valign="top">
<tbody>
<tr>
<td><p><span class="term">Requires:</span></p></td>
<td>Every bounded type must fulfill the requirements of the
<a class="link" href="../Assignable.html" title="Concept Assignable">Assignable</a>
concept.</td>
</tr>
<tr>
<td><p><span class="term">Effects:</span></p></td>
<td>Interchanges the content of <code class="computeroutput">*this</code> and
<code class="computeroutput">other</code>.</td>
</tr>
<tr>
<td><p><span class="term">Throws:</span></p></td>
<td>If the contained type of <code class="computeroutput">other</code> is the same as
the contained type of <code class="computeroutput">*this</code>, then may fail with any
exceptions arising from the <code class="computeroutput">swap</code> of the contents of
<code class="computeroutput">*this</code> and <code class="computeroutput">other</code>. Otherwise, may fail
with any exceptions arising from either of the copy constructors
of the contained types. Also, in the event of insufficient
memory, may fail with <code class="computeroutput">std::bad_alloc</code>
(<a class="link" href="../variant/design.html#variant.design.never-empty.problem" title="The Implementation Problem">why?</a>).</td>
</tr>
</tbody>
</table></div>
</li>
<li class="listitem">
<pre class="literallayout"><span class="identifier">variant</span> <span class="special">&</span> <a name="id1374533-bb"></a><span class="keyword">operator</span><span class="special">=</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">variant</span> <span class="special">&</span> rhs<span class="special">)</span><span class="special">;</span></pre>
<div class="variablelist"><table border="0">
<col align="left" valign="top">
<tbody>
<tr>
<td><p><span class="term">Requires:</span></p></td>
<td>Every bounded type must fulfill the requirements of the
<a class="link" href="../Assignable.html" title="Concept Assignable">Assignable</a>
concept.</td>
</tr>
<tr>
<td><p><span class="term">Effects:</span></p></td>
<td>If the contained type of <code class="computeroutput">rhs</code> is the same as
the contained type of <code class="computeroutput">*this</code>, then assigns the
content of <code class="computeroutput">rhs</code> into the content of
<code class="computeroutput">*this</code>. Otherwise, makes the content of
<code class="computeroutput">*this</code> a copy of the content of <code class="computeroutput">rhs</code>,
destroying the previous content of <code class="computeroutput">*this</code>.</td>
</tr>
<tr>
<td><p><span class="term">Throws:</span></p></td>
<td>If the contained type of <code class="computeroutput">rhs</code> is the same as
the contained type of <code class="computeroutput">*this</code>, then may fail with any
exceptions arising from the assignment of the content of
<code class="computeroutput">rhs</code> into the content <code class="computeroutput">*this</code>. Otherwise,
may fail with any exceptions arising from the copy constructor
of the contained type of <code class="computeroutput">rhs</code>. Also, in the event of
insufficient memory, may fail with <code class="computeroutput">std::bad_alloc</code>
(<a class="link" href="../variant/design.html#variant.design.never-empty.problem" title="The Implementation Problem">why?</a>).</td>
</tr>
</tbody>
</table></div>
</li>
<li class="listitem">
<pre class="literallayout"><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> T<span class="special">></span> <span class="identifier">variant</span> <span class="special">&</span> <a name="id668119-bb"></a><span class="keyword">operator</span><span class="special">=</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">T</span> <span class="special">&</span> rhs<span class="special">)</span><span class="special">;</span></pre>
<div class="variablelist"><table border="0">
<col align="left" valign="top">
<tbody>
<tr>
<td><p><span class="term">Requires:</span></p></td>
<td><div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
<code class="computeroutput">T</code> must be unambiguously convertible to
one of the bounded types (i.e., <code class="computeroutput">T1</code>,
<code class="computeroutput">T2</code>, etc.).</li>
<li class="listitem">Every bounded type must fulfill the requirements of the
<a class="link" href="../Assignable.html" title="Concept Assignable">Assignable</a>
concept.</li>
</ul></div></td>
</tr>
<tr>
<td><p><span class="term">Effects:</span></p></td>
<td>If the contained type of <code class="computeroutput">*this</code> is
<code class="computeroutput">T</code>, then assigns <code class="computeroutput">rhs</code> into the content
of <code class="computeroutput">*this</code>. Otherwise, makes the content of
<code class="computeroutput">*this</code> the best conversion of <code class="computeroutput">rhs</code> to
one of the bounded types, as determined by standard overload
resolution rules, destroying the previous content of
<code class="computeroutput">*this</code>.</td>
</tr>
<tr>
<td><p><span class="term">Throws:</span></p></td>
<td>If the contained type of <code class="computeroutput">*this</code> is
<code class="computeroutput">T</code>, then may fail with any exceptions arising from
the assignment of <code class="computeroutput">rhs</code> into the content
<code class="computeroutput">*this</code>. Otherwise, may fail with any exceptions
arising from the conversion of <code class="computeroutput">rhs</code> to one of the
bounded types. Also, in the event of insufficient memory, may
fail with <code class="computeroutput">std::bad_alloc</code>
(<a class="link" href="../variant/design.html#variant.design.never-empty.problem" title="The Implementation Problem">why?</a>).</td>
</tr>
</tbody>
</table></div>
</li>
</ol></div>
</div>
<div class="refsect2">
<a name="id3084773"></a><h3>
<a name="id1376600-bb"></a><code class="computeroutput">variant</code> queries</h3>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
<pre class="literallayout"><span class="keyword">int</span> <a name="id648404-bb"></a><span class="identifier">which</span><span class="special">(</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span></pre>
<div class="variablelist"><table border="0">
<col align="left" valign="top">
<tbody>
<tr>
<td><p><span class="term">Returns:</span></p></td>
<td>The zero-based index into the set of bounded types
of the contained type of <code class="computeroutput">*this</code>. (For instance, if
called on a <code class="computeroutput">variant<int, std::string></code> object
containing a <code class="computeroutput">std::string</code>, <code class="computeroutput">which()</code>
would return <code class="computeroutput">1</code>.)</td>
</tr>
<tr>
<td><p><span class="term">Throws:</span></p></td>
<td>Will not throw.</td>
</tr>
</tbody>
</table></div>
</li>
<li class="listitem">
<pre class="literallayout"><span class="keyword">bool</span> <a name="id1138398-bb"></a><span class="identifier">empty</span><span class="special">(</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span></pre>
<div class="variablelist"><table border="0">
<col align="left" valign="top">
<tbody>
<tr>
<td><p><span class="term">Returns:</span></p></td>
<td>
<code class="computeroutput">false</code>: <code class="computeroutput">variant</code> always contains
exactly one of its bounded types. (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 information.)</td>
</tr>
<tr>
<td><p><span class="term">Rationale:</span></p></td>
<td>Facilitates generic compatibility with
<a class="link" href="any.html" title="Class any">boost::any</a>.</td>
</tr>
<tr>
<td><p><span class="term">Throws:</span></p></td>
<td>Will not throw.</td>
</tr>
</tbody>
</table></div>
</li>
<li class="listitem">
<pre class="literallayout"><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">type_info</span> <span class="special">&</span> <a name="id1493085-bb"></a><span class="identifier">type</span><span class="special">(</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span></pre>
<div class="variablelist"><table border="0">
<col align="left" valign="top">
<tbody>
<tr>
<td><p><span class="term">Returns:</span></p></td>
<td>
<code class="computeroutput">typeid(x)</code>, where <code class="computeroutput">x</code> is the the
content of <code class="computeroutput">*this</code>.</td>
</tr>
<tr>
<td><p><span class="term">Throws:</span></p></td>
<td>Will not throw.</td>
</tr>
<tr>
<td><p><span class="term">Notes:</span></p></td>
<td>Not available when <code class="computeroutput">BOOST_NO_TYPEID</code> is
defined.</td>
</tr>
</tbody>
</table></div>
</li>
</ol></div>
</div>
<div class="refsect2">
<a name="id3085081"></a><h3>
<a name="id1364802-bb"></a><code class="computeroutput">variant</code> relational</h3>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
<pre class="literallayout"><a name="id1364806-bb"></a><span class="keyword">bool</span> <a name="id623589-bb"></a><span class="keyword">operator</span><span class="special">==</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">variant</span> <span class="special">&</span> rhs<span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> U<span class="special">></span> <span class="keyword">void</span> <a name="id663596-bb"></a><span class="keyword">operator</span><span class="special">==</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">U</span> <span class="special">&</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span></pre>Equality comparison.<div class="variablelist"><table border="0">
<col align="left" valign="top">
<tbody>
<tr>
<td><p><span class="term">Notes:</span></p></td>
<td>The overload returning <code class="computeroutput">void</code> exists only to
prohibit implicit conversion of the operator's right-hand side
to <code class="computeroutput">variant</code>; thus, its use will (purposefully)
result in a compile-time error.</td>
</tr>
<tr>
<td><p><span class="term">Requires:</span></p></td>
<td>Every bounded type of the <code class="computeroutput">variant</code> must
fulfill the requirements of the
<a class="link" href="../EqualityComparable.html" title="Concept EqualityComparable">EqualityComparable</a>
concept.</td>
</tr>
<tr>
<td><p><span class="term">Returns:</span></p></td>
<td>
<code class="computeroutput">true</code> iff <code class="computeroutput">which() == rhs.which()</code>
<span class="emphasis"><em>and</em></span>
<code class="computeroutput">content_this == content_rhs</code>, where
<code class="computeroutput">content_this</code> is the content of <code class="computeroutput">*this</code>
and <code class="computeroutput">content_rhs</code> is the content of
<code class="computeroutput">rhs</code>.</td>
</tr>
<tr>
<td><p><span class="term">Throws:</span></p></td>
<td>If <code class="computeroutput">which() == rhs.which()</code> then may fail with
any exceptions arising from <code class="computeroutput">operator==(T,T)</code>, where
<code class="computeroutput">T</code> is the contained type of
<code class="computeroutput">*this</code>.</td>
</tr>
</tbody>
</table></div>
</li>
<li class="listitem">
<pre class="literallayout"><a name="id1375017-bb"></a><span class="keyword">bool</span> <a name="id1375023-bb"></a><span class="keyword">operator</span><span class="special"><</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">variant</span> <span class="special">&</span> rhs<span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> U<span class="special">></span> <span class="keyword">void</span> <a name="id621172-bb"></a><span class="keyword">operator</span><span class="special"><</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">U</span> <span class="special">&</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span></pre>LessThan comparison.<div class="variablelist"><table border="0">
<col align="left" valign="top">
<tbody>
<tr>
<td><p><span class="term">Notes:</span></p></td>
<td>The overload returning <code class="computeroutput">void</code> exists only to
prohibit implicit conversion of the operator's right-hand side
to <code class="computeroutput">variant</code>; thus, its use will (purposefully)
result in a compile-time error.</td>
</tr>
<tr>
<td><p><span class="term">Requires:</span></p></td>
<td>Every bounded type of the <code class="computeroutput">variant</code> must
fulfill the requirements of the
<a class="link" href="../LessThanComparable.html" title="Concept LessThanComparable">LessThanComparable</a>
concept.</td>
</tr>
<tr>
<td><p><span class="term">Returns:</span></p></td>
<td>If <code class="computeroutput">which() == rhs.which()</code> then:
<code class="computeroutput">content_this < content_rhs</code>, where
<code class="computeroutput">content_this</code> is the content of <code class="computeroutput">*this</code>
and <code class="computeroutput">content_rhs</code> is the content of <code class="computeroutput">rhs</code>.
Otherwise: <code class="computeroutput">which() < rhs.which()</code>.</td>
</tr>
<tr>
<td><p><span class="term">Throws:</span></p></td>
<td>If <code class="computeroutput">which() == rhs.which()</code> then may fail with
any exceptions arising from <code class="computeroutput">operator<(T,T)</code>,
where <code class="computeroutput">T</code> is the contained type of
<code class="computeroutput">*this</code>.</td>
</tr>
</tbody>
</table></div>
</li>
</ol></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">Copyright © 2002, 2003 Eric Friedman, Itay Maman<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></td>
</tr></table>
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