boost-react-native-bundle

<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Class template variant</title> <link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.78.1"> <link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> <link rel="up" href="../variant/reference.html#header.boost.variant.variant_hpp" title="Header <boost/variant/variant.hpp>"> <link rel="prev" href="../BOOST_VARIANT_NO_FULL_RECURSIVE_VARIANT_SUPPORT.html" title="Macro BOOST_VARIANT_NO_FULL_RECURSIVE_VARIANT_SUPPORT"> <link rel="next" href="swap_idp305727776.html" title="Function template swap"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table cellpadding="2" width="100%"><tr> <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> <td align="center"><a href="../../../index.html">Home</a></td> <td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> <td align="center"><a href="../../../more/index.htm">More</a></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="../BOOST_VARIANT_NO_FULL_RECURSIVE_VARIANT_SUPPORT.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../variant/reference.html#header.boost.variant.variant_hpp"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="swap_idp305727776.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> </div> <div class="refentry"> <a name="boost.variant"></a><div class="titlepage"></div> <div class="refnamediv"> <h2>Class template variant</h2> boost::variant — Safe, generic, stack-based discriminated union container. </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">// 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>> template<typename T1, typename T2 = unspecified, ..., typename TN = unspecified> class <a class="link" href="variant.html" title="Class template variant">variant</a> { public: // types typedef unspecified <a name="boost.variant.types"></a>types; // <a class="link" href="variant.html#boost.variantconstruct-copy-destruct">construct/copy/destruct</a> <a class="link" href="variant.html#idp312345248-bb">variant</a>(); <a class="link" href="variant.html#idp302911456-bb">variant</a>(const variant &); <a class="link" href="variant.html#idp301284368-bb">variant</a>(variant &&); template<typename T> <a class="link" href="variant.html#idp308883488-bb">variant</a>(T &); template<typename T> <a class="link" href="variant.html#idp157636032-bb">variant</a>(const T &); template<typename T> <a class="link" href="variant.html#idp216393520-bb">variant</a>(T &&); template<typename U1, typename U2, ..., typename UN> <a class="link" href="variant.html#idp302423216-bb">variant</a>(variant<U1, U2, ..., UN> &); template<typename U1, typename U2, ..., typename UN> <a class="link" href="variant.html#idp157915040-bb">variant</a>(const variant<U1, U2, ..., UN> &); template<typename U1, typename U2, ..., typename UN> <a class="link" href="variant.html#idp157919952-bb">variant</a>(variant<U1, U2, ..., UN> &&); <a class="link" href="variant.html#idp223387600-bb">~variant</a>(); // <a class="link" href="variant.html#idp351566256-bb">modifiers</a> void <a class="link" href="variant.html#idp302822304-bb">swap</a>(variant &); variant & <a class="link" href="variant.html#idp309094368-bb">operator=</a>(const variant &); variant & <a class="link" href="variant.html#idp219320512-bb">operator=</a>(variant &&); template<typename T> variant & <a class="link" href="variant.html#idp350506304-bb">operator=</a>(const T &); template<typename T> variant & <a class="link" href="variant.html#idp349666256-bb">operator=</a>(T &&); // <a class="link" href="variant.html#idp150314720-bb">queries</a> int <a class="link" href="variant.html#idp352467824-bb">which</a>() const; bool <a class="link" href="variant.html#idp302912480-bb">empty</a>() const; const std::type_info & <a class="link" href="variant.html#idp309110032-bb">type</a>() const; // <a class="link" href="variant.html#idp308508656-bb">relational</a> bool <a class="link" href="variant.html#idp303815376-bb">operator==</a>(const variant &) const; template<typename U> void <a class="link" href="variant.html#idp300997552-bb">operator==</a>(const U &) const; bool <a class="link" href="variant.html#idp221431920-bb">operator<</a>(const variant &) const; template<typename U> void <a class="link" href="variant.html#idp308797968-bb">operator<</a>(const U &) const; };</pre></div> <div class="refsect1"> <a name="idp523347136"></a><h2>Description</h2> 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_idp94543392.html" title="Function get">get</a></code>, <code class="computeroutput">variant</code> supports run-time checked, type-safe value retrieval. Notes: <div class="itemizedlist"><ul class="itemizedlist" style="list-style-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 MPL-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 <a class="link" href="../variant/reference.html#variant.concepts.bounded-type" title="BoundedType">BoundedType</a> 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 MPL or compatible Sequence, thus overcoming this limitation.)</li> </ul></div> <div class="refsect2"> <a name="idp523373168"></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="idp312345248-bb"></a>variant();</pre> <div class="variablelist"><table border="0" class="variablelist compact"> <colgroup> <col align="left" valign="top"> <col> </colgroup> <tbody> <tr> <td>Requires:</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 DefaultConstructible [20.1.4] concept.</td> </tr> <tr> <td>Postconditions:</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>Throws:</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="idp302911456-bb"></a>variant(const variant & other);</pre> <div class="variablelist"><table border="0" class="variablelist compact"> <colgroup> <col align="left" valign="top"> <col> </colgroup> <tbody> <tr> <td>Postconditions:</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>Throws:</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"><a name="idp301284368-bb"></a>variant(variant && other);</pre> <div class="variablelist"><table border="0" class="variablelist compact"> <colgroup> <col align="left" valign="top"> <col> </colgroup> <tbody> <tr> <td>Requires:</td> <td>C++11 compatible compiler.</td> </tr> <tr> <td>Postconditions:</td> <td>Content of <code class="computeroutput">*this</code> is move constructed from the content of <code class="computeroutput">other</code>.</td> </tr> <tr> <td>Throws:</td> <td>May fail with any exceptions arising from the move constructor of <code class="computeroutput">other</code>'s contained type.</td> </tr> </tbody> </table></div> </li> <li class="listitem"> <pre class="literallayout">template<typename T> <a name="idp308883488-bb"></a>variant(T & operand);</pre> <div class="variablelist"><table border="0" class="variablelist compact"> <colgroup> <col align="left" valign="top"> <col> </colgroup> <tbody> <tr> <td>Requires:</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>Postconditions:</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>Throws:</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">template<typename T> <a name="idp157636032-bb"></a>variant(const T & operand);</pre> <div class="variablelist"><table border="0" class="variablelist compact"> <colgroup> <col align="left" valign="top"> <col> </colgroup> <tbody><tr> <td>Notes:</td> <td>Same semantics as previous constructor, but allows construction from temporaries.</td> </tr></tbody> </table></div> </li> <li class="listitem"> <pre class="literallayout">template<typename T> <a name="idp216393520-bb"></a>variant(T && operand);</pre> <div class="variablelist"><table border="0" class="variablelist compact"> <colgroup> <col align="left" valign="top"> <col> </colgroup> <tbody> <tr> <td>Requires:</td> <td>C++11 compatible compiler.</td> </tr> <tr> <td>Notes:</td> <td>Same semantics as previous constructor, but allows move construction if <code class="computeroutput">operand</code> is an rvalue.</td> </tr> </tbody> </table></div> </li> <li class="listitem"> <pre class="literallayout">template<typename U1, typename U2, ..., typename UN> <a name="idp302423216-bb"></a>variant(variant<U1, U2, ..., UN> & operand);</pre> <div class="variablelist"><table border="0" class="variablelist compact"> <colgroup> <col align="left" valign="top"> <col> </colgroup> <tbody> <tr> <td>Requires:</td> <td> Every 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>Postconditions:</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>Throws:</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">template<typename U1, typename U2, ..., typename UN> <a name="idp157915040-bb"></a>variant(const variant<U1, U2, ..., UN> & operand);</pre> <div class="variablelist"><table border="0" class="variablelist compact"> <colgroup> <col align="left" valign="top"> <col> </colgroup> <tbody><tr> <td>Notes:</td> <td>Same semantics as previous constructor, but allows construction from temporaries.</td> </tr></tbody> </table></div> </li> <li class="listitem"> <pre class="literallayout">template<typename U1, typename U2, ..., typename UN> <a name="idp157919952-bb"></a>variant(variant<U1, U2, ..., UN> && operand);</pre> <div class="variablelist"><table border="0" class="variablelist compact"> <colgroup> <col align="left" valign="top"> <col> </colgroup> <tbody> <tr> <td>Requires:</td> <td>C++11 compatible compiler.</td> </tr> <tr> <td>Notes:</td> <td>Same semantics as previous constructor, but allows move construction.</td> </tr> </tbody> </table></div> </li> <li class="listitem"> <pre class="literallayout"><a name="idp223387600-bb"></a>~variant();</pre> <div class="variablelist"><table border="0" class="variablelist compact"> <colgroup> <col align="left" valign="top"> <col> </colgroup> <tbody> <tr> <td>Effects:</td> <td>Destroys the content of <code class="computeroutput">*this</code>.</td> </tr> <tr> <td>Throws:</td> <td>Will not throw.</td> </tr> </tbody> </table></div> </li> </ol></div> </div> <div class="refsect2"> <a name="idp523516784"></a><h3> <a name="idp351566256-bb"></a><code class="computeroutput">variant</code> modifiers</h3> <div class="orderedlist"><ol class="orderedlist" type="1"> <li class="listitem"> <pre class="literallayout">void <a name="idp302822304-bb"></a>swap(variant & other);</pre> <div class="variablelist"><table border="0" class="variablelist compact"> <colgroup> <col align="left" valign="top"> <col> </colgroup> <tbody> <tr> <td>Requires:</td> <td>Every bounded type must fulfill the requirements of the MoveAssignable concept.</td> </tr> <tr> <td>Effects:</td> <td>Interchanges the content of <code class="computeroutput">*this</code> and <code class="computeroutput">other</code>.</td> </tr> <tr> <td>Throws:</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 move or 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">variant & <a name="idp309094368-bb"></a>operator=(const variant & rhs);</pre> <div class="variablelist"><table border="0" class="variablelist compact"> <colgroup> <col align="left" valign="top"> <col> </colgroup> <tbody> <tr> <td>Requires:</td> <td>Every bounded type must fulfill the requirements of the Assignable concept.</td> </tr> <tr> <td>Effects:</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>Throws:</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">variant & <a name="idp219320512-bb"></a>operator=(variant && rhs);</pre> <div class="variablelist"><table border="0" class="variablelist compact"> <colgroup> <col align="left" valign="top"> <col> </colgroup> <tbody> <tr> <td>Requires:</td> <td><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> <li class="listitem">C++11 compatible compiler.</li> <li class="listitem">Every bounded type must fulfill the requirements of the MoveAssignable concept.</li> </ul></div></td> </tr> <tr> <td>Effects:</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 move assigns the content of <code class="computeroutput">rhs</code> into the content of <code class="computeroutput">*this</code>. Otherwise, move constructs <code class="computeroutput">*this</code> using the content of <code class="computeroutput">rhs</code>, destroying the previous content of <code class="computeroutput">*this</code>.</td> </tr> <tr> <td>Throws:</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 move 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 move 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">template<typename T> variant & <a name="idp350506304-bb"></a>operator=(const T & rhs);</pre> <div class="variablelist"><table border="0" class="variablelist compact"> <colgroup> <col align="left" valign="top"> <col> </colgroup> <tbody> <tr> <td>Requires:</td> <td><div class="itemizedlist"><ul class="itemizedlist" style="list-style-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 Assignable concept.</li> </ul></div></td> </tr> <tr> <td>Effects:</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>Throws:</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> <li class="listitem"> <pre class="literallayout">template<typename T> variant & <a name="idp349666256-bb"></a>operator=(T && rhs);</pre> <div class="variablelist"><table border="0" class="variablelist compact"> <colgroup> <col align="left" valign="top"> <col> </colgroup> <tbody> <tr> <td>Requires:</td> <td><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> <li class="listitem">C++11 compatible compiler.</li> <li class="listitem"> <code class="computeroutput">rhs</code> is an rvalue. Otherwise previous operator will be used.</li> <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 MoveAssignable concept.</li> </ul></div></td> </tr> <tr> <td>Effects:</td> <td>If the contained type of <code class="computeroutput">*this</code> is <code class="computeroutput">T</code>, then move 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>(conversion is usually done via move construction).</td> </tr> <tr> <td>Throws:</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 move 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="idp523631264"></a><h3> <a name="idp150314720-bb"></a><code class="computeroutput">variant</code> queries</h3> <div class="orderedlist"><ol class="orderedlist" type="1"> <li class="listitem"> <pre class="literallayout">int <a name="idp352467824-bb"></a>which() const;</pre> <div class="variablelist"><table border="0" class="variablelist compact"> <colgroup> <col align="left" valign="top"> <col> </colgroup> <tbody> <tr> <td>Returns:</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>Throws:</td> <td>Will not throw.</td> </tr> </tbody> </table></div> </li> <li class="listitem"> <pre class="literallayout">bool <a name="idp302912480-bb"></a>empty() const;</pre> <div class="variablelist"><table border="0" class="variablelist compact"> <colgroup> <col align="left" valign="top"> <col> </colgroup> <tbody> <tr> <td>Returns:</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>Rationale:</td> <td>Facilitates generic compatibility with <a class="link" href="any.html" title="Class any">boost::any</a>.</td> </tr> <tr> <td>Throws:</td> <td>Will not throw.</td> </tr> </tbody> </table></div> </li> <li class="listitem"> <pre class="literallayout">const std::type_info & <a name="idp309110032-bb"></a>type() const;</pre> <div class="variablelist"><table border="0" class="variablelist compact"> <colgroup> <col align="left" valign="top"> <col> </colgroup> <tbody> <tr> <td>Returns:</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>Throws:</td> <td>Will not throw.</td> </tr> <tr> <td>Notes:</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="idp523668144"></a><h3> <a name="idp308508656-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="idp308509104-bb"></a>bool <a name="idp303815376-bb"></a>operator==(const variant & rhs) const; template<typename U> void <a name="idp300997552-bb"></a>operator==(const U &) const;</pre>Equality comparison.<div class="variablelist"><table border="0" class="variablelist compact"> <colgroup> <col align="left" valign="top"> <col> </colgroup> <tbody> <tr> <td>Notes:</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>Requires:</td> <td>Every bounded type of the <code class="computeroutput">variant</code> must fulfill the requirements of the EqualityComparable concept.</td> </tr> <tr> <td>Returns:</td> <td> <code class="computeroutput">true</code> iff <code class="computeroutput">which() == rhs.which()</code> and <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>Throws:</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="idp221431024-bb"></a>bool <a name="idp221431920-bb"></a>operator<(const variant & rhs) const; template<typename U> void <a name="idp308797