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- </head>
- <body class="article toc2 toc-left">
- <div id="header">
- <h1>Boost.Variant2: A never valueless variant type</h1>
- <div class="details">
- <span id="author" class="author">Peter Dimov</span><br>
- </div>
- <div id="toc" class="toc2">
- <div id="toctitle">Table of Contents</div>
- <ul class="sectlevel1">
- <li><a href="#overview">Overview</a>
- <ul class="sectlevel2">
- <li><a href="#overview_description">Description</a></li>
- <li><a href="#overview_usage_examples">Usage Examples</a></li>
- <li><a href="#overview_construction_and_assignment">Construction and Assignment</a></li>
- <li><a href="#overview_inspecting_the_value">Inspecting the Value</a></li>
- <li><a href="#overview_visitation">Visitation</a></li>
- <li><a href="#overview_default_construction">Default Construction</a></li>
- </ul>
- </li>
- <li><a href="#changelog">Revision History</a>
- <ul class="sectlevel2">
- <li><a href="#changelog_changes_in_1_71_0">Changes in 1.71.0</a></li>
- </ul>
- </li>
- <li><a href="#design">Design</a>
- <ul class="sectlevel2">
- <li><a href="#design_features">Features</a></li>
- <li><a href="#design_rationale">Rationale</a>
- <ul class="sectlevel3">
- <li><a href="#design_never_valueless">Never Valueless</a></li>
- <li><a href="#design_strong_exception_safety">Strong Exception Safety</a></li>
- </ul>
- </li>
- <li><a href="#design_differences_with_stdvariant">Differences with std::variant</a></li>
- <li><a href="#design_differences_with_boost_variant">Differences with Boost.Variant</a></li>
- </ul>
- </li>
- <li><a href="#implementation">Implementation</a>
- <ul class="sectlevel2">
- <li><a href="#implementation_dependencies">Dependencies</a></li>
- <li><a href="#implementation_supported_compilers">Supported Compilers</a></li>
- </ul>
- </li>
- <li><a href="#reference">Reference</a>
- <ul class="sectlevel2">
- <li><a href="#ref_boostvariant2variant_hpp"><boost/variant2/variant.hpp></a>
- <ul class="sectlevel3">
- <li><a href="#ref_synopsis">Synopsis</a></li>
- <li><a href="#ref_variant">variant</a>
- <ul class="sectlevel4">
- <li><a href="#ref_constructors">Constructors</a></li>
- <li><a href="#ref_destructor">Destructor</a></li>
- <li><a href="#ref_assignment">Assignment</a></li>
- <li><a href="#ref_modifiers">Modifiers</a></li>
- <li><a href="#ref_value_status">Value Status</a></li>
- <li><a href="#ref_swap">Swap</a></li>
- <li><a href="#ref_converting_constructors_extension">Converting Constructors (extension)</a></li>
- <li><a href="#ref_subset_extension">Subset (extension)</a></li>
- </ul>
- </li>
- <li><a href="#ref_variant_alternative">variant_alternative</a></li>
- <li><a href="#ref_holds_alternative">holds_alternative</a></li>
- <li><a href="#ref_get">get</a></li>
- <li><a href="#ref_get_if">get_if</a></li>
- <li><a href="#ref_relational_operators">Relational Operators</a></li>
- <li><a href="#ref_visit">visit</a></li>
- <li><a href="#ref_swap_2">swap</a></li>
- <li><a href="#ref_bad_variant_access">bad_variant_access</a></li>
- </ul>
- </li>
- </ul>
- </li>
- <li><a href="#copyright">Copyright and License</a></li>
- </ul>
- </div>
- </div>
- <div id="content">
- <div class="sect1">
- <h2 id="overview">Overview</h2>
- <div class="sectionbody">
- <div class="sect2">
- <h3 id="overview_description">Description</h3>
- <div class="paragraph">
- <p>This library implements a type-safe discriminated/tagged union type,
- <code>variant<T…​></code>, that is API-compatible with the C++17 Standard’s
- <a href="http://en.cppreference.com/w/cpp/utility/variant"><code>std::variant<T…​></code></a>.</p>
- </div>
- <div class="paragraph">
- <p>A <code>variant<T1, T2, …​, Tn></code> variable can hold a value of any of the
- types <code>T1</code>, <code>T2</code>, …​, <code>Tn</code>. For example,
- <code>variant<int64_t, double, std::string></code> can hold an <code>int64_t</code> value, a
- <code>double</code> value, or a <code>string</code> value.</p>
- </div>
- <div class="paragraph">
- <p>Such a type is sometimes called a "tagged union", because it’s roughly
- equivalent to</p>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>struct V
- {
- enum tag { tag_int64_t, tag_double, tag_string };
- tag tag_;
- union
- {
- int64_t i_;
- double d_;
- std::string s_;
- };
- };</code></pre>
- </div>
- </div>
- </div>
- <div class="sect2">
- <h3 id="overview_usage_examples">Usage Examples</h3>
- <div class="paragraph">
- <p>Variants can be used to represent dynamically-typed values. A configuration
- file of the form</p>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>server.host=test.example.com
- server.port=9174
- cache.max_load=0.7</code></pre>
- </div>
- </div>
- <div class="paragraph">
- <p>can be represented as <code>std::map<std::string, variant<int64_t, double,
- std::string>></code>.</p>
- </div>
- <div class="paragraph">
- <p>Variants can also represent polymorphism. To take a classic example, a
- polymorphic collection of shapes:</p>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>#define _USE_MATH_DEFINES
- #include <iostream>
- #include <vector>
- #include <memory>
- #include <cmath>
- class Shape
- {
- public:
- virtual ~Shape() = default;
- virtual double area() const = 0;
- };
- class Rectangle: public Shape
- {
- private:
- double width_, height_;
- public:
- Rectangle( double width, double height ):
- width_( width ), height_( height ) {}
- virtual double area() const { return width_ * height_; }
- };
- class Circle: public Shape
- {
- private:
- double radius_;
- public:
- explicit Circle( double radius ): radius_( radius ) {}
- virtual double area() const { return M_PI * radius_ * radius_; }
- };
- double total_area( std::vector<std::unique_ptr<Shape>> const & v )
- {
- double s = 0.0;
- for( auto const& p: v )
- {
- s += p->area();
- }
- return s;
- }
- int main()
- {
- std::vector<std::unique_ptr<Shape>> v;
- v.push_back( std::unique_ptr<Shape>( new Circle( 1.0 ) ) );
- v.push_back( std::unique_ptr<Shape>( new Rectangle( 2.0, 3.0 ) ) );
- std::cout << "Total area: " << total_area( v ) << std::endl;
- }</code></pre>
- </div>
- </div>
- <div class="paragraph">
- <p>can instead be represented as a collection of <code>variant<Rectangle, Circle></code>
- values. This requires the possible <code>Shape</code> types be known in advance, as is
- often the case. In return, we no longer need virtual functions, or to allocate
- the values on the heap with <code>new Rectangle</code> and <code>new Circle</code>:</p>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>#define _USE_MATH_DEFINES
- #include <iostream>
- #include <vector>
- #include <cmath>
- #include <boost/variant2/variant.hpp>
- using namespace boost::variant2;
- struct Rectangle
- {
- double width_, height_;
- double area() const { return width_ * height_; }
- };
- struct Circle
- {
- double radius_;
- double area() const { return M_PI * radius_ * radius_; }
- };
- double total_area( std::vector<variant<Rectangle, Circle>> const & v )
- {
- double s = 0.0;
- for( auto const& x: v )
- {
- s += visit( []( auto const& y ){ return y.area(); }, x );
- }
- return s;
- }
- int main()
- {
- std::vector<variant<Rectangle, Circle>> v;
- v.push_back( Circle{ 1.0 } );
- v.push_back( Rectangle{ 2.0, 3.0 } );
- std::cout << "Total area: " << total_area( v ) << std::endl;
- }</code></pre>
- </div>
- </div>
- </div>
- <div class="sect2">
- <h3 id="overview_construction_and_assignment">Construction and Assignment</h3>
- <div class="paragraph">
- <p>If we look at the</p>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code> v.push_back( Circle{ 1.0 } );</code></pre>
- </div>
- </div>
- <div class="paragraph">
- <p>line, we can deduce that <code>variant<Rectangle, Circle></code> can be (implicitly)
- constructed from <code>Circle</code> (and <code>Rectangle</code>), and indeed it can. It can also
- be assigned a <code>Circle</code> or a <code>Rectangle</code>:</p>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>variant<Rectangle, Circle> v = Circle{ 1.0 }; // v holds Circle
- v = Rectangle{ 2.0, 3.0 }; // v now holds Rectangle</code></pre>
- </div>
- </div>
- <div class="paragraph">
- <p>If we try to construct <code>variant<int, float></code> from something that is neither
- <code>int</code> nor <code>float</code>, say, <code>(short)1</code>, the behavior is "as if" the <code>variant</code> has
- declared two constructors,</p>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>variant::variant(int x);
- variant::variant(float x);</code></pre>
- </div>
- </div>
- <div class="paragraph">
- <p>and the standard overload resolution rules are used to pick the one that will
- be used. So <code>variant<int, float>((short)1)</code> will hold an <code>int</code>.</p>
- </div>
- </div>
- <div class="sect2">
- <h3 id="overview_inspecting_the_value">Inspecting the Value</h3>
- <div class="paragraph">
- <p>Putting values into a <code>variant</code> is easy, but taking them out is necessarily a
- bit more convoluted. It’s not possible for <code>variant<int, float></code> to define a
- member function <code>get() const</code>, because such a function will need its return
- type fixed at compile time, and whether the correct return type is <code>int</code> or
- <code>float</code> will only become known at run time.</p>
- </div>
- <div class="paragraph">
- <p>There are a few ways around that. First, there is the accessor member function</p>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>std::size_t variant::index() const noexcept;</code></pre>
- </div>
- </div>
- <div class="paragraph">
- <p>that returns the zero-based index of the current type. For <code>variant<int,
- float></code>, it will return <code>0</code> for <code>int</code> and <code>1</code> for <code>float</code>.</p>
- </div>
- <div class="paragraph">
- <p>Once we have the index, we can use the free function <code>get<N></code> to obtain the
- value. Since we’re passing the type index to <code>get</code>, it knows what to return.
- <code>get<0>(v)</code> will return <code>int</code>, and <code>get<1>(v)</code> will return <code>float</code>:</p>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>void f( variant<int, float> const& v )
- {
- switch( v.index() )
- {
- case 0:
- // use get<0>(v)
- break;
- case 1:
- // use get<1>(v)
- break;
- default:
- assert(false); // never happens
- }
- }</code></pre>
- </div>
- </div>
- <div class="paragraph">
- <p>If we call <code>get<0>(v)</code>, and <code>v.index()</code> is not currently <code>0</code>, an exception
- (of type <code>bad_variant_access</code>) will be thrown.</p>
- </div>
- <div class="paragraph">
- <p>An alternative approach is to use <code>get<int>(v)</code> or <code>get<float>(v)</code>. This
- works similarly.</p>
- </div>
- <div class="paragraph">
- <p>Another alternative that avoids the possibility of <code>bad_variant_access</code> is
- to use <code>get_if</code>. Instead of a reference to the contained value, it returns
- a pointer to it, returning <code>nullptr</code> to indicate type mismatch. <code>get_if</code>
- takes a pointer to the <code>variant</code>, so in our example we’ll use something along
- the following lines:</p>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>void f( variant<int, float> const& v )
- {
- if( int const * p = get_if<int>(&v) )
- {
- // use *p
- }
- else if( float const * p = get_if<float>(&v) )
- {
- // use *p
- }
- else
- {
- assert(false); // never happens
- }
- }</code></pre>
- </div>
- </div>
- </div>
- <div class="sect2">
- <h3 id="overview_visitation">Visitation</h3>
- <div class="paragraph">
- <p>Last but not least, there’s <code>visit</code>. <code>visit(f, v)</code> calls the a function object
- <code>f</code> with the value contained in the <code>variant</code> <code>v</code> and returns the result. When
- <code>v</code> is <code>variant<int, float></code>, it will call <code>f</code> with either an <code>int</code> or a
- <code>float</code>. The function object must be prepared to accept both.</p>
- </div>
- <div class="paragraph">
- <p>In practice, this can be achieved by having the function take a type that can
- be passed either <code>int</code> or <code>float</code>, such as <code>double</code>:</p>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>double f( double x ) { return x; }
- double g( variant<int, float> const& v )
- {
- return visit( f, v );
- }</code></pre>
- </div>
- </div>
- <div class="paragraph">
- <p>By using a function object with an overloaded <code>operator()</code>:</p>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>struct F
- {
- void operator()(int x) const { /* use x */ }
- void operator()(float x) const { /* use x */ }
- };
- void g( variant<int, float> const& v )
- {
- visit( F(), v );
- }</code></pre>
- </div>
- </div>
- <div class="paragraph">
- <p>Or by using a polymorphic lambda, as we did in our <code>Circle</code>/<code>Rectangle</code>
- example:</p>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>void g( variant<int, float> const& v )
- {
- visit( [&]( auto const& x ){ std::cout << x << std::endl; }, v );
- }</code></pre>
- </div>
- </div>
- <div class="paragraph">
- <p><code>visit</code> can also take more than one <code>variant</code>. <code>visit(f, v1, v2)</code> calls
- <code>f(x1, x2)</code>, where <code>x1</code> is the value contained in <code>v1</code> and <code>x2</code> is the value
- in <code>v2</code>.</p>
- </div>
- </div>
- <div class="sect2">
- <h3 id="overview_default_construction">Default Construction</h3>
- <div class="paragraph">
- <p>The default constructor of <code>variant</code> value-initializes the first type in
- the list. <code>variant<int, float>{}</code> holds <code>0</code> (of type <code>int</code>), and
- <code>variant<float, int>{}</code> holds <code>0.0f</code>.</p>
- </div>
- <div class="paragraph">
- <p>This is usually the desired behavior. However, in cases such as
- <code>variant<std::mutex, std::recursive_mutex></code>, one might legitimately wish to
- avoid constructing a <code>std::mutex</code> by default. A provided type, <code>monostate</code>,
- can be used as the first type in those scenarios. <code>variant<monostate,
- std::mutex, std::recursive_mutex></code> will default-construct a <code>monostate</code>,
- which is basically a no-op, as <code>monostate</code> is effectively an empty <code>struct</code>.</p>
- </div>
- </div>
- </div>
- </div>
- <div class="sect1">
- <h2 id="changelog">Revision History</h2>
- <div class="sectionbody">
- <div class="sect2">
- <h3 id="changelog_changes_in_1_71_0">Changes in 1.71.0</h3>
- <div class="paragraph">
- <p>After the Boost formal review, the implementation has been
- changed to provide the strong exception safety guarantee,
- instead of basic. <code>expected</code> has been removed.</p>
- </div>
- </div>
- </div>
- </div>
- <div class="sect1">
- <h2 id="design">Design</h2>
- <div class="sectionbody">
- <div class="sect2">
- <h3 id="design_features">Features</h3>
- <div class="paragraph">
- <p>This <code>variant</code> implementation has two distinguishing features:</p>
- </div>
- <div class="ulist">
- <ul>
- <li>
- <p>It’s never "valueless", that is, <code>variant<T1, T2, …​, Tn></code> has an
- invariant that it always contains a valid value of one of the types
- <code>T1</code>, <code>T2</code>, …​, <code>Tn</code>.</p>
- </li>
- <li>
- <p>It provides the strong exception safety guarantee on assignment and
- <code>emplace</code>.</p>
- </li>
- </ul>
- </div>
- <div class="paragraph">
- <p>This is achieved with the use of double storage, unless all of the
- contained types have a non-throwing move constructor.</p>
- </div>
- </div>
- <div class="sect2">
- <h3 id="design_rationale">Rationale</h3>
- <div class="sect3">
- <h4 id="design_never_valueless">Never Valueless</h4>
- <div class="paragraph">
- <p>It makes intuitive sense that <code>variant<X, Y, Z></code> can hold only values
- of type <code>X</code>, type <code>Y</code>, or type <code>Z</code>, and nothing else.</p>
- </div>
- <div class="paragraph">
- <p>If we think of <code>variant</code> as an extension of <code>union</code>, since a <code>union</code>
- has a state called "no active member", an argument can be made that a
- <code>variant<X, Y, Z></code> should also have such an additional state, holding
- none of <code>X</code>, <code>Y</code>, <code>Z</code>.</p>
- </div>
- <div class="paragraph">
- <p>This however makes <code>variant</code> less convenient in practice and less useful
- as a building block. If we really need a variable that only holds <code>X</code>,
- <code>Y</code>, or <code>Z</code>, the additional empty state creates complications that need
- to be worked around. And in the case where we do need this additional
- empty state, we can just use <code>variant<empty, X, Y, Z></code>, with a suitable
- <code>struct empty {};</code>.</p>
- </div>
- <div class="paragraph">
- <p>From a pure design perspective, the case for no additional empty state is
- solid. Implementation considerations, however, argue otherwise.</p>
- </div>
- <div class="paragraph">
- <p>When we replace the current value of the <code>variant</code> (of, say, type <code>X</code>) with
- another (of type <code>Y</code>), since the new value needs to occupy the same storage
- as the old one, we need to destroy the old <code>X</code> first, then construct a new
- <code>Y</code> in its place. But since this is C++, the construction can fail with an
- exception. At this point the <code>variant</code> is in the "has no active member"
- state that we’ve agreed it cannot be in.</p>
- </div>
- <div class="paragraph">
- <p>This is a legitimate problem, and it is this problem that makes having
- an empty/valueless state so appealing. We just leave the <code>variant</code> empty on
- exception and we’re done.</p>
- </div>
- <div class="paragraph">
- <p>As explained, though, this is undesirable from a design perspective as it
- makes the component less useful and less elegant.</p>
- </div>
- <div class="paragraph">
- <p>There are several ways around the issue. The most straightforward one is to
- just disallow types whose construction can throw. Since we can always create
- a temporary value first, then use the move constructor to initialize the one
- in the <code>variant</code>, it’s enough to require a nonthrowing move constructor,
- rather than all constructors to be nonthrowing.</p>
- </div>
- <div class="paragraph">
- <p>Unfortunately, under at least one popular standard library implementation,
- node based containers such as <code>std::list</code> and <code>std::map</code> have a potentially
- throwing move constructor. Disallowing <code>variant<X, std::map<Y, Z>></code> is hardly
- practical, so the exceptional case cannot be avoided.</p>
- </div>
- <div class="paragraph">
- <p>On exception, we could also construct some other value, leaving the <code>variant</code>
- valid; but in the general case, that construction can also throw. If one of
- the types has a nonthrowing default constructor, we can use it; but if not,
- we can’t.</p>
- </div>
- <div class="paragraph">
- <p>The approach Boost.Variant takes here is to allocate a temporary copy of
- the value on the heap. On exception, a pointer to that temporary copy can be
- stored into the <code>variant</code>. Pointer operations don’t throw.</p>
- </div>
- <div class="paragraph">
- <p>Another option is to use double buffering. If our <code>variant</code> occupies twice
- the storage, we can construct the new value in the unused half, then, once
- the construction succeeds, destroy the old value in the other half.</p>
- </div>
- <div class="paragraph">
- <p>When <code>std::variant</code> was standardized, none of those approaches was deemed
- palatable, as all of them either introduce overhead or are too restrictive
- with respect to the types a <code>variant</code> can contain. So as a compromise,
- <code>std::variant</code> took a way that can (noncharitably) be described as "having
- your cake and eating it too."</p>
- </div>
- <div class="paragraph">
- <p>Since the described exceptional situation is relatively rare, <code>std::variant</code>
- has a special case, called "valueless", into which it goes on exception,
- but the interface acknowledges its existence as little as possible, allowing
- users to pretend that it doesn’t exist.</p>
- </div>
- <div class="paragraph">
- <p>This is, arguably, not that bad from a practical point of view, but it leaves
- many of us wanting. Rare states that "never" occur are undertested and when
- that "never" actually happens, it’s usually in the most inconvenient of times.</p>
- </div>
- <div class="paragraph">
- <p>This implementation does not follow <code>std::variant</code>; it statically guarantees
- that <code>variant</code> is never in a valueless state. The function
- <code>valueless_by_exception</code> is provided for compatibility, but it always returns
- <code>false</code>.</p>
- </div>
- <div class="paragraph">
- <p>Instead, if the contained types are such that it’s not possible to avoid an
- exceptional situation when changing the contained value, double storage is
- used.</p>
- </div>
- </div>
- <div class="sect3">
- <h4 id="design_strong_exception_safety">Strong Exception Safety</h4>
- <div class="paragraph">
- <p>The initial submission only provided the basic exception safety guarantee.
- If an attempt to change the contained value (via assignment or <code>emplace</code>)
- failed with an exception, and a type with a nonthrowing default constructor
- existed among the alternatives, a value of that type was created into the
- <code>variant</code>. The upside of this decision was that double storage was needed
- less frequently.</p>
- </div>
- <div class="paragraph">
- <p>The reviewers were fairly united in hating it. Constructing a random type
- was deemed too unpredictable and not complying with the spirit of the
- basic guarantee. The default constructor of the chosen type, even if
- nonthrowing, may still have undesirable side effects. Or, if not that, a
- value of that type may have special significance for the surrounding code.
- Therefore, some argued, the <code>variant</code> should either remain with its
- old value, or transition into the new one, without synthesizing other
- states.</p>
- </div>
- <div class="paragraph">
- <p>At the other side of the spectrum, there were those who considered double
- storage unacceptable. But they considered it unacceptable in principle,
- regardless of the frequency with which it was used.</p>
- </div>
- <div class="paragraph">
- <p>As a result, providing the strong exception safety guarantee on assignment
- and <code>emplace</code> was declared an acceptance condition.</p>
- </div>
- <div class="paragraph">
- <p>In retrospect, this was the right decision. The reason the strong guarantee
- is generally not provided is because it doesn’t compose. When <code>X</code> and <code>Y</code>
- provide the basic guarantee on assignment, so does <code>struct { X x; Y y; };</code>.
- Similarly, when <code>X</code> and <code>Y</code> have nonthrowing assignments, so does the
- <code>struct</code>. But this doesn’t hold for the strong guarantee.</p>
- </div>
- <div class="paragraph">
- <p>The usual practice is to provide the basic guarantee on assignment and
- let the user synthesize a "strong" assignment out of either a nonthrowing
- <code>swap</code> or a nonthrowing move assignment. That is, given <code>x1</code> and <code>x2</code> of
- type <code>X</code>, instead of the "basic" <code>x1 = x2;</code>, use either <code>X(x2).swap(x1);</code>
- or <code>x1 = X(x2);</code>.</p>
- </div>
- <div class="paragraph">
- <p>Nearly all types provide a nonthrowing <code>swap</code> or a nonthrowing move
- assignment, so this works well. Nearly all, except <code>variant</code>, which in the
- general case has neither a nonthrowing <code>swap</code> nor a nonthrowing move
- assignment. If <code>variant</code> does not provide the strong guarantee itself, it’s
- impossible for the user to synthesize it.</p>
- </div>
- <div class="paragraph">
- <p>So it should, and so it does.</p>
- </div>
- </div>
- </div>
- <div class="sect2">
- <h3 id="design_differences_with_stdvariant">Differences with std::variant</h3>
- <div class="paragraph">
- <p>The main differences between this implementation and <code>std::variant</code> are:</p>
- </div>
- <div class="ulist">
- <ul>
- <li>
- <p>No valueless-by-exception state: <code>valueless_by_exception()</code> always
- returns <code>false</code>.</p>
- </li>
- <li>
- <p>Strong exception safety guarantee on assignment and <code>emplace</code>.</p>
- </li>
- <li>
- <p><code>emplace</code> first constructs the new value and then destroys the old one;
- in the single storage case, this translates to constructing a temporary
- and then moving it into place.</p>
- </li>
- <li>
- <p>A converting constructor from, e.g. <code>variant<int, float></code> to
- <code>variant<float, double, int></code> is provided as an extension.</p>
- </li>
- <li>
- <p>The reverse operation, going from <code>variant<float, double, int></code> to
- <code>variant<int, float></code> is provided as the member function <code>subset<U…​></code>.
- (This operation can throw if the current state of the variant cannot be
- represented.)</p>
- </li>
- <li>
- <p><code>variant<T…​></code> is not (yet) trivial when all contained types are trivial,
- as mandated by C++17.</p>
- </li>
- <li>
- <p>The C++20 additions and changes to <code>std::variant</code> have not yet been
- implemented.</p>
- </li>
- </ul>
- </div>
- </div>
- <div class="sect2">
- <h3 id="design_differences_with_boost_variant">Differences with Boost.Variant</h3>
- <div class="paragraph">
- <p>This library is API compatible with <code>std::variant</code>. As such, its interface
- is different from Boost.Variant’s. For example, visitation is performed via
- <code>visit</code> instead of <code>apply_visitor</code>.</p>
- </div>
- <div class="paragraph">
- <p>Recursive variants are not supported.</p>
- </div>
- <div class="paragraph">
- <p>Double storage is used instead of temporary heap backup. This <code>variant</code> is
- always "stack-based", it never allocates, and never throws <code>bad_alloc</code> on
- its own.</p>
- </div>
- </div>
- </div>
- </div>
- <div class="sect1">
- <h2 id="implementation">Implementation</h2>
- <div class="sectionbody">
- <div class="sect2">
- <h3 id="implementation_dependencies">Dependencies</h3>
- <div class="paragraph">
- <p>This implementation only depends on Boost.Config and Boost.Mp11.</p>
- </div>
- </div>
- <div class="sect2">
- <h3 id="implementation_supported_compilers">Supported Compilers</h3>
- <div class="ulist">
- <ul>
- <li>
- <p>GCC 4.8 or later with <code>-std=c++11</code> or above</p>
- </li>
- <li>
- <p>Clang 3.5 or later with <code>-std=c++11</code> or above</p>
- </li>
- <li>
- <p>Visual Studio 2015, 2017, 2019</p>
- </li>
- </ul>
- </div>
- <div class="paragraph">
- <p>Tested on <a href="https://travis-ci.org/boostorg/variant2/">Travis</a> and
- <a href="https://ci.appveyor.com/project/pdimov/variant2-fkab9">Appveyor</a>.</p>
- </div>
- </div>
- </div>
- </div>
- <div class="sect1">
- <h2 id="reference">Reference</h2>
- <div class="sectionbody">
- <div class="sect2">
- <h3 id="ref_boostvariant2variant_hpp"><boost/variant2/variant.hpp></h3>
- <div class="sect3">
- <h4 id="ref_synopsis">Synopsis</h4>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>namespace boost {
- namespace variant2 {
- // in_place_type
- template<class T> struct in_place_type_t {};
- template<class T> constexpr in_place_type_t<T> in_place_type{};
- // in_place_index
- template<std::size_t I> struct in_place_index_t {};
- template<std::size_t I> constexpr in_place_index_t<I> in_place_index{};
- // variant
- template<class... T> class variant;
- // variant_size
- template<class T> struct variant_size {};
- template<class T> struct variant_size<T const>: variant_size<T> {};
- template<class T> struct variant_size<T volatile>: variant_size<T> {};
- template<class T> struct variant_size<T const volatile>: variant_size<T> {};
- template<class T> struct variant_size<T&>: variant_size<T> {}; // extension
- template<class T> struct variant_size<T&&>: variant_size<T> {}; // extension
- template<class T>
- inline constexpr size_t variant_size_v = variant_size<T>::value;
- template<class... T>
- struct variant_size<variant<T...>>:
- std::integral_constant<std::size_t, sizeof...(T)> {};
- // variant_alternative
- template<size_t I, class T> struct variant_alternative {};
- template<size_t I, class T> struct variant_alternative<I, T const>;
- template<size_t I, class T> struct variant_alternative<I, T volatile>;
- template<size_t I, class T> struct variant_alternative<I, T const volatile>;
- template<size_t I, class T> struct variant_alternative<I, T&>; // extension
- template<size_t I, class T> struct variant_alternative<I, T&&>; // extension
- template<size_t I, class T>
- using variant_alternative_t = typename variant_alternative<I, T>::type;
- template<size_t I, class... T>
- struct variant_alternative<I, variant<T...>>;
- // variant_npos
- constexpr std::size_t variant_npos = -1;
- // holds_alternative
- template<class U, class... T>
- constexpr bool holds_alternative(const variant<T...>& v) noexcept;
- // get
- template<size_t I, class... T>
- constexpr variant_alternative_t<I, variant<T...>>&
- get(variant<T...>& v);
- template<size_t I, class... T>
- constexpr variant_alternative_t<I, variant<T...>>&&
- get(variant<T...>&& v);
- template<size_t I, class... T>
- constexpr const variant_alternative_t<I, variant<T...>>&
- get(const variant<T...>& v);
- template<size_t I, class... T>
- constexpr const variant_alternative_t<I, variant<T...>>&&
- get(const variant<T...>&& v);
- template<class U, class... T>
- constexpr U& get(variant<T...>& v);
- template<class U, class... T>
- constexpr U&& get(variant<T...>&& v);
- template<class U, class... T>
- constexpr const U& get(const variant<T...>& v);
- template<class U, class... T>
- constexpr const U&& get(const variant<T...>&& v);
- // get_if
- template<size_t I, class... T>
- constexpr add_pointer_t<variant_alternative_t<I, variant<T...>>>
- get_if(variant<T...>* v) noexcept;
- template<size_t I, class... T>
- constexpr add_pointer_t<const variant_alternative_t<I, variant<T...>>>
- get_if(const variant<T...>* v) noexcept;
- template<class U, class... T>
- constexpr add_pointer_t<U>
- get_if(variant<T...>* v) noexcept;
- template<class U, class... T>
- constexpr add_pointer_t<const U>
- get_if(const variant<T...>* v) noexcept;
- // relational operators
- template<class... T>
- constexpr bool operator==(const variant<T...>& v, const variant<T...>& w);
- template<class... T>
- constexpr bool operator!=(const variant<T...>& v, const variant<T...>& w);
- template<class... T>
- constexpr bool operator<(const variant<T...>& v, const variant<T...>& w);
- template<class... T>
- constexpr bool operator>(const variant<T...>& v, const variant<T...>& w);
- template<class... T>
- constexpr bool operator<=(const variant<T...>& v, const variant<T...>& w);
- template<class... T>
- constexpr bool operator>=(const variant<T...>& v, const variant<T...>& w);
- // visit
- template<class F, class... V>
- constexpr /*see below*/ visit(F&& f, V&&... v);
- // monostate
- struct monostate {};
- constexpr bool operator==(monostate, monostate) noexcept { return true; }
- constexpr bool operator!=(monostate, monostate) noexcept { return false; }
- constexpr bool operator<(monostate, monostate) noexcept { return false; }
- constexpr bool operator>(monostate, monostate) noexcept { return false; }
- constexpr bool operator<=(monostate, monostate) noexcept { return true; }
- constexpr bool operator>=(monostate, monostate) noexcept { return true; }
- // swap
- template<class... T>
- void swap(variant<T...>& v, variant<T...>& w) noexcept( /*see below*/ );
- // bad_variant_access
- class bad_variant_access;
- } // namespace variant2
- } // namespace boost</code></pre>
- </div>
- </div>
- </div>
- <div class="sect3">
- <h4 id="ref_variant">variant</h4>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>namespace boost {
- namespace variant2 {
- template<class... T> class variant
- {
- public:
- // constructors
- constexpr variant() noexcept( /*see below*/ );
- constexpr variant( variant const & r ) noexcept( /*see below*/ );
- constexpr variant( variant&& r ) noexcept( /*see below*/ );
- template<class U>
- constexpr variant( U&& u ) noexcept( /*see below*/ );
- template<class U, class... A>
- constexpr explicit variant( in_place_type_t<U>, A&&... a );
- template<class U, class V, class... A>
- constexpr explicit variant( in_place_type_t<U>,
- std::initializer_list<V> il, A&&... a );
- template<size_t I, class... A>
- constexpr explicit variant( in_place_index_t<I>, A&&... a );
- template<size_t I, class V, class... A>
- constexpr explicit variant( in_place_index_t<I>,
- std::initializer_list<V> il, A&&... a );
- // destructor
- ~variant();
- // assignment
- constexpr variant& operator=( variant const & r ) noexcept( /*see below*/ );
- constexpr variant& operator=( variant&& r ) noexcept( /*see below*/ );
- template<class U> constexpr variant& operator=( U&& u ) noexcept( /*see below*/ );
- // modifiers
- template<class U, class... A>
- constexpr U& emplace( A&&... a );
- template<class U, class V, class... A>
- constexpr U& emplace( std::initializer_list<V> il, A&&... a );
- template<size_t I, class... A>
- constexpr variant_alternative_t<I, variant<T...>>&
- emplace( A&&... a );
- template<size_t I, class V, class... A>
- constexpr variant_alternative_t<I, variant<T...>>&
- emplace( std::initializer_list<V> il, A&&... a );
- // value status
- constexpr bool valueless_by_exception() const noexcept;
- constexpr size_t index() const noexcept;
- // swap
- void swap( variant& r ) noexcept( /*see below*/ );
- // converting constructors (extension)
- template<class... U> variant( variant<U...> const& r )
- noexcept( /*see below*/ );
- template<class... U> variant( variant<U...>&& r )
- noexcept( /*see below*/ );
- // subset (extension)
- template<class... U> constexpr variant<U...> subset() & ;
- template<class... U> constexpr variant<U...> subset() && ;
- template<class... U> constexpr variant<U...> subset() const& ;
- template<class... U> constexpr variant<U...> subset() const&& ;
- };
- } // namespace variant2
- } // namespace boost</code></pre>
- </div>
- </div>
- <div class="paragraph">
- <p>In the descriptions that follow, let <code>i</code> be in the range <code>[0, sizeof…​(T))</code>,
- and <code>Ti</code> be the <code>i</code>-th type in <code>T…​</code>.</p>
- </div>
- <div class="sect4">
- <h5 id="ref_constructors">Constructors</h5>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>constexpr variant() noexcept( std::is_nothrow_default_constructible_v<T0> );</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Effects: </dt>
- <dd>
- <p>Constructs a <code>variant</code> holding a value-initialized value of
- type <code>T0</code>.</p>
- </dd>
- <dt class="hdlist1">Ensures: </dt>
- <dd>
- <p><code>index() == 0</code>.</p>
- </dd>
- <dt class="hdlist1">Throws: </dt>
- <dd>
- <p>Any exception thrown by the value-initialization of <code>T0</code>.</p>
- </dd>
- <dt class="hdlist1">Remarks: </dt>
- <dd>
- <p>This function does not participate in overload resolution unless
- <code>std::is_default_constructible_v<T0></code> is <code>true</code>.</p>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>constexpr variant( variant const & w )
- noexcept( mp_all<std::is_nothrow_copy_constructible<T>...>::value );</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Effects: </dt>
- <dd>
- <p>Initializes the variant to hold the same alternative and value as
- <code>w</code>.</p>
- </dd>
- <dt class="hdlist1">Throws: </dt>
- <dd>
- <p>Any exception thrown by the initialization of the contained value.</p>
- </dd>
- <dt class="hdlist1">Remarks: </dt>
- <dd>
- <p>This function does not participate in overload resolution unless
- <code>std::is_copy_constructible_v<Ti></code> is <code>true</code> for all <code>i</code>.</p>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>constexpr variant( variant&& w )
- noexcept( mp_all<std::is_nothrow_move_constructible<T>...>::value );</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Effects: </dt>
- <dd>
- <p>Initializes the variant to hold the same alternative and value as
- <code>w</code>.</p>
- </dd>
- <dt class="hdlist1">Throws: </dt>
- <dd>
- <p>Any exception thrown by the move-initialization of the contained
- value.</p>
- </dd>
- <dt class="hdlist1">Remarks: </dt>
- <dd>
- <p>This function does not participate in overload resolution unless
- <code>std::is_move_constructible_v<Ti></code> is <code>true</code> for all <code>i</code>.</p>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<class U> constexpr variant( U&& u ) noexcept(/*see below*/);</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="paragraph">
- <p>Let <code>Tj</code> be a type that is determined as follows: build an imaginary function
- <code>FUN(Ti)</code> for each alternative type <code>Ti</code>. The overload <code>FUN(Tj)</code> selected by
- overload resolution for the expression <code>FUN(std::forward<U>(u))</code> defines the
- alternative <code>Tj</code> which is the type of the contained value after construction.</p>
- </div>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Effects: </dt>
- <dd>
- <p>Initializes <code>*this</code> to hold the alternative type <code>Tj</code> and
- initializes the contained value from <code>std::forward<U>(u)</code>.</p>
- </dd>
- <dt class="hdlist1">Ensures: </dt>
- <dd>
- <p><code>holds_alternative<Tj>(*this)</code>.</p>
- </dd>
- <dt class="hdlist1">Throws: </dt>
- <dd>
- <p>Any exception thrown by the initialization of the contained value.</p>
- </dd>
- <dt class="hdlist1">Remarks: </dt>
- <dd>
- <p>The expression inside <code>noexcept</code> is equivalent to
- <code>std::is_nothrow_constructible_v<Tj, U></code>. This function does not participate in
- overload resolution unless</p>
- <div class="ulist">
- <ul>
- <li>
- <p><code>sizeof…​(T)</code> is nonzero,</p>
- </li>
- <li>
- <p><code>std::is_same_v<std::remove_cvref_t<U>, variant></code> is <code>false</code>,</p>
- </li>
- <li>
- <p><code>std::remove_cvref_t<U></code> is neither a specialization of <code>in_place_type_t</code> nor a
- specialization of <code>in_place_index_t</code>,</p>
- </li>
- <li>
- <p><code>std::is_constructible_v<Tj, U></code> is <code>true</code>, and</p>
- </li>
- <li>
- <p>the expression <code>FUN(std::forward<U>(u))</code> is well-formed.</p>
- </li>
- </ul>
- </div>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<class U, class... A>
- constexpr explicit variant( in_place_type_t<U>, A&&... a );</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Effects: </dt>
- <dd>
- <p>Initializes the contained value of type <code>U</code> with the arguments
- <code>std::forward<A>(a)…​</code>.</p>
- </dd>
- <dt class="hdlist1">Ensures: </dt>
- <dd>
- <p><code>holds_alternative<U>(*this)</code>.</p>
- </dd>
- <dt class="hdlist1">Throws: </dt>
- <dd>
- <p>Any exception thrown by the initialization of the contained value.</p>
- </dd>
- <dt class="hdlist1">Remarks: </dt>
- <dd>
- <p>This function does not participate in overload resolution unless
- there is exactly one occurrence of <code>U</code> in <code>T…​</code> and
- <code>std::is_constructible_v<U, A…​></code> is true.</p>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<class U, class V, class... A>
- constexpr explicit variant( in_place_type_t<U>, std::initializer_list<V> il,
- A&&... a );</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Effects: </dt>
- <dd>
- <p>Initializes the contained value of type <code>U</code> with the arguments <code>il</code>,
- <code>std::forward<A>(a)…​</code>.</p>
- </dd>
- <dt class="hdlist1">Ensures: </dt>
- <dd>
- <p><code>holds_alternative<U>(*this)</code>.</p>
- </dd>
- <dt class="hdlist1">Throws: </dt>
- <dd>
- <p>Any exception thrown by the initialization of the contained value.</p>
- </dd>
- <dt class="hdlist1">Remarks: </dt>
- <dd>
- <p>This function does not participate in overload resolution unless
- there is exactly one occurrence of <code>U</code> in <code>T…​</code> and
- <code>std::is_constructible_v<U, initializer_list<V>&, A…​></code> is <code>true</code>.</p>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<size_t I, class... A>
- constexpr explicit variant( in_place_index_t<I>, A&&... a );</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Effects: </dt>
- <dd>
- <p>Initializes the contained value of type <code>TI</code> with the arguments
- <code>std::forward<A>(a)…​</code>.</p>
- </dd>
- <dt class="hdlist1">Ensures: </dt>
- <dd>
- <p><code>index() == I</code>.</p>
- </dd>
- <dt class="hdlist1">Throws: </dt>
- <dd>
- <p>Any exception thrown by the initialization of the contained value.</p>
- </dd>
- <dt class="hdlist1">Remarks: </dt>
- <dd>
- <p>This function does not participate in overload resolution unless
- <code>I < sizeof…​(T)</code> and <code>std::is_constructible_v<TI, A…​></code> is <code>true</code>.</p>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<size_t I, class V, class... A>
- constexpr explicit variant( in_place_index_t<I>, std::initializer_list<V> il,
- A&&... a );</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Effects: </dt>
- <dd>
- <p>Initializes the contained value of type <code>TI</code> with the arguments
- <code>il</code>, <code>std::forward<A>(a)…​</code>.</p>
- </dd>
- <dt class="hdlist1">Ensures: </dt>
- <dd>
- <p><code>index() == I</code>.</p>
- </dd>
- <dt class="hdlist1">Throws: </dt>
- <dd>
- <p>Any exception thrown by the initialization of the contained value.</p>
- </dd>
- <dt class="hdlist1">Remarks: </dt>
- <dd>
- <p>This function does not participate in overload resolution unless
- <code>I < sizeof…​(T)</code> and
- <code>std::is_constructible_v<TI, initializer_list<V>&, A…​></code> is <code>true</code>.</p>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- </div>
- <div class="sect4">
- <h5 id="ref_destructor">Destructor</h5>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>~variant();</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Effects: </dt>
- <dd>
- <p>Destroys the currently contained value.</p>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- </div>
- <div class="sect4">
- <h5 id="ref_assignment">Assignment</h5>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>constexpr variant& operator=( const variant& r )
- noexcept( mp_all<std::is_nothrow_copy_constructible<T>...>::value );</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="paragraph">
- <p>Let <code>j</code> be <code>r.index()</code>.</p>
- </div>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Effects: </dt>
- <dd>
- <p><code>emplace<j>(get<j>(r))</code>.</p>
- </dd>
- <dt class="hdlist1">Returns: </dt>
- <dd>
- <p><code>*this</code>.</p>
- </dd>
- <dt class="hdlist1">Ensures: </dt>
- <dd>
- <p><code>index() == r.index()</code>.</p>
- </dd>
- <dt class="hdlist1">Remarks: </dt>
- <dd>
- <p>This operator does not participate in overload resolution unless
- <code>std::is_copy_constructible_v<Ti> && std::is_copy_assignable_v<Ti></code> is
- <code>true</code> for all <code>i</code>.</p>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>constexpr variant& operator=( variant&& r )
- noexcept( mp_all<std::is_nothrow_move_constructible<T>...>::value );</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="paragraph">
- <p>Let <code>j</code> be <code>r.index()</code>.</p>
- </div>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Effects: </dt>
- <dd>
- <p><code>emplace<j>(get<j>(std::move(r)))</code>.</p>
- </dd>
- <dt class="hdlist1">Returns: </dt>
- <dd>
- <p><code>*this</code>.</p>
- </dd>
- <dt class="hdlist1">Ensures: </dt>
- <dd>
- <p><code>index() == r.index()</code>.</p>
- </dd>
- <dt class="hdlist1">Remarks: </dt>
- <dd>
- <p>This operator does not participate in overload resolution unless
- <code>std::is_move_constructible_v<Ti> && std::is_move_assignable_v<Ti></code> is
- <code>true</code> for all <code>i</code>.</p>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<class U> constexpr variant& operator=( U&& u )
- noexcept( /*see below*/ );</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="paragraph">
- <p>Let <code>Tj</code> be a type that is determined as follows: build an imaginary function
- <code>FUN(Ti)</code> for each alternative type <code>Ti</code>. The overload <code>FUN(Tj)</code> selected by
- overload resolution for the expression <code>FUN(std::forward<U>(u))</code> defines the
- alternative <code>Tj</code> which is the type of the contained value after construction.</p>
- </div>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Effects: </dt>
- <dd>
- <p><code>emplace<j>(std::forward<U>(u))</code>.</p>
- </dd>
- <dt class="hdlist1">Returns: </dt>
- <dd>
- <p><code>*this</code>.</p>
- </dd>
- <dt class="hdlist1">Ensures: </dt>
- <dd>
- <p><code>index() == j</code>.</p>
- </dd>
- <dt class="hdlist1">Remarks: </dt>
- <dd>
- <p>The expression inside <code>noexcept</code> is <code>std::is_nothrow_constructible_v<Tj, U&&></code>.
- This operator does not participate in overload resolution unless</p>
- <div class="ulist">
- <ul>
- <li>
- <p><code>std::is_same_v<std::remove_cvref_t<T>, variant></code> is <code>false</code>,</p>
- </li>
- <li>
- <p><code>std::is_constructible_v<Tj, U&&> && std::is_assignable_v<Tj&, U&&></code> is
- <code>true</code>, and</p>
- </li>
- <li>
- <p>the expression <code>FUN(std::forward<U>(u))</code> (with <code>FUN</code> being the
- above-mentioned set of imaginary functions) is well-formed.</p>
- </li>
- </ul>
- </div>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- </div>
- <div class="sect4">
- <h5 id="ref_modifiers">Modifiers</h5>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<class U, class... A>
- constexpr U& emplace( A&&... a );</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="paragraph">
- <p>Let <code>I</code> be the zero-based index of <code>U</code> in <code>T…​</code>.</p>
- </div>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Effects: </dt>
- <dd>
- <p>Equivalent to: <code>return emplace<I>(std::forward<A>(a)…​);</code></p>
- </dd>
- <dt class="hdlist1">Remarks: </dt>
- <dd>
- <p>This function shall not participate in overload resolution unless
- <code>std::is_constructible_v<U, A&&…​></code> is <code>true</code> and <code>U</code> occurs exactly once
- in <code>T…​</code>.</p>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<class U, class V, class... A>
- constexpr U& emplace( std::initializer_list<V> il, A&&... a );</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="paragraph">
- <p>Let <code>I</code> be the zero-based index of <code>U</code> in <code>T…​</code>.</p>
- </div>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Effects: </dt>
- <dd>
- <p>Equivalent to: <code>return emplace<I>(il, std::forward<A>(a)…​);</code></p>
- </dd>
- <dt class="hdlist1">Remarks: </dt>
- <dd>
- <p>This function shall not participate in overload resolution unless
- <code>std::is_constructible_v<U, std::initializer_list<V>&, A&&…​></code> is <code>true</code>
- and <code>U</code> occurs exactly once in <code>T…​</code>.</p>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<size_t I, class... A>
- constexpr variant_alternative_t<I, variant<T...>>&
- emplace( A&&... a );</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Requires: </dt>
- <dd>
- <p><code>I < sizeof…​(T)</code>.</p>
- </dd>
- <dt class="hdlist1">Effects: </dt>
- <dd>
- <p>Destroys the currently contained value, then initializes a new contained
- value as if using the expression <code>Ti(std::forward<A>(a)…​)</code>.</p>
- </dd>
- <dt class="hdlist1">Ensures: </dt>
- <dd>
- <p><code>index() == I</code>.</p>
- </dd>
- <dt class="hdlist1">Returns: </dt>
- <dd>
- <p>A reference to the new contained value.</p>
- </dd>
- <dt class="hdlist1">Throws: </dt>
- <dd>
- <p>Nothing unless the initialization of the new contained value throws.</p>
- </dd>
- <dt class="hdlist1">Exception Safety: </dt>
- <dd>
- <p>Strong. On exception, the contained value is unchanged.</p>
- </dd>
- <dt class="hdlist1">Remarks: </dt>
- <dd>
- <p>This function shall not participate in overload resolution unless
- <code>std::is_constructible_v<Ti, A&&…​></code> is <code>true</code>.</p>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<size_t I, class V, class... A>
- constexpr variant_alternative_t<I, variant<T...>>&
- emplace( std::initializer_list<V> il, A&&... a );</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Requires: </dt>
- <dd>
- <p><code>I < sizeof…​(T)</code>.</p>
- </dd>
- <dt class="hdlist1">Effects: </dt>
- <dd>
- <p>Destroys the currently contained value, then initializes a new contained
- value as if using the expression <code>Ti(il, std::forward<A>(a)…​)</code>.</p>
- </dd>
- <dt class="hdlist1">Ensures: </dt>
- <dd>
- <p><code>index() == I</code>.</p>
- </dd>
- <dt class="hdlist1">Returns: </dt>
- <dd>
- <p>A reference to the new contained value.</p>
- </dd>
- <dt class="hdlist1">Throws: </dt>
- <dd>
- <p>Nothing unless the initialization of the new contained value throws.</p>
- </dd>
- <dt class="hdlist1">Exception Safety: </dt>
- <dd>
- <p>Strong. On exception, the contained value is unchanged.</p>
- </dd>
- <dt class="hdlist1">Remarks: </dt>
- <dd>
- <p>This function shall not participate in overload resolution unless
- <code>std::is_constructible_v<Ti, std::initializer_list<V>&, A&&…​></code> is <code>true</code>.</p>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- </div>
- <div class="sect4">
- <h5 id="ref_value_status">Value Status</h5>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>constexpr bool valueless_by_exception() const noexcept;</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Returns: </dt>
- <dd>
- <p><code>false</code>.</p>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- <div class="admonitionblock note">
- <table>
- <tr>
- <td class="icon">
- <div class="title">Note</div>
- </td>
- <td class="content">
- This function is provided purely for compatibility with <code>std::variant</code>.
- </td>
- </tr>
- </table>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>constexpr size_t index() const noexcept;</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Returns: </dt>
- <dd>
- <p>The zero-based index of the active alternative.</p>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- </div>
- <div class="sect4">
- <h5 id="ref_swap">Swap</h5>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>void swap( variant& r ) noexcept( mp_all<std::is_nothrow_move_constructible<T>...,
- is_nothrow_swappable<T>...>::value );</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Effects: </dt>
- <dd>
- <div class="ulist">
- <ul>
- <li>
- <p>If <code>index() == r.index()</code>, calls <code>swap(get<I>(*this), get<I>(r))</code>,
- where <code>I</code> is <code>index()</code>.</p>
- </li>
- <li>
- <p>Otherwise, as if
- <code>variant tmp(std::move(*this)); *this = std::move(r); r = std::move(tmp);</code></p>
- </li>
- </ul>
- </div>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- </div>
- <div class="sect4">
- <h5 id="ref_converting_constructors_extension">Converting Constructors (extension)</h5>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<class... U> variant( variant<U...> const& r )
- noexcept( mp_all<std::is_nothrow_copy_constructible<U>...>::value );</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Effects: </dt>
- <dd>
- <p>Initializes the contained value from the contained value of <code>r</code>.</p>
- </dd>
- <dt class="hdlist1">Throws: </dt>
- <dd>
- <p>Any exception thrown by the initialization of the contained value.</p>
- </dd>
- <dt class="hdlist1">Remarks: </dt>
- <dd>
- <p>This function does not participate in overload resolution unless
- all types in <code>U…​</code> are in <code>T…​</code> and
- <code>std::is_copy_constructible_v<Ui>::value</code> is <code>true</code> for all <code>Ui</code>.</p>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<class... U> variant( variant<U...>&& r )
- noexcept( mp_all<std::is_nothrow_move_constructible<U>...>::value );</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Effects: </dt>
- <dd>
- <p>Initializes the contained value from the contained value of
- <code>std::move(r)</code>.</p>
- </dd>
- <dt class="hdlist1">Throws: </dt>
- <dd>
- <p>Any exception thrown by the initialization of the contained value.</p>
- </dd>
- <dt class="hdlist1">Remarks: </dt>
- <dd>
- <p>This function does not participate in overload resolution unless
- all types in <code>U…​</code> are in <code>T…​</code> and
- <code>std::is_move_constructible_v<Ui>::value</code> is <code>true</code> for all <code>Ui</code>.</p>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- </div>
- <div class="sect4">
- <h5 id="ref_subset_extension">Subset (extension)</h5>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<class... U> constexpr variant<U...> subset() & ;</code></pre>
- </div>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<class... U> constexpr variant<U...> subset() const& ;</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Returns: </dt>
- <dd>
- <p>A <code>variant<U…​></code> whose contained value is copy-initialized from
- the contained value of <code>*this</code> and has the same type.</p>
- </dd>
- <dt class="hdlist1">Throws: </dt>
- <dd>
- <div class="ulist">
- <ul>
- <li>
- <p>If the active alternative of <code>*this</code> is not among the types in <code>U…​</code>,
- <code>bad_variant_access</code>.</p>
- </li>
- <li>
- <p>Otherwise, any exception thrown by the initialization of the contained value.</p>
- </li>
- </ul>
- </div>
- </dd>
- <dt class="hdlist1">Remarks: </dt>
- <dd>
- <p>This function does not participate in overload resolution unless
- all types in <code>U…​</code> are in <code>T…​</code> and
- <code>std::is_copy_constructible_v<Ui>::value</code> is <code>true</code> for all <code>Ui</code>.</p>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<class... U> constexpr variant<U...> subset() && ;</code></pre>
- </div>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<class... U> constexpr variant<U...> subset() const&& ;</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Returns: </dt>
- <dd>
- <p>A <code>variant<U…​></code> whose contained value is move-initialized from
- the contained value of <code>*this</code> and has the same type.</p>
- </dd>
- <dt class="hdlist1">Throws: </dt>
- <dd>
- <div class="ulist">
- <ul>
- <li>
- <p>If the active alternative of <code>*this</code> is not among the types in <code>U…​</code>,
- <code>bad_variant_access</code>.</p>
- </li>
- <li>
- <p>Otherwise, any exception thrown by the initialization of the contained value.</p>
- </li>
- </ul>
- </div>
- </dd>
- <dt class="hdlist1">Remarks: </dt>
- <dd>
- <p>This function does not participate in overload resolution unless
- all types in <code>U…​</code> are in <code>T…​</code> and
- <code>std::is_move_constructible_v<Ui>::value</code> is <code>true</code> for all <code>Ui</code>.</p>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- </div>
- </div>
- <div class="sect3">
- <h4 id="ref_variant_alternative">variant_alternative</h4>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<size_t I, class T> struct variant_alternative<I, T const>;</code></pre>
- </div>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<size_t I, class T> struct variant_alternative<I, T volatile>;</code></pre>
- </div>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<size_t I, class T> struct variant_alternative<I, T const volatile>;</code></pre>
- </div>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<size_t I, class T> struct variant_alternative<I, T&>; // extension</code></pre>
- </div>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<size_t I, class T> struct variant_alternative<I, T&&>; // extension</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="openblock">
- <div class="content">
- <div class="paragraph">
- <p>If <code>typename variant_alternative<I, T>::type</code> exists and is <code>U</code>,</p>
- </div>
- <div class="ulist">
- <ul>
- <li>
- <p><code>variant_alternative<I, T const>::type</code> is <code>U const</code>;</p>
- </li>
- <li>
- <p><code>variant_alternative<I, T volatile>::type</code> is <code>U volatile</code>;</p>
- </li>
- <li>
- <p><code>variant_alternative<I, T const volatile>::type</code> is <code>U const volatile</code>.</p>
- </li>
- <li>
- <p><code>variant_alternative<I, T&>::type</code> is <code>U&</code>.</p>
- </li>
- <li>
- <p><code>variant_alternative<I, T&&>::type</code> is <code>U&&</code>.</p>
- </li>
- </ul>
- </div>
- <div class="paragraph">
- <p>Otherwise, these structs have no member <code>type</code>.</p>
- </div>
- </div>
- </div>
- </li>
- </ul>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<size_t I, class... T>
- struct variant_alternative<I, variant<T...>>;</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="paragraph">
- <p>When <code>I < sizeof…​(T)</code>, the nested type <code>type</code> is an alias for the <code>I</code>-th
- (zero-based) type in <code>T…​</code>. Otherwise, there is no member <code>type</code>.</p>
- </div>
- </li>
- </ul>
- </div>
- </div>
- <div class="sect3">
- <h4 id="ref_holds_alternative">holds_alternative</h4>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<class U, class... T>
- constexpr bool holds_alternative(const variant<T...>& v) noexcept;</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Requires: </dt>
- <dd>
- <p>The type <code>U</code> occurs exactly once in <code>T…​</code>. Otherwise, the
- program is ill-formed.</p>
- </dd>
- <dt class="hdlist1">Returns: </dt>
- <dd>
- <p><code>true</code> if <code>index()</code> is equal to the zero-based index of <code>U</code>
- in <code>T…​</code>.</p>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- </div>
- <div class="sect3">
- <h4 id="ref_get">get</h4>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<size_t I, class... T>
- constexpr variant_alternative_t<I, variant<T...>>&
- get(variant<T...>& v);</code></pre>
- </div>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<size_t I, class... T>
- constexpr variant_alternative_t<I, variant<T...>>&&
- get(variant<T...>&& v);</code></pre>
- </div>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<size_t I, class... T>
- constexpr const variant_alternative_t<I, variant<T...>>&
- get(const variant<T...>& v);</code></pre>
- </div>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<size_t I, class... T>
- constexpr const variant_alternative_t<I, variant<T...>>&&
- get(const variant<T...>&& v);</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Effects: </dt>
- <dd>
- <p>If <code>v.index()</code> is <code>I</code>, returns a reference to the object stored in
- the variant. Otherwise, throws <code>bad_variant_access</code>.</p>
- </dd>
- <dt class="hdlist1">Remarks: </dt>
- <dd>
- <p>These functions do not participate in overload resolution
- unless <code>I</code> < <code>sizeof…​(T)</code>.</p>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<class U, class... T>
- constexpr U& get(variant<T...>& v);</code></pre>
- </div>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<class U, class... T>
- constexpr U&& get(variant<T...>&& v);</code></pre>
- </div>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<class U, class... T>
- constexpr const U& get(const variant<T...>& v);</code></pre>
- </div>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<class U, class... T>
- constexpr const U&& get(const variant<T...>&& v);</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Requires: </dt>
- <dd>
- <p>The type <code>U</code> occurs exactly once in <code>T…​</code>. Otherwise, the
- program is ill-formed.</p>
- </dd>
- <dt class="hdlist1">Effects: </dt>
- <dd>
- <p>If <code>v</code> holds a value of type <code>U</code>, returns a reference to that value.
- Otherwise, throws <code>bad_variant_access</code>.</p>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- </div>
- <div class="sect3">
- <h4 id="ref_get_if">get_if</h4>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<size_t I, class... T>
- constexpr add_pointer_t<variant_alternative_t<I, variant<T...>>>
- get_if(variant<T...>* v) noexcept;</code></pre>
- </div>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<size_t I, class... T>
- constexpr add_pointer_t<const variant_alternative_t<I, variant<T...>>>
- get_if(const variant<T...>* v) noexcept;</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Effects: </dt>
- <dd>
- <p>A pointer to the value stored in the variant, if
- <code>v != nullptr && v->index() == I</code>. Otherwise, <code>nullptr</code>.</p>
- </dd>
- <dt class="hdlist1">Remarks: </dt>
- <dd>
- <p>These functions do not participate in overload resolution
- unless <code>I</code> < <code>sizeof…​(T)</code>.</p>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<class U, class... T>
- constexpr add_pointer_t<U>
- get_if(variant<T...>* v) noexcept;</code></pre>
- </div>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<class U, class... T>
- constexpr add_pointer_t<const U>
- get_if(const variant<T...>* v) noexcept;</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Requires: </dt>
- <dd>
- <p>The type <code>U</code> occurs exactly once in <code>T…​</code>. Otherwise, the
- program is ill-formed.</p>
- </dd>
- <dt class="hdlist1">Effects: </dt>
- <dd>
- <p>Equivalent to: <code>return get_if<I>(v);</code> with <code>I</code> being
- the zero-based index of <code>U</code> in <code>T…​</code>.</p>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- </div>
- <div class="sect3">
- <h4 id="ref_relational_operators">Relational Operators</h4>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<class... T>
- constexpr bool operator==(const variant<T...>& v, const variant<T...>& w);</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Returns: </dt>
- <dd>
- <p><code>v.index() == w.index() && get<I>(v) == get<I>(w)</code>, where <code>I</code>
- is <code>v.index()</code>.</p>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<class... T>
- constexpr bool operator!=(const variant<T...>& v, const variant<T...>& w);</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Returns: </dt>
- <dd>
- <p><code>!(v == w)</code>.</p>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<class... T>
- constexpr bool operator<(const variant<T...>& v, const variant<T...>& w);</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Returns: </dt>
- <dd>
- <p><code>v.index() < w.index() || (v.index() == w.index() && get<I>(v) < get<I>(w))</code>,
- where <code>I</code> is <code>v.index()</code>.</p>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<class... T>
- constexpr bool operator>(const variant<T...>& v, const variant<T...>& w);</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Returns: </dt>
- <dd>
- <p><code>w < v</code>.</p>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<class... T>
- constexpr bool operator<=(const variant<T...>& v, const variant<T...>& w);</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Returns: </dt>
- <dd>
- <p><code>v.index() < w.index() || (v.index() == w.index() && get<I>(v) <= get<I>(w))</code>,
- where <code>I</code> is <code>v.index()</code>.</p>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<class... T>
- constexpr bool operator>=(const variant<T...>& v, const variant<T...>& w);</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Returns: </dt>
- <dd>
- <p><code>w <= v</code>.</p>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- </div>
- <div class="sect3">
- <h4 id="ref_visit">visit</h4>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<class F, class... V>
- constexpr /*see below*/ visit(F&& f, V&&... v);</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Returns: </dt>
- <dd>
- <p><code>std::forward<F>(f)(get<I>(std::forward<V>(v))…​)</code>, where
- <code>I…​</code> is <code>v.index()…​</code>.</p>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- </div>
- <div class="sect3">
- <h4 id="ref_swap_2">swap</h4>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>template<class... T>
- void swap(variant<T...>& v, variant<T...>& w) noexcept( /*see below*/ );</code></pre>
- </div>
- </div>
- <div class="ulist none">
- <ul class="none">
- <li>
- <p></p>
- <div class="dlist">
- <dl>
- <dt class="hdlist1">Effects: </dt>
- <dd>
- <p>Equivalent to <code>v.swap(w)</code>.</p>
- </dd>
- </dl>
- </div>
- </li>
- </ul>
- </div>
- </div>
- <div class="sect3">
- <h4 id="ref_bad_variant_access">bad_variant_access</h4>
- <div class="listingblock">
- <div class="content">
- <pre class="highlight"><code>class bad_variant_access: public std::exception
- {
- public:
- bad_variant_access() noexcept = default;
- char const * what() const noexcept
- {
- return "bad_variant_access";
- }
- };</code></pre>
- </div>
- </div>
- </div>
- </div>
- </div>
- </div>
- <div class="sect1">
- <h2 id="copyright">Copyright and License</h2>
- <div class="sectionbody">
- <div class="paragraph">
- <p>This documentation is copyright 2018, 2019 Peter Dimov and is distributed under
- the <a href="http://www.boost.org/LICENSE_1_0.txt">Boost Software License, Version 1.0</a>.</p>
- </div>
- </div>
- </div>
- </div>
- <div id="footer">
- <div id="footer-text">
- Last updated 2019-12-10 00:20:09 UTC
- </div>
- </div>
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- *:not(pre)>code { background: none; color: #600000; }
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