////
Copyright 2018-2021 Peter Dimov
Distributed under the Boost Software License, Version 1.0.
https://www.boost.org/LICENSE_1_0.txt
////

[#reference]
# Reference
:idprefix: ref_

## <boost/variant2/variant.hpp>

### Synopsis

```
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;

// unsafe_get (extension)

template<size_t I, class... T>
  constexpr variant_alternative_t<I, variant<T...>>&
    unsafe_get(variant<T...>& v);
template<size_t I, class... T>
  constexpr variant_alternative_t<I, variant<T...>>&&
    unsafe_get(variant<T...>&& v);
template<size_t I, class... T>
  constexpr const variant_alternative_t<I, variant<T...>>&
    unsafe_get(const variant<T...>& v);
template<size_t I, class... T>
  constexpr const variant_alternative_t<I, variant<T...>>&&
    unsafe_get(const variant<T...>&& v);

// 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);

// swap

template<class... T>
  void swap(variant<T...>& v, variant<T...>& w) noexcept( /*see below*/ );

// visit

template<class R = /*unspecified*/, class F, class... V>
  constexpr /*see below*/ visit(F&& f, V&&... v);

// visit_by_index (extension)

template<class R = /*unspecified*/, class V, class... F>
  constexpr /*see below*/ visit_by_index(V&& v, F&&.. f);

// 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; }

// stream insertion (extension)

template<class Ch, class Tr, class... T>
  std::basic_ostream<Ch, Tr>&
    operator<<( std::basic_ostream<Ch, Tr>& os, variant<T...> const& v );

template<class Ch, class Tr>
  std::basic_ostream<Ch, Tr>&
    operator<<( std::basic_ostream<Ch, Tr>& os, monostate const& v );

// bad_variant_access

class bad_variant_access;

} // namespace variant2
} // namespace boost
```

### variant

```
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;

  static constexpr bool uses_double_storage() 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
```

In the descriptions that follow, let `i` be in the range `[0, sizeof...(T))`,
and `Ti` be the `i`-th type in `T...`.

#### Constructors

```
constexpr variant() noexcept( std::is_nothrow_default_constructible_v<T0> );
```
[none]
* {blank}
+
Effects: :: Constructs a `variant` holding a value-initialized value of
  type `T0`.
Ensures: :: `index() == 0`.
Throws: :: Any exception thrown by the value-initialization of `T0`.
Remarks: :: This function does not participate in overload resolution unless
  `std::is_default_constructible_v<T0>` is `true`.

```
constexpr variant( variant const & w )
  noexcept( mp_all<std::is_nothrow_copy_constructible<T>...>::value );
```
[none]
* {blank}
+
Effects: :: Initializes the variant to hold the same alternative and value as
  `w`.
Throws: :: Any exception thrown by the initialization of the contained value.
Remarks: :: This function does not participate in overload resolution unless
  `std::is_copy_constructible_v<Ti>` is `true` for all `i`.

```
constexpr variant( variant&& w )
  noexcept( mp_all<std::is_nothrow_move_constructible<T>...>::value );
```
[none]
* {blank}
+
Effects: :: Initializes the variant to hold the same alternative and value as
  `w`.
Throws: :: Any exception thrown by the move-initialization of the contained
  value.
Remarks: :: This function does not participate in overload resolution unless
  `std::is_move_constructible_v<Ti>` is `true` for all `i`.

```
template<class U> constexpr variant( U&& u ) noexcept(/*see below*/);
```
[none]
* {blank}
+
Let `Tj` be a type that is determined as follows: build an imaginary function
`FUN(Ti)` for each alternative type `Ti`. The overload `FUN(Tj)` selected by
overload resolution for the expression `FUN(std::forward<U>(u))` defines the
alternative `Tj` which is the type of the contained value after construction.

Effects: :: Initializes `*this` to hold the alternative type `Tj` and
  initializes the contained value from `std::forward<U>(u)`.
Ensures: :: `holds_alternative<Tj>(*this)`.
Throws: :: Any exception thrown by the initialization of the contained value.
Remarks: :: The expression inside `noexcept` is equivalent to
`std::is_nothrow_constructible_v<Tj, U>`. This function does not participate in
overload resolution unless
- `sizeof...(T)` is nonzero,
- `std::is_same_v<std::remove_cvref_t<U>, variant>` is `false`,
- `std::remove_cvref_t<U>` is neither a specialization of `in_place_type_t` nor a
specialization of `in_place_index_t`,
- `std::is_constructible_v<Tj, U>` is `true`, and
- the expression `FUN(std::forward<U>(u))` is well-formed.

```
template<class U, class... A>
  constexpr explicit variant( in_place_type_t<U>, A&&... a );
```
[none]
* {blank}
+
Effects: :: Initializes the contained value of type `U` with the arguments
  `std::forward<A>(a)...`.
Ensures: :: `holds_alternative<U>(*this)`.
Throws: :: Any exception thrown by the initialization of the contained value.
Remarks: :: This function does not participate in overload resolution unless
  there is exactly one occurrence of `U` in `T...` and
  `std::is_constructible_v<U, A...>` is true.

```
template<class U, class V, class... A>
  constexpr explicit variant( in_place_type_t<U>, std::initializer_list<V> il,
    A&&... a );
```
[none]
* {blank}
+
Effects: :: Initializes the contained value of type `U` with the arguments `il`,
  `std::forward<A>(a)...`.
Ensures: :: `holds_alternative<U>(*this)`.
Throws: :: Any exception thrown by the initialization of the contained value.
Remarks: :: This function does not participate in overload resolution unless
  there is exactly one occurrence of `U` in `T...` and
  `std::is_constructible_v<U, initializer_list<V>&, A...>` is `true`.

```
template<size_t I, class... A>
  constexpr explicit variant( in_place_index_t<I>, A&&... a );
```
[none]
* {blank}
+
Effects: :: Initializes the contained value of type `TI` with the arguments
  `std::forward<A>(a)...`.
Ensures: :: `index() == I`.
Throws: :: Any exception thrown by the initialization of the contained value.
Remarks: :: This function does not participate in overload resolution unless
 `I < sizeof...(T)` and `std::is_constructible_v<TI, A...>` is `true`.

```
template<size_t I, class V, class... A>
  constexpr explicit variant( in_place_index_t<I>, std::initializer_list<V> il,
    A&&... a );
```
[none]
* {blank}
+
Effects: :: Initializes the contained value of type `TI` with the arguments
  `il`, `std::forward<A>(a)...`.
Ensures: :: `index() == I`.
Throws: :: Any exception thrown by the initialization of the contained value.
Remarks: :: This function does not participate in overload resolution unless
 `I < sizeof...(T)` and
 `std::is_constructible_v<TI, initializer_list<V>&, A...>` is `true`.

#### Destructor

```
~variant();
```
[none]
* {blank}
+
Effects: ::
  Destroys the currently contained value.

#### Assignment

```
constexpr variant& operator=( const variant& r )
  noexcept( mp_all<std::is_nothrow_copy_constructible<T>...>::value );
```
[none]
* {blank}
+
Let `j` be `r.index()`.

Effects: :: `emplace<j>(get<j>(r))`.
Returns: :: `*this`.
Ensures: :: `index() == r.index()`.
Remarks: :: This operator does not participate in overload resolution unless
  `std::is_copy_constructible_v<Ti> && std::is_copy_assignable_v<Ti>` is
  `true` for all `i`.

```
constexpr variant& operator=( variant&& r )
  noexcept( mp_all<std::is_nothrow_move_constructible<T>...>::value );
```
[none]
* {blank}
+
Let `j` be `r.index()`.

Effects: :: `emplace<j>(get<j>(std::move(r)))`.
Returns: :: `*this`.
Ensures: :: `index() == r.index()`.
Remarks: :: This operator does not participate in overload resolution unless
  `std::is_move_constructible_v<Ti> && std::is_move_assignable_v<Ti>` is
  `true` for all `i`.

```
template<class U> constexpr variant& operator=( U&& u )
  noexcept( /*see below*/ );
```
[none]
* {blank}
+
Let `Tj` be a type that is determined as follows: build an imaginary function
`FUN(Ti)` for each alternative type `Ti`. The overload `FUN(Tj)` selected by
overload resolution for the expression `FUN(std::forward<U>(u))` defines the
alternative `Tj` which is the type of the contained value after construction.

Effects: :: `emplace<j>(std::forward<U>(u))`.
Returns: :: `*this`.
Ensures: :: `index() == j`.
Remarks: ::
  The expression inside `noexcept` is `std::is_nothrow_constructible_v<Tj, U&&>`.
  This operator does not participate in overload resolution unless
  - `std::is_same_v<std::remove_cvref_t<T>, variant>` is `false`,
  - `std::is_constructible_v<Tj, U&&> && std::is_assignable_v<Tj&, U&&>` is
    `true`,  and
  - the expression `FUN(std::forward<U>(u))` (with `FUN` being the
    above-mentioned set of imaginary functions) is well-formed.

#### Modifiers

```
template<class U, class... A>
  constexpr U& emplace( A&&... a );
```
[none]
* {blank}
+
Let `I` be the zero-based index of `U` in `T...`.

Effects: :: Equivalent to: `return emplace<I>(std::forward<A>(a)...);`
Remarks: ::
  This function shall not participate in overload resolution unless
  `std::is_constructible_v<U, A&&...>` is `true` and `U` occurs exactly once
  in `T...`.

```
template<class U, class V, class... A>
  constexpr U& emplace( std::initializer_list<V> il, A&&... a );
```
[none]
* {blank}
+
Let `I` be the zero-based index of `U` in `T...`.

Effects: :: Equivalent to: `return emplace<I>(il, std::forward<A>(a)...);`
Remarks: ::
  This function shall not participate in overload resolution unless
  `std::is_constructible_v<U, std::initializer_list<V>&, A&&...>` is `true`
  and `U` occurs exactly once in `T...`.


```
template<size_t I, class... A>
  constexpr variant_alternative_t<I, variant<T...>>&
    emplace( A&&... a );
```
[none]
* {blank}
+
Requires: :: `I < sizeof...(T)`.
Effects:  ::
  Initializes a new contained value as if using the expression
  `Ti(std::forward<A>(a)...)`, then destroys the currently contained value.
Ensures:  :: `index() == I`.
Returns:  :: A reference to the new contained value.
Throws:   ::
  Nothing unless the initialization of the new contained value throws.
Exception Safety: :: Strong. On exception, the contained value is unchanged.
Remarks:  ::
  This function shall not participate in overload resolution unless
  `std::is_constructible_v<Ti, A&&...>` is `true`.

```
template<size_t I, class V, class... A>
  constexpr variant_alternative_t<I, variant<T...>>&
    emplace( std::initializer_list<V> il, A&&... a );
```
[none]
* {blank}
+
Requires: :: `I < sizeof...(T)`.
Effects:  ::
  Initializes a new contained value as if using the expression
  `Ti(il, std::forward<A>(a)...)`, then destroys the currently contained value.
Ensures:  :: `index() == I`.
Returns:  :: A reference to the new contained value.
Throws:   ::
  Nothing unless the initialization of the new contained value throws.
Exception Safety: :: Strong. On exception, the contained value is unchanged.
Remarks:  ::
  This function shall not participate in overload resolution unless
  `std::is_constructible_v<Ti, std::initializer_list<V>&, A&&...>` is `true`.

#### Value Status

```
constexpr bool valueless_by_exception() const noexcept;
```
[none]
* {blank}
+
Returns:  :: `false`.

NOTE: This function is provided purely for compatibility with `std::variant`.

```
constexpr size_t index() const noexcept;
```
[none]
* {blank}
+
Returns:  ::
  The zero-based index of the active alternative.

```
static constexpr bool uses_double_storage() noexcept;
```
[none]
* {blank}
+
Returns:  ::
  `true` if the variant uses double storage to meet the never valueless
  guarantee because one of the alternatives is not nothrow move constructible,
  and `false` otherwise.


#### Swap

```
void swap( variant& r ) noexcept( mp_all<std::is_nothrow_move_constructible<T>...,
  is_nothrow_swappable<T>...>::value );
```
[none]
* {blank}
+
Effects:  ::
- If `index() == r.index()`, calls `swap(get<I>(*this), get<I>(r))`,
  where `I` is `index()`.
- Otherwise, as if
  `variant tmp(std::move(*this)); *this = std::move(r); r = std::move(tmp);`

#### Converting Constructors (extension)

```
template<class... U> variant( variant<U...> const& r )
  noexcept( mp_all<std::is_nothrow_copy_constructible<U>...>::value );
```
[none]
* {blank}
+
Effects: :: Initializes the contained value from the contained value of `r`.
Throws:  :: Any exception thrown by the initialization of the contained value.
Remarks: :: This function does not participate in overload resolution unless
  all types in `U...` are in `T...` and
  `std::is_copy_constructible_v<Ui>::value` is `true` for all `Ui`.

```
template<class... U> variant( variant<U...>&& r )
  noexcept( mp_all<std::is_nothrow_move_constructible<U>...>::value );
```
[none]
* {blank}
+
Effects: :: Initializes the contained value from the contained value of
  `std::move(r)`.
Throws:  :: Any exception thrown by the initialization of the contained value.
Remarks: :: This function does not participate in overload resolution unless
  all types in `U...` are in `T...` and
  `std::is_move_constructible_v<Ui>::value` is `true` for all `Ui`.

#### Subset (extension)

```
template<class... U> constexpr variant<U...> subset() & ;
```
```
template<class... U> constexpr variant<U...> subset() const& ;
```
[none]
* {blank}
+
Returns: :: A `variant<U...>` whose contained value is copy-initialized from
  the contained value of `*this` and has the same type.
Throws:  ::
- If the active alternative of `*this` is not among the types in `U...`,
  `bad_variant_access`.
- Otherwise, any exception thrown by the initialization of the contained value.
Remarks: :: This function does not participate in overload resolution unless
  all types in `U...` are in `T...` and
  `std::is_copy_constructible_v<Ui>::value` is `true` for all `Ui`.

```
template<class... U> constexpr variant<U...> subset() && ;
```
```
template<class... U> constexpr variant<U...> subset() const&& ;
```
[none]
* {blank}
+
Returns: :: A `variant<U...>` whose contained value is move-initialized from
  the contained value of `*this` and has the same type.
Throws:  ::
- If the active alternative of `*this` is not among the types in `U...`,
  `bad_variant_access`.
- Otherwise, any exception thrown by the initialization of the contained value.
Remarks: :: This function does not participate in overload resolution unless
  all types in `U...` are in `T...` and
  `std::is_move_constructible_v<Ui>::value` is `true` for all `Ui`.

### 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
```
[none]
* {blank}
+
--
If `typename variant_alternative<I, T>::type` exists and is `U`,

* `variant_alternative<I, T const>::type` is `U const`;
* `variant_alternative<I, T volatile>::type` is `U volatile`;
* `variant_alternative<I, T const volatile>::type` is `U const volatile`.
* `variant_alternative<I, T&>::type` is `U&`.
* `variant_alternative<I, T&&>::type` is `U&&`.

Otherwise, these structs have no member `type`.
--

```
template<size_t I, class... T>
  struct variant_alternative<I, variant<T...>>;
```
[none]
* {blank}
+
When `I < sizeof...(T)`, the nested type `type` is an alias for the `I`-th
(zero-based) type in `T...`. Otherwise, there is no member `type`.

### holds_alternative

```
template<class U, class... T>
  constexpr bool holds_alternative(const variant<T...>& v) noexcept;
```
[none]
* {blank}
+
Requires: :: The type `U` occurs exactly once in `T...`. Otherwise, the
  program is ill-formed.
Returns:  :: `true` if `index()` is equal to the zero-based index of `U`
  in `T...`.

### 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);
```
[none]
* {blank}
+
Effects: :: If `v.index()` is `I`, returns a reference to the object stored in
  the variant. Otherwise, throws `bad_variant_access`.
Remarks: :: These functions do not participate in overload resolution
  unless `I` < `sizeof...(T)`.

```
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);
```
[none]
* {blank}
+
Requires: :: The type `U` occurs exactly once in `T...`. Otherwise, the
  program is ill-formed.
Effects:  :: If `v` holds a value of type `U`, returns a reference to that value.
  Otherwise, throws `bad_variant_access`.

### 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;
```
[none]
* {blank}
+
Effects:  :: A pointer to the value stored in the variant, if
  `v != nullptr && v\->index() == I`. Otherwise, `nullptr`.
Remarks: :: These functions do not participate in overload resolution
  unless `I` < `sizeof...(T)`.

```
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;
```
[none]
* {blank}
+
Requires: :: The type `U` occurs exactly once in `T...`. Otherwise, the
  program is ill-formed.
Effects:  :: Equivalent to: `return get_if<I>(v);` with `I` being
  the zero-based index of `U` in `T...`.

### unsafe_get (extension)

```
template<size_t I, class... T>
  constexpr variant_alternative_t<I, variant<T...>>&
    unsafe_get(variant<T...>& v);
```
```
template<size_t I, class... T>
  constexpr variant_alternative_t<I, variant<T...>>&&
    unsafe_get(variant<T...>&& v);
```
```
template<size_t I, class... T>
  constexpr const variant_alternative_t<I, variant<T...>>&
    unsafe_get(const variant<T...>& v);
```
```
template<size_t I, class... T>
  constexpr const variant_alternative_t<I, variant<T...>>&&
    unsafe_get(const variant<T...>&& v);
```
[none]
* {blank}
+
Requires: :: `v.index() == I`.
Returns: :: a reference to the object stored in the variant.

### Relational Operators

```
template<class... T>
  constexpr bool operator==(const variant<T...>& v, const variant<T...>& w);
```
[none]
* {blank}
+
Returns: :: `v.index() == w.index() && get<I>(v) == get<I>(w)`, where `I`
  is `v.index()`.

```
template<class... T>
  constexpr bool operator!=(const variant<T...>& v, const variant<T...>& w);
```
[none]
* {blank}
+
Returns: :: `!(v == w)`.

```
template<class... T>
  constexpr bool operator<(const variant<T...>& v, const variant<T...>& w);
```
[none]
* {blank}
+
Returns: :: `v.index() < w.index() || (v.index() == w.index() && get<I>(v) < get<I>(w))`,
  where `I` is `v.index()`.

```
template<class... T>
  constexpr bool operator>(const variant<T...>& v, const variant<T...>& w);
```
[none]
* {blank}
+
Returns: :: `w < v`.

```
template<class... T>
  constexpr bool operator<=(const variant<T...>& v, const variant<T...>& w);
```
[none]
* {blank}
+
Returns: :: `v.index() < w.index() || (v.index() == w.index() && get<I>(v) \<= get<I>(w))`,
  where `I` is `v.index()`.

```
template<class... T>
  constexpr bool operator>=(const variant<T...>& v, const variant<T...>& w);
```
[none]
* {blank}
+
Returns: ::
  `w \<= v`.

### swap

```
template<class... T>
  void swap(variant<T...>& v, variant<T...>& w) noexcept( /*see below*/ );
```
[none]
* {blank}
+
Effects: ::
  Equivalent to `v.swap(w)`.

### visit

```
template<class R = /*unspecified*/, class F, class... V>
  constexpr /*see below*/ visit(F&& f, V&&... v);
```
[none]
* {blank}
+
Returns: :: `std::forward<F>(f)(get<I>(std::forward<V>(v))...)`, where
  `I...` is `v.index()...`.
Remarks: :: If `R` is given explicitly, as in `visit<int>`, the return
  type is `R`. Otherwise, it's deduced from `F`. All possible applications
  of `F` to the variant alternatives must have the same return type for
  this deduction to succeed.

### visit_by_index (extension)

```
template<class R = /*unspecified*/, class V, class... F>
  constexpr /*see below*/ visit_by_index(V&& v, F&&.. f);
```
[none]
* {blank}
+
Requires: :: `variant_size<V>::value == sizeof...(F)`, or the program is ill-formed.
Returns: :: `std::forward<Fi>(fi)(get<i>(std::forward<V>(v)))`, where
  `i` is `v.index()` and `Fi` and `fi` are the `i`-th element of `F...` and `f...`
  accordingly.
Remarks: :: If `R` is given explicitly, as in `visit_by_index<int>`, the return
  type is `R`. Otherwise, it's deduced from `F...` and `V`. All the applications
  of `Fi` to the corresponding variant alternatives must have the same return type
  for this deduction to succeed.

### Stream Insertion (extension)
```
template<class Ch, class Tr, class... T>
  std::basic_ostream<Ch, Tr>&
    operator<<( std::basic_ostream<Ch, Tr>& os, variant<T...> const& v );
```
[none]
* {blank}
+
Requires: ::
  `sizeof...(T) != 0`.
Returns: ::
  `os << get<I>(v)`, where `I` is `v.index()`.

```
template<class Ch, class Tr>
  std::basic_ostream<Ch, Tr>&
    operator<<( std::basic_ostream<Ch, Tr>& os, monostate const& v );
```
[none]
* {blank}
+
Effects: ::
  `os << "monostate"`.
Returns: ::
  `os`.

### bad_variant_access

```
class bad_variant_access: public std::exception
{
public:

    bad_variant_access() noexcept = default;

    char const * what() const noexcept
    {
        return "bad_variant_access";
    }
};
```

## <boost/variant2.hpp>

This convenience header includes `<boost/variant2/variant.hpp>`.
