22 General utilities library [utilities]

22.6 Variants [variant]

22.6.1 General [variant.general]

A variant object holds and manages the lifetime of a value.

If the variant holds a value, that value's type has to be one of the template argument types given to variant.

These template arguments are called alternatives.

In [variant], GET denotes a set of exposition-only function templates ([variant.get]).

22.6.2 Header <variant> synopsis [variant.syn]

// mostly freestanding #include <compare> // see [compare.syn] namespace std { // [variant.variant], class template variant template<class... Types> class variant; // [variant.helper], variant helper classes template<class T> struct variant_size; // not defined template<class T> struct variant_size<const T>; template<class T> constexpr size_t variant_size_v = variant_size<T>::value; template<class... Types> struct variant_size<variant<Types...>>; template<size_t I, class T> struct variant_alternative; // not defined template<size_t I, class T> struct variant_alternative<I, const T>; template<size_t I, class T> using variant_alternative_t = typename variant_alternative<I, T>::type; template<size_t I, class... Types> struct variant_alternative<I, variant<Types...>>; inline constexpr size_t variant_npos = -1; // [variant.get], value access template<class T, class... Types> constexpr bool holds_alternative(const variant<Types...>&) noexcept; template<size_t I, class... Types> constexpr variant_alternative_t<I, variant<Types...>>& get(variant<Types...>&); // freestanding-deleted template<size_t I, class... Types> constexpr variant_alternative_t<I, variant<Types...>>&& get(variant<Types...>&&); // freestanding-deleted template<size_t I, class... Types> constexpr const variant_alternative_t<I, variant<Types...>>& get(const variant<Types...>&); // freestanding-deleted template<size_t I, class... Types> constexpr const variant_alternative_t<I, variant<Types...>>&& get(const variant<Types...>&&); // freestanding-deleted template<class T, class... Types> constexpr T& get(variant<Types...>&); // freestanding-deleted template<class T, class... Types> constexpr T&& get(variant<Types...>&&); // freestanding-deleted template<class T, class... Types> constexpr const T& get(const variant<Types...>&); // freestanding-deleted template<class T, class... Types> constexpr const T&& get(const variant<Types...>&&); // freestanding-deleted template<size_t I, class... Types> constexpr add_pointer_t<variant_alternative_t<I, variant<Types...>>> get_if(variant<Types...>*) noexcept; template<size_t I, class... Types> constexpr add_pointer_t<const variant_alternative_t<I, variant<Types...>>> get_if(const variant<Types...>*) noexcept; template<class T, class... Types> constexpr add_pointer_t<T> get_if(variant<Types...>*) noexcept; template<class T, class... Types> constexpr add_pointer_t<const T> get_if(const variant<Types...>*) noexcept; // [variant.relops], relational operators template<class... Types> constexpr bool operator==(const variant<Types...>&, const variant<Types...>&); template<class... Types> constexpr bool operator!=(const variant<Types...>&, const variant<Types...>&); template<class... Types> constexpr bool operator<(const variant<Types...>&, const variant<Types...>&); template<class... Types> constexpr bool operator>(const variant<Types...>&, const variant<Types...>&); template<class... Types> constexpr bool operator<=(const variant<Types...>&, const variant<Types...>&); template<class... Types> constexpr bool operator>=(const variant<Types...>&, const variant<Types...>&); template<class... Types> requires (three_way_comparable<Types> && ...) constexpr common_comparison_category_t<compare_three_way_result_t<Types>...> operator<=>(const variant<Types...>&, const variant<Types...>&); // [variant.visit], visitation template<class Visitor, class... Variants> constexpr see below visit(Visitor&&, Variants&&...); template<class R, class Visitor, class... Variants> constexpr R visit(Visitor&&, Variants&&...); // [variant.monostate], class monostate struct monostate; // [variant.monostate.relops], monostate relational operators constexpr bool operator==(monostate, monostate) noexcept; constexpr strong_ordering operator<=>(monostate, monostate) noexcept; // [variant.specalg], specialized algorithms template<class... Types> constexpr void swap(variant<Types...>&, variant<Types...>&) noexcept(see below); // [variant.bad.access], class bad_variant_access class bad_variant_access; // [variant.hash], hash support template<class T> struct hash; template<class... Types> struct hash<variant<Types...>>; template<> struct hash<monostate>; }

22.6.3 Class template variant [variant.variant]

22.6.3.1 General [variant.variant.general]

namespace std { template<class... Types> class variant { public: // [variant.ctor], constructors constexpr variant() noexcept(see below); constexpr variant(const variant&); constexpr variant(variant&&) noexcept(see below); template<class T> constexpr variant(T&&) noexcept(see below); template<class T, class... Args> constexpr explicit variant(in_place_type_t<T>, Args&&...); template<class T, class U, class... Args> constexpr explicit variant(in_place_type_t<T>, initializer_list, Args&&...); template<size_t I, class... Args> constexpr explicit variant(in_place_index_t, Args&&...); template<size_t I, class U, class... Args> constexpr explicit variant(in_place_index_t, initializer_list, Args&&...); // [variant.dtor], destructor constexpr ~variant(); // [variant.assign], assignment constexpr variant& operator=(const variant&); constexpr variant& operator=(variant&&) noexcept(see below); template<class T> constexpr variant& operator=(T&&) noexcept(see below); // [variant.mod], modifiers template<class T, class... Args> constexpr T& emplace(Args&&...); template<class T, class U, class... Args> constexpr T& emplace(initializer_list, Args&&...); template<size_t I, class... Args> constexpr variant_alternative_t<I, variant<Types...>>& emplace(Args&&...); template<size_t I, class U, class... Args> constexpr variant_alternative_t<I, variant<Types...>>& emplace(initializer_list, Args&&...); // [variant.status], value status constexpr bool valueless_by_exception() const noexcept; constexpr size_t index() const noexcept; // [variant.swap], swap constexpr void swap(variant&) noexcept(see below); // [variant.visit], visitation template<class Self, class Visitor> constexpr decltype(auto) visit(this Self&&, Visitor&&); template<class R, class Self, class Visitor> constexpr R visit(this Self&&, Visitor&&); }; }

Any instance of variant at any given time either holds a value of one of its alternative types or holds no value.

When an instance of variant holds a value of alternative type T, it means that a value of type T, referred to as the variant object's contained value, is nested within ([intro.object]) the variant object.

All types in Types shall meet the Cpp17Destructible requirements (Table 35).

A program that instantiates the definition of variant with no template arguments is ill-formed.

If a program declares an explicit or partial specialization of variant, the program is ill-formed, no diagnostic required.

22.6.3.2 Constructors [variant.ctor]

In the descriptions that follow, let i be in the range [0, sizeof...(Types)), and

T_{i}

be the

i^{th}

type in Types.

🔗

constexpr variant() noexcept(see below);

Constraints: is_default_constructible_v<

T_{0}

> is true.

Effects: Constructs a variant holding a value-initialized value of type

T_{0}

Postconditions: valueless_by_exception() is false and index() is 0.

Throws: Any exception thrown by the value-initialization of

T_{0}

Remarks: This function is constexpr if and only if the value-initialization of the alternative type

T_{0}

would be constexpr-suitable ([dcl.constexpr]).

The exception specification is equivalent to is_nothrow_default_constructible_v<

T_{0}

[Note 1:

22.6.3.3 Destructor [variant.dtor]

🔗

constexpr ~variant();

Effects: If valueless_by_exception() is false, destroys the currently contained value.

Remarks: If is_trivially_destructible_v<

T_{i}

> is true for all

T_{i}

, then this destructor is trivial.

22.6.3.4 Assignment [variant.assign]

🔗

constexpr variant& operator=(const variant& rhs);

Let j be rhs.index().

Effects:

(2.1)
If neither *this nor rhs holds a value, there is no effect.
(2.2)
Otherwise, if *this holds a value but rhs does not, destroys the value contained in *this and sets *this to not hold a value.
(2.3)
Otherwise, if index() == j, assigns the value contained in rhs to the value contained in *this.
(2.4)
Otherwise, if either is_nothrow_copy_constructible_v< $T_{j}$ > is true or is_nothrow_move_constructible_v< $T_{j}$ > is false, equivalent to emplace<j>(GET<j>(rhs)).
(2.5)
Otherwise, equivalent to operator=(variant(rhs)).

Postconditions: index() == rhs.index().

Returns: *this.

Remarks: This operator is defined as deleted unless is_copy_constructible_v<

T_{i}

> && is_copy_assignable_v<

T_{i}

> is true for all i.

If is_trivially_copy_constructible_v<

T_{i}

> && is_trivially_copy_assignable_v<

T_{i}

> && is_trivially_destructible_v<

T_{i}

> is true for all i, this assignment operator is trivial.

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constexpr variant& operator=(variant&& rhs) noexcept(see below);

Let j be rhs.index().

Constraints: is_move_constructible_v<

T_{i}

> && is_move_assignable_v<

T_{i}

> is true for all i.

Effects:

(8.1)
If neither *this nor rhs holds a value, there is no effect.
(8.2)
Otherwise, if *this holds a value but rhs does not, destroys the value contained in *this and sets *this to not hold a value.
(8.3)
Otherwise, if index() == j, assigns GET<j>(std::move(rhs)) to the value contained in *this.
(8.4)
Otherwise, equivalent to emplace<j>(GET<j>(std::move(rhs))).

Returns: *this.

Remarks: If is_trivially_move_constructible_v<

T_{i}

> && is_trivially_move_assignable_v<

T_{i}

> && is_trivially_destructible_v<

T_{i}

> is true for all i, this assignment operator is trivial.

The exception specification is equivalent to is_nothrow_move_constructible_v<

T_{i}

> && is_nothrow_move_assignable_v<

T_{i}

> for all i.

(10.1)
If an exception is thrown during the call to $T_{j}$ 's move construction (with j being rhs.index()), the variant will hold no value.
(10.2)
If an exception is thrown during the call to $T_{j}$ 's move assignment, the state of the contained value is as defined by the exception safety guarantee of $T_{j}$ 's move assignment; index() will be j.

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template<class T> constexpr variant& operator=(T&& t) noexcept(see below);

Let

T_{j}

be a type that is determined as follows: build an imaginary function FUN(

T_{i}

) for each alternative type

T_{i}

for which

T_{i}

x[] = {std::forward<T>(t)}; is well-formed for some invented variable x.

The overload FUN(

T_{j}

) selected by overload resolution for the expression FUN(std::forward<T>(t)) defines the alternative

T_{j}

which is the type of the contained value after assignment.

Constraints:

(12.1)
is_same_v<remove_cvref_t<T>, variant> is false,
(12.2)
is_assignable_v< $T_{j}$ &, T> && is_constructible_v< $T_{j}$ , T> is true, and
(12.3)
the expression FUN(std::forward<T>(t)) (with FUN being the above-mentioned set of imaginary functions) is well-formed.
[Note 1:
variant<string, string> v; v = "abc"; is ill-formed, as both alternative types have an equally viable constructor for the argument.
— end note]

Effects:

(13.1)
If *this holds a $T_{j}$ , assigns std::forward<T>(t) to the value contained in *this.
(13.2)
Otherwise, if is_nothrow_constructible_v< $T_{j}$ , T> || !is_nothrow_move_constructible_v< $T_{j}$ > is true, equivalent to emplace<j>(std::forward<T>(t)).
(13.3)
Otherwise, equivalent to emplace<j>( $T_{j}$ (std::forward<T>(t))).

Postconditions: holds_alternative<

T_{j}

>(*this) is true, with

T_{j}

selected by the imaginary function overload resolution described above.

Returns: *this.

Remarks: The exception specification is equivalent to: is_nothrow_assignable_v<T

_{j}

&, T> && is_nothrow_constructible_v<T

_{j}

, T>

(16.1)
If an exception is thrown during the assignment of std::forward<T>(t) to the value contained in *this, the state of the contained value and t are as defined by the exception safety guarantee of the assignment expression; valueless_by_exception() will be false.
(16.2)
If an exception is thrown during the initialization of the contained value, the variant object is permitted to not hold a value.

22.6.3.5 Modifiers [variant.mod]

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template<class T, class... Args> constexpr T& emplace(Args&&... args);

Constraints: is_constructible_v<T, Args...> is true, and T occurs exactly once in Types.

Effects: Equivalent to: return emplace(std::forward<Args>(args)...); where I is the zero-based index of T in Types.

🔗

template<class T, class U, class... Args>
  constexpr T& emplace(initializer_list<U> il, Args&&... args);

Constraints: is_constructible_v<T, initializer_list&, Args...> is true, and T occurs exactly once in Types.

Effects: Equivalent to: return emplace(il, std::forward<Args>(args)...); where I is the zero-based index of T in Types.

🔗

template<size_t I, class... Args>
  constexpr variant_alternative_t<I, variant<Types...>>& emplace(Args&&... args);

Mandates: I < sizeof...(Types).

Constraints: is_constructible_v<

T_{I}

, Args...> is true.

Effects: Destroys the currently contained value if valueless_by_exception() is false.

Then direct-non-list-initializes the contained value of type

T_{I}

with the arguments std::forward<Args>(args)....

Postconditions: index() is I.

Returns: A reference to the new contained value.

Throws: Any exception thrown during the initialization of the contained value.

Remarks: If an exception is thrown during the initialization of the contained value, the variant is permitted to not hold a value.

🔗

template<size_t I, class U, class... Args>
  constexpr variant_alternative_t<I, variant<Types...>>&
    emplace(initializer_list<U> il, Args&&... args);

Mandates: I < sizeof...(Types).

Constraints: is_constructible_v<

T_{I}

, initializer_list&, Args...> is true.

Effects: Destroys the currently contained value if valueless_by_exception() is false.

Then direct-non-list-initializes the contained value of type

T_{I}

with il, std::forward<Args>(args)....

Postconditions: index() is I.

Returns: A reference to the new contained value.

Throws: Any exception thrown during the initialization of the contained value.

Remarks: If an exception is thrown during the initialization of the contained value, the variant is permitted to not hold a value.

22.6.3.6 Value status [variant.status]

🔗

constexpr bool valueless_by_exception() const noexcept;

Effects: Returns false if and only if the variant holds a value.

[Note 1:

It is possible for a variant to hold no value if an exception is thrown during a type-changing assignment or emplacement.

The latter means that even a variant<float, int> can become valueless_by_exception(), for instance by struct S { operator int() { throw 42; }}; variant<float, int> v{12.f}; v.emplace<1>(S());

— end note]

🔗

constexpr size_t index() const noexcept;

Effects: If valueless_by_exception() is true, returns variant_npos.

Otherwise, returns the zero-based index of the alternative of the contained value.

22.6.3.7 Swap [variant.swap]

🔗

constexpr void swap(variant& rhs) noexcept(see below);

Mandates: is_move_constructible_v<

T_{i}

> is true for all i.

Preconditions: Each

T_{i}

meets the Cpp17Swappable requirements ([swappable.requirements]).

Effects:

(3.1)
If valueless_by_exception() && rhs.valueless_by_exception() no effect.
(3.2)
Otherwise, if index() == rhs.index(), calls swap(GET(*this), GET(rhs)) where i is index().
(3.3)
Otherwise, exchanges values of rhs and *this.

Throws: If index() == rhs.index(), any exception thrown by swap(GET(*this), GET(rhs)) with i being index().

Otherwise, any exception thrown by the move constructor of

T_{i}

T_{j}

with i being index() and j being rhs.index().

Remarks: If an exception is thrown during the call to function swap(GET(*this), GET(rhs)), the states of the contained values of *this and of rhs are determined by the exception safety guarantee of swap for lvalues of

T_{i}

with i being index().

If an exception is thrown during the exchange of the values of *this and rhs, the states of the values of *this and of rhs are determined by the exception safety guarantee of variant's move constructor.

The exception specification is equivalent to the logical and of is_nothrow_move_constructible_v<

T_{i}

> && is_nothrow_swappable_v<

T_{i}

> for all i.

22.6.4 variant helper classes [variant.helper]

🔗

template<class T> struct variant_size;

All specializations of variant_size meet the Cpp17UnaryTypeTrait requirements ([meta.rqmts]) with a base characteristic of integral_constant<size_t, N> for some N.

🔗

template<class T> struct variant_size<const T>;

Let VS denote variant_size<T> of the cv-unqualified type T.

Then each specialization of the template meets the Cpp17UnaryTypeTrait requirements ([meta.rqmts]) with a base characteristic of integral_constant<size_t, VS::value>.

🔗

template<class... Types>
  struct variant_size<variant<Types...>> : integral_constant<size_t, sizeof...(Types)> { };

🔗

template<size_t I, class T> struct variant_alternative<I, const T>;

Let VA denote variant_alternative<I, T> of the cv-unqualified type T.

Then each specialization of the template meets the Cpp17TransformationTrait requirements ([meta.rqmts]) with a member typedef type that names the type add_const_t<VA::type>.

🔗

variant_alternative<I, variant<Types...>>::type

Mandates: I < sizeof...(Types).

Type: The type

T_{I}

22.6.5 Value access [variant.get]

🔗

template<class T, class... Types>
  constexpr bool holds_alternative(const variant<Types...>& v) noexcept;

Mandates: The type T occurs exactly once in Types.

Returns: true if index() is equal to the zero-based index of T in Types.

🔗

template<size_t I, class... Types>
  constexpr variant_alternative_t<I, variant<Types...>>&
    GET(variant<Types...>& v);                                  // exposition only
template<size_t I, class... Types>
  constexpr variant_alternative_t<I, variant<Types...>>&&
    GET(variant<Types...>&& v);                                 // exposition only
template<size_t I, class... Types>
  constexpr const variant_alternative_t<I, variant<Types...>>&
    GET(const variant<Types...>& v);                            // exposition only
template<size_t I, class... Types>
  constexpr const variant_alternative_t<I, variant<Types...>>&&
    GET(const variant<Types...>&& v);                           // exposition only

Mandates: I < sizeof...(Types).

Preconditions: v.index() is I.

Returns: A reference to the object stored in the variant.

🔗

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

Mandates: I < sizeof...(Types).

Effects: If v.index() is I, returns a reference to the object stored in the variant.

Otherwise, throws an exception of type bad_variant_access.

🔗

template<class T, class... Types> constexpr T& get(variant<Types...>& v);
template<class T, class... Types> constexpr T&& get(variant<Types...>&& v);
template<class T, class... Types> constexpr const T& get(const variant<Types...>& v);
template<class T, class... Types> constexpr const T&& get(const variant<Types...>&& v);

Mandates: The type T occurs exactly once in Types.

Effects: If v holds a value of type T, returns a reference to that value.

Otherwise, throws an exception of type bad_variant_access.

🔗

template<size_t I, class... Types>
  constexpr add_pointer_t<variant_alternative_t<I, variant<Types...>>>
    get_if(variant<Types...>* v) noexcept;
template<size_t I, class... Types>
  constexpr add_pointer_t<const variant_alternative_t<I, variant<Types...>>>
    get_if(const variant<Types...>* v) noexcept;

Mandates: I < sizeof...(Types).

Returns: A pointer to the value stored in the variant, if v != nullptr and v->index() == I.

Otherwise, returns nullptr.

🔗

template<class T, class... Types>
  constexpr add_pointer_t<T>
    get_if(variant<Types...>* v) noexcept;
template<class T, class... Types>
  constexpr add_pointer_t<const T>
    get_if(const variant<Types...>* v) noexcept;

Mandates: The type T occurs exactly once in Types.

Effects: Equivalent to: return get_if(v); with i being the zero-based index of T in Types.

22.6.6 Relational operators [variant.relops]

🔗

template<class... Types>
  constexpr bool operator==(const variant<Types...>& v, const variant<Types...>& w);

Constraints: GET(v) == GET(w) is a valid expression that is convertible to bool, for all i.

Returns: If v.index() != w.index(), false; otherwise if v.valueless_by_exception(), true; otherwise GET(v) == GET(w) with i being v.index().

🔗

template<class... Types>
  constexpr bool operator!=(const variant<Types...>& v, const variant<Types...>& w);

Constraints: GET(v) != GET(w) is a valid expression that is convertible to bool, for all i.

Returns: If v.index() != w.index(), true; otherwise if v.valueless_by_exception(), false; otherwise GET(v) != GET(w) with i being v.index().

🔗

template<class... Types>
  constexpr bool operator<(const variant<Types...>& v, const variant<Types...>& w);

Constraints: GET(v) < GET(w) is a valid expression that is convertible to bool, for all i.

Returns: If w.valueless_by_exception(), false; otherwise if v.valueless_by_exception(), true; otherwise, if v.index() < w.index(), true; otherwise if v.index() > w.index(), false; otherwise GET(v) < GET(w) with i being v.index().

🔗

template<class... Types>
  constexpr bool operator>(const variant<Types...>& v, const variant<Types...>& w);

Constraints: GET(v) > GET(w) is a valid expression that is convertible to bool, for all i.

Returns: If v.valueless_by_exception(), false; otherwise if w.valueless_by_exception(), true; otherwise, if v.index() > w.index(), true; otherwise if v.index() < w.index(), false; otherwise GET(v) > GET(w) with i being v.index().

🔗

template<class... Types>
  constexpr bool operator<=(const variant<Types...>& v, const variant<Types...>& w);

Constraints: GET(v) <= GET(w) is a valid expression that is convertible to bool, for all i.

Returns: If v.valueless_by_exception(), true; otherwise if w.valueless_by_exception(), false; otherwise, if v.index() < w.index(), true; otherwise if v.index() > w.index(), false; otherwise GET(v) <= GET(w) with i being v.index().

🔗

template<class... Types>
  constexpr bool operator>=(const variant<Types...>& v, const variant<Types...>& w);

Constraints: GET(v) >= GET(w) is a valid expression that is convertible to bool, for all i.

Returns: If w.valueless_by_exception(), true; otherwise if v.valueless_by_exception(), false; otherwise, if v.index() > w.index(), true; otherwise if v.index() < w.index(), false; otherwise GET(v) >= GET(w) with i being v.index().

🔗

template<class... Types> requires (three_way_comparable<Types> && ...)
  constexpr common_comparison_category_t<compare_three_way_result_t<Types>...>
    operator<=>(const variant<Types...>& v, const variant<Types...>& w);

Effects: Equivalent to: if (v.valueless_by_exception() && w.valueless_by_exception()) return strong_ordering::equal; if (v.valueless_by_exception()) return strong_ordering::less; if (w.valueless_by_exception()) return strong_ordering::greater; if (auto c = v.index() <=> w.index(); c != 0) return c; return GET(v) <=> GET(w); with i being v.index().

22.6.7 Visitation [variant.visit]

🔗

template<class Visitor, class... Variants>
  constexpr see below visit(Visitor&& vis, Variants&&... vars);
template<class R, class Visitor, class... Variants>
  constexpr R visit(Visitor&& vis, Variants&&... vars);

Let as-variant denote the following exposition-only function templates: template<class... Ts> constexpr auto&& as-variant(variant<Ts...>& var) { return var; } template<class... Ts> constexpr auto&& as-variant(const variant<Ts...>& var) { return var; } template<class... Ts> constexpr auto&& as-variant(variant<Ts...>&& var) { return std::move(var); } template<class... Ts> constexpr auto&& as-variant(const variant<Ts...>&& var) { return std::move(var); }

Let n be sizeof...(Variants).

For each

0 \leq i < n

, let

V_{i}

denote the type
decltype(as-variant(std::forward<

{Variants}_{i}

{vars}_{i}

))).

Constraints:

V_{i}

is a valid type for all

0 \leq i < n

Let V denote the pack of types

V_{i}

Let m be a pack of n values of type size_t.

Such a pack is valid if

0 \leq m_{i} < {variant_size_v<remove_reference_t<V}_{i} >>

for all

0 \leq i < n

For each valid pack m, let e(m) denote the expression: INVOKE(std::forward<Visitor>(vis), GET<m>(std::forward<V>(vars))...) // see [func.require] for the first form and INVOKE<R>(std::forward<Visitor>(vis), GET<m>(std::forward<V>(vars))...) // see [func.require] for the second form.

Mandates: For each valid pack m, e(m) is a valid expression.

All such expressions are of the same type and value category.

Returns: e(m), where m is the pack for which

m_{i}

is as-variant(vars

_{i}

).index() for all

0 \leq i < n

The return type is decltype(e(m)) for the first form.

Throws: bad_variant_access if (as-variant(vars).valueless_by_exception() || ...) is true.

Complexity: For n ≤ 1, the invocation of the callable object is implemented in constant time, i.e., for

n = 1

, it does not depend on the number of alternative types of

V_{0}

For

n > 1

, the invocation of the callable object has no complexity requirements.

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template<class Self, class Visitor>
  constexpr decltype(auto) visit(this Self&& self, Visitor&& vis);

Let V be OVERRIDE_REF(Self&&, COPY_CONST(remove_reference_t<Self>, variant)) ([forward]).

Constraints: The call to visit does not use an explicit template-argument-list that begins with a type template-argument.

Effects: Equivalent to: return std::visit(std::forward<Visitor>(vis), (V)self);

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template<class R, class Self, class Visitor>
  constexpr R visit(this Self&& self, Visitor&& vis);

Let V be OVERRIDE_REF(Self&&, COPY_CONST(remove_reference_t<Self>, variant)) ([forward]).

Effects: Equivalent to: return std::visit<R>(std::forward<Visitor>(vis), (V)self);

22.6.8 Class monostate [variant.monostate]

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struct monostate{};

The class monostate can serve as a first alternative type for a variant to make the variant type default constructible.

22.6.9 monostate relational operators [variant.monostate.relops]

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constexpr bool operator==(monostate, monostate) noexcept { return true; }
constexpr strong_ordering operator<=>(monostate, monostate) noexcept
{ return strong_ordering::equal; }

[Note 1:

monostate objects have only a single state; they thus always compare equal.

— end note]

22.6.10 Specialized algorithms [variant.specalg]

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template<class... Types>
  constexpr void swap(variant<Types...>& v, variant<Types...>& w) noexcept(see below);

Constraints: is_move_constructible_v<

T_{i}

> && is_swappable_v<

T_{i}

> is true for all i.

Effects: Equivalent to v.swap(w).

Remarks: The exception specification is equivalent to noexcept(v.swap(w)).

22.6.11 Class bad_variant_access [variant.bad.access]

namespace std { class bad_variant_access : public exception { public: // see [exception] for the specification of the special member functions const char* what() const noexcept override; }; }

Objects of type bad_variant_access are thrown to report invalid accesses to the value of a variant object.

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const char* what() const noexcept override;

Returns: An implementation-defined ntbs.

22.6.12 Hash support [variant.hash]

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template<class... Types> struct hash<variant<Types...>>;

The specialization hash<variant<Types...>> is enabled ([unord.hash]) if and only if every specialization in hash<remove_const_t<Types>>... is enabled.

The member functions are not guaranteed to be noexcept.

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template<> struct hash<monostate>;

The specialization is enabled ([unord.hash]).