[variant.variant]

# 22 General utilities library [[utilities]](./#utilities)

## 22.6 Variants [[variant]](variant#variant)

### 22.6.3 Class template variant [variant.variant]

#### [22.6.3.1](#general) General [[variant.variant.general]](variant.variant.general)

namespace std {template<class... Types>class variant {public:// [[variant.ctor]](#variant.ctor "22.6.3.2 Constructors"), constructorsconstexpr 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<U>, Args&&...); template<size_t I, class... Args>constexpr explicit variant(in_place_index_t<I>, Args&&...); template<size_t I, class U, class... Args>constexpr explicit variant(in_place_index_t<I>, initializer_list<U>, Args&&...); // [[variant.dtor]](#variant.dtor "22.6.3.3 Destructor"), destructorconstexpr ~variant(); // [[variant.assign]](#variant.assign "22.6.3.4 Assignment"), assignmentconstexpr variant& operator=(const variant&); constexpr variant& operator=(variant&&) noexcept(*see below*); template<class T> constexpr variant& operator=(T&&) noexcept(*see below*); // [[variant.mod]](#variant.mod "22.6.3.5 Modifiers"), modifierstemplate<class T, class... Args>constexpr T& emplace(Args&&...); template<class T, class U, class... Args>constexpr T& emplace(initializer_list<U>, 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<U>, Args&&...); // [[variant.status]](#variant.status "22.6.3.6 Value status"), value statusconstexpr bool valueless_by_exception() const noexcept; constexpr size_t index() const noexcept; // [[variant.swap]](#variant.swap "22.6.3.7 Swap"), swapconstexpr void swap(variant&) noexcept(*see below*); // [[variant.visit]](variant.visit "22.6.7 Visitation"), visitationtemplate<class Self, class Visitor>constexpr decltype(auto) visit(this Self&&, Visitor&&); template<class R, class Self, class Visitor>constexpr R visit(this Self&&, Visitor&&); };}

[1](#general-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5722)

Any instance of variant at any given time either holds a value
of one of its alternative types or holds no value[.](#general-1.sentence-1)

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*](#def:contained_value,variant "22.6.3.1 General [variant.variant.general]"),
is nested within ([[intro.object]](intro.object "6.8.2 Object model")) thevariant object[.](#general-1.sentence-2)

[2](#general-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5731)

All types in Types shall meet
the [*Cpp17Destructible*](utility.arg.requirements#:Cpp17Destructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") requirements (Table [35](utility.arg.requirements#tab:cpp17.destructible "Table 35: Cpp17Destructible requirements"))[.](#general-2.sentence-1)

[3](#general-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5735)

A program that instantiates the definition of variant with
no template arguments is ill-formed[.](#general-3.sentence-1)

[4](#general-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5739)

If a program declares an explicit or partial specialization of variant,
the program is ill-formed, no diagnostic required[.](#general-4.sentence-1)

#### [22.6.3.2](#variant.ctor) Constructors [[variant.ctor]](variant.ctor)

[1](#variant.ctor-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5745)

In the descriptions that follow, let i be in the range [0, sizeof...(Types)),
and Ti be the ith type in Types[.](#variant.ctor-1.sentence-1)

[🔗](#lib:variant,constructor)

`constexpr variant() noexcept(see below);
`

[2](#variant.ctor-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5755)

*Constraints*: is_default_constructible_v<T0> is true[.](#variant.ctor-2.sentence-1)

[3](#variant.ctor-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5759)

*Effects*: Constructs a variant holding a value-initialized value of type T0[.](#variant.ctor-3.sentence-1)

[4](#variant.ctor-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5763)

*Postconditions*: valueless_by_exception() is false and index() is 0[.](#variant.ctor-4.sentence-1)

[5](#variant.ctor-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5767)

*Throws*: Any exception thrown by the value-initialization of T0[.](#variant.ctor-5.sentence-1)

[6](#variant.ctor-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5771)

*Remarks*: This function is constexpr if and only if the
value-initialization of the alternative type T0 would be constexpr-suitable ([[dcl.constexpr]](dcl.constexpr "9.2.6 The constexpr and consteval specifiers"))[.](#variant.ctor-6.sentence-1)

The exception specification is equivalent tois_nothrow_default_constructible_v<T0>[.](#variant.ctor-6.sentence-2)

[*Note [1](#variant.ctor-note-1)*:

See also class monostate[.](#variant.ctor-6.sentence-3)

— *end note*]

[🔗](#lib:variant,constructor_)

`constexpr variant(const variant& w);
`

[7](#variant.ctor-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5789)

*Effects*: If w holds a value, initializes the variant to hold the same
alternative as w and direct-initializes the contained value
with *GET*<j>(w), where j is w.index()[.](#variant.ctor-7.sentence-1)

Otherwise, initializes the variant to not hold a value[.](#variant.ctor-7.sentence-2)

[8](#variant.ctor-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5796)

*Throws*: Any exception thrown by direct-initializing any Ti for all i[.](#variant.ctor-8.sentence-1)

[9](#variant.ctor-9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5800)

*Remarks*: This constructor is defined as deleted unlessis_copy_constructible_v<Ti> is true for all i[.](#variant.ctor-9.sentence-1)

If is_trivially_copy_constructible_v<Ti> is true for all i, this constructor is trivial[.](#variant.ctor-9.sentence-2)

[🔗](#lib:variant,constructor__)

`constexpr variant(variant&& w) noexcept(see below);
`

[10](#variant.ctor-10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5814)

*Constraints*: is_move_constructible_v<Ti> is true for all i[.](#variant.ctor-10.sentence-1)

[11](#variant.ctor-11)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5818)

*Effects*: If w holds a value, initializes the variant to hold the same
alternative as w and direct-initializes the contained value with*GET*<j>(std​::​move(w)), where j is w.index()[.](#variant.ctor-11.sentence-1)

Otherwise, initializes the variant to not hold a value[.](#variant.ctor-11.sentence-2)

[12](#variant.ctor-12)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5825)

*Throws*: Any exception thrown by move-constructing any Ti for all i[.](#variant.ctor-12.sentence-1)

[13](#variant.ctor-13)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5829)

*Remarks*: The exception specification is equivalent to the logical and ofis_nothrow_move_constructible_v<Ti> for all i[.](#variant.ctor-13.sentence-1)

If is_trivially_move_constructible_v<Ti> is true for all i, this constructor is trivial[.](#variant.ctor-13.sentence-2)

[🔗](#lib:variant,constructor___)

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

[14](#variant.ctor-14)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5843)

Let Tj be a type that is determined as follows:
build an imaginary function *FUN*(Ti) for each alternative type Ti for which Ti x[] = {std​::​forward<T>(t)}; is well-formed for some invented variable x[.](#variant.ctor-14.sentence-1)

The overload *FUN*(Tj) selected by overload
resolution for the expression *FUN*(std​::​forward<T>(​t)) defines
the alternative Tj which is the type of the contained value after
construction[.](#variant.ctor-14.sentence-2)

[15](#variant.ctor-15)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5854)

*Constraints*: 

- [(15.1)](#variant.ctor-15.1)

sizeof...(Types) is nonzero,

- [(15.2)](#variant.ctor-15.2)

is_same_v<remove_cvref_t<T>, variant> is false,

- [(15.3)](#variant.ctor-15.3)

remove_cvref_t<T> is neither
a specialization of in_place_type_t nor
a specialization of in_place_index_t,

- [(15.4)](#variant.ctor-15.4)

is_constructible_v<Tj, T> is true, and

- [(15.5)](#variant.ctor-15.5)

the expression *FUN*(​std​::​forward<T>(t)) (with *FUN* being the above-mentioned set of
imaginary functions) is well-formed[.](#variant.ctor-15.sentence-1)
  [*Note [2](#variant.ctor-note-2)*:
  variant<string, string> v("abc"); is ill-formed, as both alternative types have an equally viable constructor
for the argument[.](#variant.ctor-15.5.sentence-2)
 — *end note*]

[16](#variant.ctor-16)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5884)

*Effects*: Initializes *this to hold the alternative type Tj and
direct-non-list-initializes the contained value with std​::​forward<T>(t)[.](#variant.ctor-16.sentence-1)

[17](#variant.ctor-17)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5889)

*Postconditions*: holds_alternative<Tj>(*this) is true[.](#variant.ctor-17.sentence-1)

[18](#variant.ctor-18)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5893)

*Throws*: Any exception thrown by the initialization of the selected alternative Tj[.](#variant.ctor-18.sentence-1)

[19](#variant.ctor-19)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5897)

*Remarks*: The exception specification is equivalent tois_nothrow_constructible_v<Tj, T>[.](#variant.ctor-19.sentence-1)

If Tj's selected constructor is a constexpr constructor,
this constructor is a constexpr constructor[.](#variant.ctor-19.sentence-2)

[🔗](#lib:variant,constructor____)

`template<class T, class... Args> constexpr explicit variant(in_place_type_t<T>, Args&&... args);
`

[20](#variant.ctor-20)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5911)

*Constraints*: 

- [(20.1)](#variant.ctor-20.1)

There is exactly one occurrence of T in Types... and

- [(20.2)](#variant.ctor-20.2)

is_constructible_v<T, Args...> is true[.](#variant.ctor-20.sentence-1)

[21](#variant.ctor-21)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5918)

*Effects*: Direct-non-list-initializes the contained value of type T with std​::​forward<Args>(args)...[.](#variant.ctor-21.sentence-1)

[22](#variant.ctor-22)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5923)

*Postconditions*: holds_alternative<T>(*this) is true[.](#variant.ctor-22.sentence-1)

[23](#variant.ctor-23)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5927)

*Throws*: Any exception thrown by calling the selected constructor of T[.](#variant.ctor-23.sentence-1)

[24](#variant.ctor-24)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5931)

*Remarks*: If T's selected constructor is a constexpr constructor, this
constructor is a constexpr constructor[.](#variant.ctor-24.sentence-1)

[🔗](#lib:variant,constructor_____)

`template<class T, class U, class... Args>
  constexpr explicit variant(in_place_type_t<T>, initializer_list<U> il, Args&&... args);
`

[25](#variant.ctor-25)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5944)

*Constraints*: 

- [(25.1)](#variant.ctor-25.1)

There is exactly one occurrence of T in Types... and

- [(25.2)](#variant.ctor-25.2)

is_constructible_v<T, initializer_list<U>&, Args...> is true[.](#variant.ctor-25.sentence-1)

[26](#variant.ctor-26)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5951)

*Effects*: Direct-non-list-initializes the contained value of type T with il, std​::​forward<Args>(​args)...[.](#variant.ctor-26.sentence-1)

[27](#variant.ctor-27)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5956)

*Postconditions*: holds_alternative<T>(*this) is true[.](#variant.ctor-27.sentence-1)

[28](#variant.ctor-28)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5960)

*Throws*: Any exception thrown by calling the selected constructor of T[.](#variant.ctor-28.sentence-1)

[29](#variant.ctor-29)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5964)

*Remarks*: If T's selected constructor is a constexpr constructor, this
constructor is a constexpr constructor[.](#variant.ctor-29.sentence-1)

[🔗](#lib:variant,constructor______)

`template<size_t I, class... Args> constexpr explicit variant(in_place_index_t<I>, Args&&... args);
`

[30](#variant.ctor-30)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5976)

*Constraints*: 

- [(30.1)](#variant.ctor-30.1)

I is less than sizeof...(Types) and

- [(30.2)](#variant.ctor-30.2)

is_constructible_v<TI, Args...> is true[.](#variant.ctor-30.sentence-1)

[31](#variant.ctor-31)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5985)

*Effects*: Direct-non-list-initializes the contained value of type TI with std​::​forward<Args>(args)...[.](#variant.ctor-31.sentence-1)

[32](#variant.ctor-32)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5990)

*Postconditions*: index() is I[.](#variant.ctor-32.sentence-1)

[33](#variant.ctor-33)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5994)

*Throws*: Any exception thrown by calling the selected constructor of TI[.](#variant.ctor-33.sentence-1)

[34](#variant.ctor-34)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L5998)

*Remarks*: If TI's selected constructor is a constexpr constructor, this
constructor is a constexpr constructor[.](#variant.ctor-34.sentence-1)

[🔗](#lib:variant,constructor_______)

`template<size_t I, class U, class... Args>
  constexpr explicit variant(in_place_index_t<I>, initializer_list<U> il, Args&&... args);
`

[35](#variant.ctor-35)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6011)

*Constraints*: 

- [(35.1)](#variant.ctor-35.1)

I is less than sizeof...(Types) and

- [(35.2)](#variant.ctor-35.2)

is_constructible_v<TI, initializer_list<U>&, Args...> is true[.](#variant.ctor-35.sentence-1)

[36](#variant.ctor-36)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6020)

*Effects*: Direct-non-list-initializes the contained value of type TI with il, std​::​forward<Args>(​args)...[.](#variant.ctor-36.sentence-1)

[37](#variant.ctor-37)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6025)

*Postconditions*: index() is I[.](#variant.ctor-37.sentence-1)

[38](#variant.ctor-38)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6029)

*Remarks*: If TI's selected constructor is a constexpr constructor, this
constructor is a constexpr constructor[.](#variant.ctor-38.sentence-1)

#### [22.6.3.3](#variant.dtor) Destructor [[variant.dtor]](variant.dtor)

[🔗](#lib:variant,destructor)

`constexpr ~variant();
`

[1](#variant.dtor-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6043)

*Effects*: If valueless_by_exception() is false,
destroys the currently contained value[.](#variant.dtor-1.sentence-1)

[2](#variant.dtor-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6048)

*Remarks*: If is_trivially_destructible_v<Ti> is true for all Ti,
then this destructor is trivial[.](#variant.dtor-2.sentence-1)

#### [22.6.3.4](#variant.assign) Assignment [[variant.assign]](variant.assign)

[🔗](#lib:operator=,variant)

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

[1](#variant.assign-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6062)

Let j be rhs.index()[.](#variant.assign-1.sentence-1)

[2](#variant.assign-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6065)

*Effects*: 

- [(2.1)](#variant.assign-2.1)

  If neither *this nor rhs holds a value, there is no effect[.](#variant.assign-2.1.sentence-1)

- [(2.2)](#variant.assign-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[.](#variant.assign-2.2.sentence-1)

- [(2.3)](#variant.assign-2.3)

  Otherwise, if index() == j, assigns the value contained in rhs to the value contained in *this[.](#variant.assign-2.3.sentence-1)

- [(2.4)](#variant.assign-2.4)

  Otherwise, if either is_nothrow_copy_constructible_v<Tj> is true oris_nothrow_move_constructible_v<Tj> is false,
equivalent to emplace<j>(*GET*<j>(rhs))[.](#variant.assign-2.4.sentence-1)

- [(2.5)](#variant.assign-2.5)

  Otherwise, equivalent to operator=(variant(rhs))[.](#variant.assign-2.5.sentence-1)

[3](#variant.assign-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6085)

*Postconditions*: index() == rhs.index()[.](#variant.assign-3.sentence-1)

[4](#variant.assign-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6089)

*Returns*: *this[.](#variant.assign-4.sentence-1)

[5](#variant.assign-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6093)

*Remarks*: This operator is defined as deleted unlessis_copy_constructible_v<Ti> &&is_copy_assignable_v<Ti> is true for all i[.](#variant.assign-5.sentence-1)

If is_trivially_copy_constructible_v<Ti> &&is_trivially_copy_assignable_v<Ti> &&is_trivially_destructible_v<Ti> is true for all i, this assignment operator is trivial[.](#variant.assign-5.sentence-2)

[🔗](#lib:operator=,variant_)

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

[6](#variant.assign-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6111)

Let j be rhs.index()[.](#variant.assign-6.sentence-1)

[7](#variant.assign-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6114)

*Constraints*: is_move_constructible_v<Ti> &&is_move_assignable_v<Ti> istrue for all i[.](#variant.assign-7.sentence-1)

[8](#variant.assign-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6120)

*Effects*: 

- [(8.1)](#variant.assign-8.1)

  If neither *this nor rhs holds a value, there is no effect[.](#variant.assign-8.1.sentence-1)

- [(8.2)](#variant.assign-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[.](#variant.assign-8.2.sentence-1)

- [(8.3)](#variant.assign-8.3)

  Otherwise, if index() == j, assigns *GET*<j>(std​::​move(rhs)) to
the value contained in *this[.](#variant.assign-8.3.sentence-1)

- [(8.4)](#variant.assign-8.4)

  Otherwise, equivalent to emplace<j>(*GET*<j>(std​::​move(rhs)))[.](#variant.assign-8.4.sentence-1)

[9](#variant.assign-9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6135)

*Returns*: *this[.](#variant.assign-9.sentence-1)

[10](#variant.assign-10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6139)

*Remarks*: If is_trivially_move_constructible_v<Ti> &&is_trivially_move_assignable_v<Ti> &&is_trivially_destructible_v<Ti> is true for all i, this assignment operator is trivial[.](#variant.assign-10.sentence-1)

The exception specification is equivalent tois_nothrow_move_constructible_v<Ti> && is_nothrow_move_assignable_v<Ti> for all i[.](#variant.assign-10.sentence-2)

- [(10.1)](#variant.assign-10.1)

  If an exception is thrown during the call to Tj's move construction
(with j being rhs.index()), the variant will hold no value[.](#variant.assign-10.1.sentence-1)

- [(10.2)](#variant.assign-10.2)

  If an exception is thrown during the call to Tj's move assignment,
the state of the contained value is as defined by the exception safety
guarantee of Tj's move assignment; index() will be j[.](#variant.assign-10.2.sentence-1)

[🔗](#lib:operator=,variant__)

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

[11](#variant.assign-11)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6162)

Let Tj be a type that is determined as follows:
build an imaginary function *FUN*(Ti) for each alternative type Ti for which Ti x[] = {std​::​forward<T>(t)}; is well-formed for some invented variable x[.](#variant.assign-11.sentence-1)

The overload *FUN*(Tj) selected by overload
resolution for the expression *FUN*(std​::​forward<T>(​t)) defines
the alternative Tj which is the type of the contained value after
assignment[.](#variant.assign-11.sentence-2)

[12](#variant.assign-12)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6173)

*Constraints*: 

- [(12.1)](#variant.assign-12.1)

is_same_v<remove_cvref_t<T>, variant> is false,

- [(12.2)](#variant.assign-12.2)

is_assignable_v<Tj&, T> && is_constructible_v<Tj, T> is true, and

- [(12.3)](#variant.assign-12.3)

the expression *FUN*(std​::​forward<T>(t)) (with *FUN* being the above-mentioned set
of imaginary functions) is well-formed[.](#variant.assign-12.sentence-1)
  [*Note [1](#variant.assign-note-1)*:
  variant<string, string> v;
v = "abc"; is ill-formed, as both alternative types have an equally viable constructor
for the argument[.](#variant.assign-12.3.sentence-2)
 — *end note*]

[13](#variant.assign-13)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6197)

*Effects*: 

- [(13.1)](#variant.assign-13.1)

  If *this holds a Tj, assigns std​::​forward<T>(t) to
the value contained in *this[.](#variant.assign-13.1.sentence-1)

- [(13.2)](#variant.assign-13.2)

  Otherwise, if is_nothrow_constructible_v<Tj, T> ||!is_nothrow_move_constructible_v<Tj> is true,
equivalent to emplace<j>(std​::​forward<T>(t))[.](#variant.assign-13.2.sentence-1)

- [(13.3)](#variant.assign-13.3)

  Otherwise, equivalent to emplace<j>(Tj(std​::​forward<T>(t)))[.](#variant.assign-13.3.sentence-1)

[14](#variant.assign-14)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6211)

*Postconditions*: holds_alternative<Tj>(*this) is true, with Tj selected by the imaginary function overload resolution described above[.](#variant.assign-14.sentence-1)

[15](#variant.assign-15)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6216)

*Returns*: *this[.](#variant.assign-15.sentence-1)

[16](#variant.assign-16)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6220)

*Remarks*: The exception specification is equivalent to:is_nothrow_assignable_v<Tj&, T> && is_nothrow_constructible_v<Tj, T>

- [(16.1)](#variant.assign-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 andt are as defined by the exception safety guarantee of the assignment
expression; valueless_by_exception() will be false[.](#variant.assign-16.1.sentence-1)

- [(16.2)](#variant.assign-16.2)

  If an exception is thrown during the initialization of the contained value,
the variant object is permitted to not hold a value[.](#variant.assign-16.2.sentence-1)

#### [22.6.3.5](#variant.mod) Modifiers [[variant.mod]](variant.mod)

[🔗](#lib:emplace,variant)

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

[1](#variant.mod-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6244)

*Constraints*: is_constructible_v<T, Args...> is true, andT occurs exactly once in Types[.](#variant.mod-1.sentence-1)

[2](#variant.mod-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6249)

*Effects*: Equivalent to:return emplace<I>(std::forward<Args>(args)...); where I is the zero-based index of T in Types[.](#variant.mod-2.sentence-1)

[🔗](#lib:emplace,variant_)

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

[3](#variant.mod-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6265)

*Constraints*: is_constructible_v<T, initializer_list<U>&, Args...> is true,
and T occurs exactly once in Types[.](#variant.mod-3.sentence-1)

[4](#variant.mod-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6270)

*Effects*: Equivalent to:return emplace<I>(il, std::forward<Args>(args)...); where I is the zero-based index of T in Types[.](#variant.mod-4.sentence-1)

[🔗](#lib:emplace,variant__)

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

[5](#variant.mod-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6286)

*Mandates*: I < sizeof...(Types)[.](#variant.mod-5.sentence-1)

[6](#variant.mod-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6290)

*Constraints*: is_constructible_v<TI, Args...> is true[.](#variant.mod-6.sentence-1)

[7](#variant.mod-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6294)

*Effects*: Destroys the currently contained value if valueless_by_exception() is false[.](#variant.mod-7.sentence-1)

Then direct-non-list-initializes the contained value of type TI with the arguments std​::​forward<Args>(args)...[.](#variant.mod-7.sentence-2)

[8](#variant.mod-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6301)

*Postconditions*: index() is I[.](#variant.mod-8.sentence-1)

[9](#variant.mod-9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6305)

*Returns*: A reference to the new contained value[.](#variant.mod-9.sentence-1)

[10](#variant.mod-10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6309)

*Throws*: Any exception thrown during the initialization of the contained value[.](#variant.mod-10.sentence-1)

[11](#variant.mod-11)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6313)

*Remarks*: If an exception is thrown during the initialization of the contained value,
the variant is permitted to not hold a value[.](#variant.mod-11.sentence-1)

[🔗](#lib:emplace,variant___)

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

[12](#variant.mod-12)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6327)

*Mandates*: I < sizeof...(Types)[.](#variant.mod-12.sentence-1)

[13](#variant.mod-13)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6331)

*Constraints*: is_constructible_v<TI, initializer_list<U>&, Args...> is true[.](#variant.mod-13.sentence-1)

[14](#variant.mod-14)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6335)

*Effects*: Destroys the currently contained value if valueless_by_exception() is false[.](#variant.mod-14.sentence-1)

Then direct-non-list-initializes the contained value of type TI with il, std​::​forward<Args>(args)...[.](#variant.mod-14.sentence-2)

[15](#variant.mod-15)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6342)

*Postconditions*: index() is I[.](#variant.mod-15.sentence-1)

[16](#variant.mod-16)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6346)

*Returns*: A reference to the new contained value[.](#variant.mod-16.sentence-1)

[17](#variant.mod-17)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6350)

*Throws*: Any exception thrown during the initialization of the contained value[.](#variant.mod-17.sentence-1)

[18](#variant.mod-18)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6354)

*Remarks*: If an exception is thrown during the initialization of the contained value,
the variant is permitted to not hold a value[.](#variant.mod-18.sentence-1)

#### [22.6.3.6](#variant.status) Value status [[variant.status]](variant.status)

[🔗](#lib:valueless_by_exception,variant)

`constexpr bool valueless_by_exception() const noexcept;
`

[1](#variant.status-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6368)

*Effects*: Returns false if and only if the variant holds a value[.](#variant.status-1.sentence-1)

[2](#variant.status-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6372)

[*Note [1](#variant.status-note-1)*:

It is possible for a variant to hold no value
if an exception is thrown during a
type-changing assignment or emplacement[.](#variant.status-2.sentence-1)

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

— *end note*]

[🔗](#lib:index,variant)

`constexpr size_t index() const noexcept;
`

[3](#variant.status-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6393)

*Effects*: If valueless_by_exception() is true, returns variant_npos[.](#variant.status-3.sentence-1)

Otherwise, returns the zero-based index of the alternative of the contained value[.](#variant.status-3.sentence-2)

#### [22.6.3.7](#variant.swap) Swap [[variant.swap]](variant.swap)

[🔗](#lib:swap,variant)

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

[1](#variant.swap-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6407)

*Mandates*: is_move_constructible_v<Ti> is true for all i[.](#variant.swap-1.sentence-1)

[2](#variant.swap-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6411)

*Preconditions*: Each Ti meets the [*Cpp17Swappable*](swappable.requirements#:Cpp17Swappable "16.4.4.3 Swappable requirements [swappable.requirements]") requirements ([[swappable.requirements]](swappable.requirements "16.4.4.3 Swappable requirements"))[.](#variant.swap-2.sentence-1)

[3](#variant.swap-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6415)

*Effects*: 

- [(3.1)](#variant.swap-3.1)

  If valueless_by_exception() && rhs.valueless_by_exception() no effect[.](#variant.swap-3.1.sentence-1)

- [(3.2)](#variant.swap-3.2)

  Otherwise, if index() == rhs.index(), calls swap(*GET*<i>(*this), *GET*<i>(rhs)) where i is index()[.](#variant.swap-3.2.sentence-1)

- [(3.3)](#variant.swap-3.3)

  Otherwise, exchanges values of rhs and *this[.](#variant.swap-3.3.sentence-1)

[4](#variant.swap-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6426)

*Throws*: If index() == rhs.index(),
any exception thrown by swap(*GET*<i>(*this), *GET*<i>(rhs)) with i being index()[.](#variant.swap-4.sentence-1)

Otherwise, any exception thrown by the move constructor
of Ti or Tj with i being index() and j being rhs.index()[.](#variant.swap-4.sentence-2)

[5](#variant.swap-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6435)

*Remarks*: If an exception is thrown during the call to function swap(*GET*<i>(*this), *GET*<i>(rhs)),
the states of the contained values of *this and of rhs are
determined by the exception safety guarantee of swap for lvalues ofTi with i being index()[.](#variant.swap-5.sentence-1)

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[.](#variant.swap-5.sentence-2)

The exception specification is equivalent to the logical and ofis_nothrow_move_constructible_v<Ti> && is_nothrow_swappable_v<Ti> for all i[.](#variant.swap-5.sentence-3)