22 General utilities library [utilities]

22.6 Variants [variant]

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:

See also class monostate.

— end note]

🔗

constexpr variant(const variant& w);

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().

Otherwise, initializes the variant to not hold a value.

Throws: Any exception thrown by direct-initializing any

T_{i}

for all i.

Remarks: This constructor is defined as deleted unless is_copy_constructible_v<

T_{i}

> is true for all i.

If is_trivially_copy_constructible_v<

T_{i}

> is true for all i, this constructor is trivial.

🔗

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

Constraints: is_move_constructible_v<

T_{i}

> is true for all i.

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().

Otherwise, initializes the variant to not hold a value.

Throws: Any exception thrown by move-constructing any

T_{i}

for all i.

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

T_{i}

> for all i.

If is_trivially_move_constructible_v<

T_{i}

> is true for all i, this constructor is trivial.

🔗

template<class T> constexpr variant(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 construction.

Constraints:

(15.1)
sizeof...(Types) is nonzero,
(15.2)
is_same_v<remove_cvref_t<T>, variant> is false,
(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)
is_constructible_v< $T_{j}$ , T> is true, and
(15.5)
the expression FUN(std::forward<T>(t)) (with FUN being the above-mentioned set of imaginary functions) is well-formed.
[Note 2:
variant<string, string> v("abc"); is ill-formed, as both alternative types have an equally viable constructor for the argument.
— end note]

Effects: Initializes *this to hold the alternative type

T_{j}

and direct-non-list-initializes the contained value with std::forward<T>(t).

Postconditions: holds_alternative<

T_{j}

>(*this) is true.

Throws: Any exception thrown by the initialization of the selected alternative

T_{j}

Remarks: The exception specification is equivalent to is_nothrow_constructible_v<

T_{j}

, T>.

T_{j}

's selected constructor is a constexpr constructor, this constructor is a constexpr constructor.

🔗

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

Constraints:

(20.1)
There is exactly one occurrence of T in Types... and
(20.2)
is_constructible_v<T, Args...> is true.

Effects: Direct-non-list-initializes the contained value of type T with std::forward<Args>(args)....

Postconditions: holds_alternative<T>(*this) is true.

Throws: Any exception thrown by calling the selected constructor of T.

Remarks: If T's selected constructor is a constexpr constructor, this constructor is a constexpr constructor.

🔗

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

Constraints:

(25.1)
There is exactly one occurrence of T in Types... and
(25.2)
is_constructible_v<T, initializer_list&, Args...> is true.

Effects: Direct-non-list-initializes the contained value of type T with il, std::forward<Args>(args)....

Postconditions: holds_alternative<T>(*this) is true.

Throws: Any exception thrown by calling the selected constructor of T.

Remarks: If T's selected constructor is a constexpr constructor, this constructor is a constexpr constructor.

🔗

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

Constraints:

(30.1)
I is less than sizeof...(Types) and
(30.2)
is_constructible_v< $T_{I}$ , Args...> is true.

Effects: Direct-non-list-initializes the contained value of type

T_{I}

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

Postconditions: index() is I.

Throws: Any exception thrown by calling the selected constructor of

T_{I}

Remarks: If

T_{I}

's selected constructor is a constexpr constructor, this constructor is a constexpr constructor.

🔗

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

Constraints:

(35.1)
I is less than sizeof...(Types) and
(35.2)
is_constructible_v< $T_{I}$ , initializer_list&, Args...> is true.

Effects: Direct-non-list-initializes the contained value of type

T_{I}

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

Postconditions: index() is I.

Remarks: If

T_{I}

's selected constructor is a constexpr constructor, this constructor is a constexpr constructor.

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.

🔗

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.

🔗

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]

🔗

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.