23 General utilities library [utilities]

23.7 Variants [variant]

23.7.3 Class template variant [variant.variant]

namespace std {
  template <class... Types>
    class variant {
    public:
      // [variant.ctor], constructors
      constexpr variant() noexcept(see below);
      variant(const variant&);
      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&&...);

      // allocator-extended constructors
      template <class Alloc>
        variant(allocator_arg_t, const Alloc&);
      template <class Alloc>
        variant(allocator_arg_t, const Alloc&, const variant&);
      template <class Alloc>
        variant(allocator_arg_t, const Alloc&, variant&&);
      template <class Alloc, class T>
        variant(allocator_arg_t, const Alloc&, T&&);
      template <class Alloc, class T, class... Args>
        variant(allocator_arg_t, const Alloc&, in_place_type_t<T>, Args&&...);
      template <class Alloc, class T, class U, class... Args>
        variant(allocator_arg_t, const Alloc&, in_place_type_t<T>,
                initializer_list<U>, Args&&...);
      template <class Alloc, size_t I, class... Args>
        variant(allocator_arg_t, const Alloc&, in_place_index_t<I>, Args&&...);
      template <class Alloc, size_t I, class U, class... Args>
        variant(allocator_arg_t, const Alloc&, in_place_index_t<I>,
                initializer_list<U>, Args&&...);

      // [variant.dtor], destructor
      ~variant();

      // [variant.assign], assignment
      variant& operator=(const variant&);
      variant& operator=(variant&&) noexcept(see below);

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

      // [variant.mod], modifiers
      template <class T, class... Args>
        T& emplace(Args&&...);
      template <class T, class U, class... Args>
        T& emplace(initializer_list<U>, Args&&...);
      template <size_t I, class... Args>
        variant_alternative_t<I, variant<Types...>>& emplace(Args&&...);
      template <size_t I, class U, class... Args>
        variant_alternative_t<I, variant<Types...>>& emplace(initializer_list<U>, Args&&...);

      // [variant.status], value status
      constexpr bool valueless_by_exception() const noexcept;
      constexpr size_t index() const noexcept;

      // [variant.swap], swap
      void swap(variant&) noexcept(see below);
    };
}

Any instance of variant at any given time either holds a value of one of its alternative types, or it 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 allocated within the storage of the variant object.

Implementations are not permitted to use additional storage, such as dynamic memory, to allocate the contained value.

The contained value shall be allocated in a region of the variant storage suitably aligned for all types in Types....

It is implementation-defined whether over-aligned types are supported.

All types in Types... shall be (possibly cv-qualified) object types that are not arrays.

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

23.7.3.1 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);

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 shall be constexpr if and only if the value-initialization of the alternative type T

_{0}

would satisfy the requirements for a constexpr function.

The expression inside noexcept is equivalent to is_nothrow_default_constructible_v<T

_{0}

This function shall not participate in overload resolution unless is_default_constructible_v<T

_{0}

> is true.

[ Note

See also class monostate.

— end note

]

🔗

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 function shall not participate in overload resolution unless is_copy_constructible_v<T

_{i}

> is true for all i.

🔗

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

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 expression inside noexcept is equivalent to the logical AND of is_nothrow_move_constructible_v<T

_{i}

> for all i.

This function shall not participate in overload resolution unless is_move_constructible_v<T

_{i}

> is true for all i.

🔗

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}

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.

Effects: Initializes *this to hold the alternative type T

_{j}

and direct-initializes the contained value as if direct-non-list-initializing it 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: This function shall not participate in overload resolution unless is_same_v<decay_t<T>, variant> is false, unless decay_t<T> is neither a specialization of in_place_type_t nor a specialization of in_place_index_t, unless is_constructible_v<T

_{j}

, T> is true, and unless the expression FUN(std::forward<T>(t)) (with FUN being the above-mentioned set of imaginary functions) is well formed.

[ Note

variant<string, string> v("abc");

is ill-formed, as both alternative types have an equally viable constructor for the argument.

— end note

]

The expression inside noexcept is equivalent to is_nothrow_constructible_v<T

_{j}

, T>.

If T

_{j}

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

🔗

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

Effects: Initializes the contained value as if direct-non-list-initializing an object of type T with the arguments std::forward<Args>(args)....

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

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

Remarks: This function shall not participate in overload resolution unless there is exactly one occurrence of T in Types... and is_constructible_v<T, Args...> is true.

If T's selected constructor is a constexpr constructor, this constructor shall be a constexpr constructor.

🔗

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

Effects: Initializes the contained value as if direct-non-list-initializing an object of type T with the arguments il, std::forward<Args>(args)....

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

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

Remarks: This function shall not participate in overload resolution unless there is exactly one occurrence of T in Types... and is_constructible_v<T, initializer_list&, Args...> is true.

If T's selected constructor is a constexpr constructor, this constructor shall be a constexpr constructor.

🔗

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

Effects: Initializes the contained value as if direct-non-list-initializing an object of type T

_{I}

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

Postconditions: index() is I.

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

_{I}

Remarks: This function shall not participate in overload resolution unless

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

If T

_{I}

's selected constructor is a constexpr constructor, this constructor shall be 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);

Effects: Initializes the contained value as if direct-non-list-initializing an object of type T

_{I}

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

Postconditions: index() is I.

Remarks: This function shall not participate in overload resolution unless

(33.1)
I is less than sizeof...(Types) and
(33.2)
is_constructible_v<T $_{I}$ , initializer_list&, Args...> is true.

If T

_{I}

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

🔗

// allocator-extended constructors
template <class Alloc>
  variant(allocator_arg_t, const Alloc& a);
template <class Alloc>
  variant(allocator_arg_t, const Alloc& a, const variant& v);
template <class Alloc>
  variant(allocator_arg_t, const Alloc& a, variant&& v);
template <class Alloc, class T>
  variant(allocator_arg_t, const Alloc& a, T&& t);
template <class Alloc, class T, class... Args>
  variant(allocator_arg_t, const Alloc& a, in_place_type_t<T>, Args&&... args);
template <class Alloc, class T, class U, class... Args>
  variant(allocator_arg_t, const Alloc& a, in_place_type_t<T>,
          initializer_list<U> il, Args&&... args);
template <class Alloc, size_t I, class... Args>
  variant(allocator_arg_t, const Alloc& a, in_place_index_t<I>, Args&&... args);
template <class Alloc, size_t I, class U, class... Args>
  variant(allocator_arg_t, const Alloc& a, in_place_index_t<I>,
          initializer_list<U> il, Args&&... args);

Requires: Alloc shall meet the requirements for an Allocator ([allocator.requirements]).

Effects: Equivalent to the preceding constructors except that the contained value is constructed with uses-allocator construction ([allocator.uses.construction]).

23.7.3.2 Destructor [variant.dtor]

🔗

~variant();

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

Remarks: If is_trivially_destructible_v<T

_{i}

> == true for all T

_{i}

then this destructor shall be a trivial destructor.

23.7.3.3 Assignment [variant.assign]

🔗

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.

Otherwise,
(2.2)
if *this holds a value but rhs does not, destroys the value contained in *this and sets *this to not hold a value.

Otherwise,
(2.3)
if index() == j, assigns the value contained in rhs to the value contained in *this.

Otherwise,
(2.4)
if either is_nothrow_copy_constructible_v<T $_{j}$ > or !is_nothrow_move_constructible_v<T $_{j}$ > is true, equivalent to emplace<j>(get<j>(rhs)).

Otherwise,
(2.5)
equivalent to operator=(variant(rhs)).

Returns: *this.

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

Remarks: This function shall not participate in overload resolution unless is_copy_constructible_v<T

_{i}

> && is_copy_assignable_v<T

_{i}

> is true for all i.

🔗

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

Let j be rhs.index().

Effects:

(7.1)
If neither *this nor rhs holds a value, there is no effect.

Otherwise,
(7.2)
if *this holds a value but rhs does not, destroys the value contained in *this and sets *this to not hold a value.

Otherwise,
(7.3)
if index() == j, assigns get<j>(std::move(rhs)) to the value contained in *this.

Otherwise,
(7.4)
equivalent to emplace<j>(get<j>(std::move(rhs))).

Returns: *this.

Remarks: This function shall not participate in overload resolution unless is_move_constructible_v<T

_{i}

> && is_move_assignable_v<T

_{i}

> is true for all i.

The expression inside noexcept is equivalent to: is_nothrow_move_constructible_v<T

_{i}

> && is_nothrow_move_assignable_v<T

_{i}

> for all i.

(9.1)
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.
(9.2)
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> 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}

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.

Effects:

(11.1)
If *this holds a T $_{j}$ , assigns std::forward<T>(t) to the value contained in *this.

Otherwise,
(11.2)
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)).

Otherwise,
(11.3)
equivalent to operator=(variant(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: This function shall not participate in overload resolution unless is_same_v<decay_t<T>, variant> is false, unless is_assignable_v<T

_{j}

&, T> && is_constructible_v<T

_{j}

, T> is true, and unless the expression FUN(std::forward<T>(t)) (with FUN being the above-mentioned set of imaginary functions) is well formed.

[ Note

variant<string, string> v;
v = "abc";

is ill-formed, as both alternative types have an equally viable constructor for the argument.

— end note

]

The expression inside noexcept 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 might not hold a value.

23.7.3.4 Modifiers [variant.mod]

🔗

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

Let I be the zero-based index of T in Types....

Effects: Equivalent to: return emplace(std::forward<Args>(args)...);

Remarks: This function shall not participate in overload resolution unless is_constructible_v<T, Args...> is true, and T occurs exactly once in Types....

🔗

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

Let I be the zero-based index of T in Types....

Effects: Equivalent to: return emplace(il, std::forward<Args>(args)...);

Remarks: This function shall not participate in overload resolution unless is_constructible_v<T, initializer_list&, Args...> is true, and T occurs exactly once in Types....

🔗

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

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

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

Then initializes the contained value as if direct-non-list-initializing a 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: This function shall not participate in overload resolution unless is_constructible_v<T

_{I}

, Args...> is true.

If an exception is thrown during the initialization of the contained value, the variant might not hold a value.

🔗

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

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

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

Then initializes the contained value as if direct-non-list-initializing a value of type T

_{I}

with the arguments 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: This function shall not participate in overload resolution unless is_constructible_v<T

_{I}

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

If an exception is thrown during the initialization of the contained value, the variant might not hold a value.

23.7.3.5 Value status [variant.status]

🔗

constexpr bool valueless_by_exception() const noexcept;

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

[ Note

A variant might not hold a 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.

23.7.3.6 Swap [variant.swap]

🔗

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

Requires: Lvalues of type T

_{i}

shall be swappable ([swappable.requirements]) and is_move_constructible_v<T

_{i}

> shall be true for all i.

Effects:

(2.1)
if valueless_by_exception() && rhs.valueless_by_exception() no effect.

Otherwise,
(2.2)
if index() == rhs.index(), calls swap(get(*this), get(rhs)) where i is index().

Otherwise,
(2.3)
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}

or 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 expression inside noexcept is equivalent to the logical AND of is_nothrow_move_constructible_v<T

_{i}

> && is_nothrow_swappable_v<T

_{i}

> for all i.