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> struct variant_size<volatile T>; template <class T> struct variant_size<const volatile T>; template <class T> inline 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> struct variant_alternative<I, volatile T>; template <size_t I, class T> struct variant_alternative<I, const volatile 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...>&); template <size_t I, class... Types> constexpr variant_alternative_t<I, variant<Types...>>&& get(variant<Types...>&&); template <size_t I, class... Types> constexpr const variant_alternative_t<I, variant<Types...>>& get(const variant<Types...>&); template <size_t I, class... Types> constexpr const variant_alternative_t<I, variant<Types...>>&& get(const variant<Types...>&&); template <class T, class... Types> constexpr T& get(variant<Types...>&); template <class T, class... Types> constexpr T&& get(variant<Types...>&&); template <class T, class... Types> constexpr const T& get(const variant<Types...>&); template <class T, class... Types> constexpr const T&& get(const variant<Types...>&&); 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...>&); // [variant.visit], visitation template <class Visitor, class... Variants> constexpr see below visit(Visitor&&, Variants&&...); // [variant.monostate], class monostate struct monostate; // [variant.monostate.relops], monostate relational operators constexpr bool operator<(monostate, monostate) noexcept; constexpr bool operator>(monostate, monostate) noexcept; constexpr bool operator<=(monostate, monostate) noexcept; constexpr bool operator>=(monostate, monostate) noexcept; constexpr bool operator==(monostate, monostate) noexcept; constexpr bool operator!=(monostate, monostate) noexcept; // [variant.specalg], specialized algorithms template <class... Types> 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>; // [variant.traits], allocator-related traits template <class T, class Alloc> struct uses_allocator; template <class... Types, class Alloc> struct uses_allocator<variant<Types...>, Alloc>; }
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.
In the descriptions that follow, let i be in the range [0, sizeof...(Types)), and Ti be the ith type in Types....
constexpr variant() noexcept(see below);
Remarks: This function shall be constexpr if and only if the value-initialization of the alternative type T0 would satisfy the requirements for a constexpr function. The expression inside noexcept is equivalent to is_nothrow_default_constructible_v<T0>. This function shall not participate in overload resolution unless is_default_constructible_v<T0> 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.
Remarks: This function shall not participate in overload resolution unless is_copy_constructible_v<Ti> 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.
Remarks: The expression inside noexcept is equivalent to the logical AND of is_nothrow_move_constructible_v<Ti> for all i. This function shall not participate in overload resolution unless is_move_constructible_v<Ti> is true for all i.
template <class T> constexpr variant(T&& t) noexcept(see below);
Let Tj be a type that is determined as follows: build an imaginary function FUN(Ti) for each alternative type Ti. The overload FUN(Tj) selected by overload resolution for the expression FUN(std::forward<T>(t)) defines the alternative Tj which is the type of the contained value after construction.
Effects: Initializes *this to hold the alternative type Tj and direct-initializes the contained value as if direct-non-list-initializing it with std::forward<T>(t).
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<Tj, 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<Tj, T>. If Tj'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)....
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)....
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<U>&, 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 TI with the arguments std::forward<Args>(args)....
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 TI with the arguments il, std::forward<Args>(args)....
// 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);
Effects: Equivalent to the preceding constructors except that the contained value is constructed with uses-allocator construction.
~variant();
variant& operator=(const variant& rhs);
Effects:
If neither *this nor rhs holds a value, there is no effect. Otherwise,
if *this holds a value but rhs does not, destroys the value contained in *this and sets *this to not hold a value. Otherwise,
if index() == j, assigns the value contained in rhs to the value contained in *this. Otherwise,
if either is_nothrow_copy_constructible_v<Tj> or !is_nothrow_move_constructible_v<Tj> is true, equivalent to emplace<j>(get<j>(rhs)). Otherwise,
equivalent to operator=(variant(rhs)).
Remarks: This function shall not participate in overload resolution unless is_copy_constructible_v<Ti> && is_copy_assignable_v<Ti> is true for all i.
variant& operator=(variant&& rhs) noexcept(see below);
Effects:
If neither *this nor rhs holds a value, there is no effect. Otherwise,
if *this holds a value but rhs does not, destroys the value contained in *this and sets *this to not hold a value. Otherwise,
if index() == j, assigns get<j>(std::move(rhs)) to the value contained in *this. Otherwise,
equivalent to emplace<j>(get<j>(std::move(rhs))).
Remarks: This function shall not participate in overload resolution unless is_move_constructible_v<Ti> && is_move_assignable_v<Ti> is true for all i. The expression inside noexcept is equivalent to: is_nothrow_move_constructible_v<Ti> && is_nothrow_move_assignable_v<Ti> for all i.
If an exception is thrown during the call to Tj's move construction (with j being rhs.index()), the variant will hold no value.
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.
template <class T> variant& operator=(T&& t) noexcept(see below);
Let Tj be a type that is determined as follows: build an imaginary function FUN(Ti) for each alternative type Ti. The overload FUN(Tj) selected by overload resolution for the expression FUN(std::forward<T>(t)) defines the alternative Tj which is the type of the contained value after assignment.
Effects:
If *this holds a Tj, assigns std::forward<T>(t) to the value contained in *this. Otherwise,
if is_nothrow_constructible_v<Tj, T> || !is_nothrow_move_constructible_v<Tj> is true, equivalent to emplace<j>(std::forward<T>(t)). Otherwise,
equivalent to operator=(variant(std::forward<T>(t))).
Postconditions: holds_alternative<Tj>(*this) is true, with Tj selected by the imaginary function overload resolution described above.
Remarks: This function shall not participate in overload resolution unless is_same_v<decay_t<T>, variant> is false, unless is_assignable_v<Tj&, T> && is_constructible_v<Tj, 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<Tj&, T> && is_nothrow_constructible_v<Tj, T>
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.
If an exception is thrown during the initialization of the contained value, the variant object might not hold a value.
template <class T, class... Args> T& emplace(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);
Remarks: This function shall not participate in overload resolution unless is_constructible_v<T, initializer_list<U>&, 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);
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 TI with the arguments std::forward<Args>(args)....
Remarks: This function shall not participate in overload resolution unless is_constructible_v<TI, 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);
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 TI with the arguments il, std::forward<Args>(args)....
constexpr bool valueless_by_exception() const noexcept;
[ 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;
void swap(variant& rhs) noexcept(see below);
Requires: Lvalues of type Ti shall be swappable ([swappable.requirements]) and is_move_constructible_v<Ti> shall be true for all i.
Throws: If index() == rhs.index(), any exception thrown by swap(get<i>(*this), get<i>(rhs)) with i being index(). Otherwise, any exception thrown by the move constructor of Ti or Tj with i being index() and j being rhs.index().
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 of Ti 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<Ti> && is_nothrow_swappable_v<Ti> for all i.
template <class T> struct variant_size;
Remarks: All specializations of variant_size shall meet the UnaryTypeTrait requirements with a base characteristic of integral_constant<size_t, N> for some N.
template <class T> class variant_size<const T>;
template <class T> class variant_size<volatile T>;
template <class T> class variant_size<const volatile T>;
Let VS denote variant_size<T> of the cv-unqualified type T. Then each of the three templates shall meet the UnaryTypeTrait requirements 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> class variant_alternative<I, const T>;
template <size_t I, class T> class variant_alternative<I, volatile T>;
template <size_t I, class T> class variant_alternative<I, const volatile T>;
Let VA denote variant_alternative<I, T> of the cv-unqualified type T. Then each of the three templates shall meet the TransformationTrait requirements with a member typedef type that names the following type:
for the first specialization, add_const_t<VA::type>,
for the second specialization, add_volatile_t<VA::type>, and
for the third specialization, add_cv_t<VA::type>.
variant_alternative<I, variant<Types...>>::type
template <class T, class... Types>
constexpr bool holds_alternative(const variant<Types...>& v) noexcept;
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);
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);
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;
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;
template <class... Types>
constexpr bool operator==(const variant<Types...>& v, const variant<Types...>& w);
Requires: get<i>(v) == get<i>(w) is a valid expression returning a type that is convertible to bool, for all i.
Returns: If v.index() != w.index(), false; otherwise if v.valueless_by_exception(), true; otherwise get<i>(v) == get<i>(w) with i being v.index().
template <class... Types>
constexpr bool operator!=(const variant<Types...>& v, const variant<Types...>& w);
Requires: get<i>(v) != get<i>(w) is a valid expression returning a type that is convertible to bool, for all i.
Returns: If v.index() != w.index(), true; otherwise if v.valueless_by_exception(), false; otherwise get<i>(v) != get<i>(w) with i being v.index().
template <class... Types>
constexpr bool operator<(const variant<Types...>& v, const variant<Types...>& w);
Requires: get<i>(v) < get<i>(w) is a valid expression returning a type 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<i>(v) < get<i>(w) with i being v.index().
template <class... Types>
constexpr bool operator>(const variant<Types...>& v, const variant<Types...>& w);
Requires: get<i>(v) > get<i>(w) is a valid expression returning a type 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<i>(v) > get<i>(w) with i being v.index().
template <class... Types>
constexpr bool operator<=(const variant<Types...>& v, const variant<Types...>& w);
Requires: get<i>(v) <= get<i>(w) is a valid expression returning a type 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<i>(v) <= get<i>(w) with i being v.index().
template <class... Types>
constexpr bool operator>=(const variant<Types...>& v, const variant<Types...>& w);
Requires: get<i>(v) >= get<i>(w) is a valid expression returning a type that is convertible to bool, for all i.
template <class Visitor, class... Variants>
constexpr see below visit(Visitor&& vis, Variants&&... vars);
Requires: The expression in the Effects: element shall be a valid expression of the same type and value category, for all combinations of alternative types of all variants. Otherwise, the program is ill-formed.
Effects: Let is... be vars.index().... Returns INVOKE(forward<Visitor>(vis), get<is>( forward<Variants>(vars))...);.
Remarks: The return type is the common type of all possible INVOKE expressions of the Effects: element.
struct monostate{};
constexpr bool operator<(monostate, monostate) noexcept { return false; }
constexpr bool operator>(monostate, monostate) noexcept { return false; }
constexpr bool operator<=(monostate, monostate) noexcept { return true; }
constexpr bool operator>=(monostate, monostate) noexcept { return true; }
constexpr bool operator==(monostate, monostate) noexcept { return true; }
constexpr bool operator!=(monostate, monostate) noexcept { return false; }
template <class... Types>
void swap(variant<Types...>& v, variant<Types...>& w) noexcept(see below);
Remarks: This function shall not participate in overload resolution unless is_move_constructible_v<Ti> && is_swappable_v<Ti> is true for all i. The expression inside noexcept is equivalent to noexcept(v.swap(w)).
class bad_variant_access : public exception { public: bad_variant_access() noexcept; 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.
bad_variant_access() noexcept;
const char* what() const noexcept override;
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.
template <> struct hash<monostate>;
The specialization is enabled ([unord.hash]).
template <class... Types, class Alloc>
struct uses_allocator<variant<Types...>, Alloc> : true_type { };