[optional.optional] (original) (raw)
22 General utilities library [utilities]
22.5 Optional objects [optional]
22.5.3 Class template optional [optional.optional]
22.5.3.1 General [optional.optional.general]
22.5.3.2 Constructors [optional.ctor]
22.5.3.3 Destructor [optional.dtor]
22.5.3.4 Assignment [optional.assign]
22.5.3.5 Swap [optional.swap]
22.5.3.6 Iterator support [optional.iterators]
22.5.3.7 Observers [optional.observe]
22.5.3.8 Monadic operations [optional.monadic]
22.5.3.9 Modifiers [optional.mod]
22.5.3.1 General [optional.optional.general]
namespace std { template<class T> class optional { public: using value_type = T;using iterator = implementation-defined; using const_iterator = implementation-defined; constexpr optional() noexcept;constexpr optional(nullopt_t) noexcept;constexpr optional(const optional&);constexpr optional(optional&&) noexcept(see below);template<class... Args> constexpr explicit optional(in_place_t, Args&&...);template<class U, class... Args> constexpr explicit optional(in_place_t, initializer_list<U>, Args&&...);template<class U = remove_cv_t<T>> constexpr explicit(see below) optional(U&&);template<class U> constexpr explicit(see below) optional(const optional<U>&);template<class U> constexpr explicit(see below) optional(optional<U>&&);constexpr ~optional();constexpr optional& operator=(nullopt_t) noexcept;constexpr optional& operator=(const optional&);constexpr optional& operator=(optional&&) noexcept(see below);template<class U = remove_cv_t<T>> constexpr optional& operator=(U&&);template<class U> constexpr optional& operator=(const optional<U>&);template<class U> constexpr optional& operator=(optional<U>&&);template<class... Args> constexpr T& emplace(Args&&...);template<class U, class... Args> constexpr T& emplace(initializer_list<U>, Args&&...);constexpr void swap(optional&) noexcept(see below);constexpr iterator begin() noexcept;constexpr const_iterator begin() const noexcept;constexpr iterator end() noexcept;constexpr const_iterator end() const noexcept;constexpr const T* operator->() const noexcept;constexpr T* operator->() noexcept;constexpr const T& operator*() const & noexcept;constexpr T& operator*() & noexcept;constexpr T&& operator*() && noexcept;constexpr const T&& operator*() const && noexcept;constexpr explicit operator bool() const noexcept;constexpr bool has_value() const noexcept;constexpr const T& value() const &; constexpr T& value() &; constexpr T&& value() &&; constexpr const T&& value() const &&; template<class U = remove_cv_t<T>> constexpr T value_or(U&&) const &;template<class U = remove_cv_t<T>> constexpr T value_or(U&&) &&;template<class F> constexpr auto and_then(F&& f) &;template<class F> constexpr auto and_then(F&& f) &&;template<class F> constexpr auto and_then(F&& f) const &;template<class F> constexpr auto and_then(F&& f) const &&;template<class F> constexpr auto transform(F&& f) &;template<class F> constexpr auto transform(F&& f) &&;template<class F> constexpr auto transform(F&& f) const &;template<class F> constexpr auto transform(F&& f) const &&;template<class F> constexpr optional or_else(F&& f) &&;template<class F> constexpr optional or_else(F&& f) const &;constexpr void reset() noexcept;private: union { remove_cv_t<T> val; };};template<class T> optional(T) -> optional<T>;}
An instance of optional<T> is said tocontain a valuewhen and only when its member val is active ([class.union.general]);val is referred to as itscontained value.
An optional object's contained value is nested within ([intro.object]) the optional object.
A type X is avalid contained type for optionalif X is an lvalue reference type or a complete non-array object type, and remove_cvref_t<X> is a type other than in_place_t or nullopt_t.
If a specialization of optional is instantiated with a type Tthat is not a valid contained type for optional, the program is ill-formed.
If T is an object type,T shall meet the Cpp17Destructible requirements (Table 35).
22.5.3.2 Constructors [optional.ctor]
The exposition-only variable template _converts-from-any-cvref_is used by some constructors for optional.
template<class T, class W> constexpr bool converts-from-any-cvref = disjunction_v<is_constructible<T, W&>, is_convertible<W&, T>, is_constructible<T, W>, is_convertible<W, T>, is_constructible<T, const W&>, is_convertible<const W&, T>, is_constructible<T, const W>, is_convertible<const W, T>>;
constexpr optional() noexcept;constexpr optional(nullopt_t) noexcept;
Postconditions: *this does not contain a value.
Remarks: No contained value is initialized.
For every object type T these constructors are constexpr constructors ([dcl.constexpr]).
constexpr optional(const optional& rhs);
Effects: If rhs contains a value, direct-non-list-initializes _val_with rhs.val.
Postconditions: rhs.has_value() == this->has_value().
Throws: Any exception thrown by the selected constructor of T.
Remarks: This constructor is defined as deleted unlessis_copy_constructible_v<T> is true.
If is_trivially_copy_constructible_v<T> is true, this constructor is trivial.
constexpr optional(optional&& rhs) noexcept(_see below_);
Constraints: is_move_constructible_v<T> is true.
Effects: If rhs contains a value, direct-non-list-initializes _val_with std::move(rhs.val).
rhs.has_value() is unchanged.
Postconditions: rhs.has_value() == this->has_value().
Throws: Any exception thrown by the selected constructor of T.
Remarks: The exception specification is equivalent tois_nothrow_move_constructible_v<T>.
If is_trivially_move_constructible_v<T> is true, this constructor is trivial.
template<class... Args> constexpr explicit optional(in_place_t, Args&&... args);
Constraints: is_constructible_v<T, Args...> is true.
Effects: Direct-non-list-initializes _val_with std::forward<Args>(args)....
Postconditions: *this contains a value.
Throws: Any exception thrown by the selected constructor of T.
Remarks: If T's constructor selected for the initialization is a constexpr constructor, this constructor is a constexpr constructor.
template<class U, class... Args> constexpr explicit optional(in_place_t, initializer_list<U> il, Args&&... args);
Constraints: is_constructible_v<T, initializer_list<U>&, Args...> is true.
Effects: Direct-non-list-initializes _val_with il, std::forward<Args>(args)....
Postconditions: *this contains a value.
Throws: Any exception thrown by the selected constructor of T.
Remarks: If T's constructor selected for the initialization is a constexpr constructor, this constructor is a constexpr constructor.
template<class U = remove_cv_t<T>> constexpr explicit(_see below_) optional(U&& v);
Constraints:
- is_constructible_v<T, U> is true,
- is_same_v<remove_cvref_t<U>, in_place_t> is false,
- is_same_v<remove_cvref_t<U>, optional> is false, and
- if T is cv bool,remove_cvref_t<U> is not a specialization of optional.
Effects: Direct-non-list-initializes _val_with std::forward<U>(v).
Postconditions: *this contains a value.
Throws: Any exception thrown by the selected constructor of T.
Remarks: If T's selected constructor is a constexpr constructor, this constructor is a constexpr constructor.
The expression inside explicit is equivalent to:!is_convertible_v<U, T>
template<class U> constexpr explicit(_see below_) optional(const optional<U>& rhs);
Constraints:
- is_constructible_v<T, const U&> is true, and
- if T is not cv bool,converts-from-any-cvref<T, optional<U>> is false.
Effects: If rhs contains a value, direct-non-list-initializes _val_with rhs.operator*().
Postconditions: rhs.has_value() == this->has_value().
Throws: Any exception thrown by the selected constructor of T.
Remarks: The expression inside explicit is equivalent to:!is_convertible_v<const U&, T>
template<class U> constexpr explicit(_see below_) optional(optional<U>&& rhs);
Constraints:
- is_constructible_v<T, U> is true, and
- if T is not cv bool,converts-from-any-cvref<T, optional<U>> is false.
Effects: If rhs contains a value, direct-non-list-initializes _val_with std::move(rhs).operator*().
rhs.has_value() is unchanged.
Postconditions: rhs.has_value() == this->has_value().
Throws: Any exception thrown by the selected constructor of T.
Remarks: The expression inside explicit is equivalent to:!is_convertible_v<U, T>
22.5.3.3 Destructor [optional.dtor]
Effects: If is_trivially_destructible_v<T> is falseand *this contains a value, calls val.T::~T().
Remarks: If is_trivially_destructible_v<T> is true, then this destructor is trivial.
22.5.3.4 Assignment [optional.assign]
constexpr optional<T>& operator=(nullopt_t) noexcept;
Effects: If *this contains a value, calls val.T::~T() to destroy the contained value; otherwise no effect.
Postconditions: *this does not contain a value.
constexpr optional<T>& operator=(const optional& rhs);
Table 67 — optional::operator=(const optional&) effects [tab:optional.assign.copy]
| 🔗 | *this contains a value | *this does not contain a value |
|---|---|---|
| 🔗rhs contains a value | assigns rhs.val to val | direct-non-list-initializes val with rhs.val |
| 🔗rhs does not contain a value | destroys the contained value by calling val.T::~T() | no effect |
Postconditions: rhs.has_value() == this->has_value().
Remarks: If any exception is thrown, the result of the expression this->has_value() remains unchanged.
If an exception is thrown during the call to T's copy constructor, no effect.
If an exception is thrown during the call to T's copy assignment, the state of its contained value is as defined by the exception safety guarantee of T's copy assignment.
This operator is defined as deleted unlessis_copy_constructible_v<T> is true andis_copy_assignable_v<T> is true.
If is_trivially_copy_constructible_v<T> && is_trivially_copy_assignable_v<T> && is_trivially_destructible_v<T> is true, this assignment operator is trivial.
constexpr optional& operator=(optional&& rhs) noexcept(_see below_);
Constraints: is_move_constructible_v<T> is true andis_move_assignable_v<T> is true.
The result of the expression rhs.has_value() remains unchanged.
Table 68 — optional::operator=(optional&&) effects [tab:optional.assign.move]
| 🔗 | *this contains a value | *this does not contain a value |
|---|---|---|
| 🔗rhs contains a value | assigns std::move(rhs.val) to val | direct-non-list-initializes val with std::move(rhs.val) |
| 🔗rhs does not contain a value | destroys the contained value by calling val.T::~T() | no effect |
Postconditions: rhs.has_value() == this->has_value().
Remarks: The exception specification is equivalent to:is_nothrow_move_assignable_v<T> && is_nothrow_move_constructible_v<T>
If any exception is thrown, the result of the expression this->has_value() remains unchanged.
If an exception is thrown during the call to T's move constructor, the state of rhs.val is determined by the exception safety guarantee of T's move constructor.
If an exception is thrown during the call to T's move assignment, the states of val and rhs.val are determined by the exception safety guarantee of T's move assignment.
If is_trivially_move_constructible_v<T> && is_trivially_move_assignable_v<T> && is_trivially_destructible_v<T> is true, this assignment operator is trivial.
template<class U = remove_cv_t<T>> constexpr optional& operator=(U&& v);
Constraints:
- is_same_v<remove_cvref_t<U>, optional> is false,
- conjunction_v<is_scalar<T>, is_same<T, decay_t<U>>> is false,
- is_constructible_v<T, U> is true, and
- is_assignable_v<T&, U> is true.
Effects: If *this contains a value, assigns std::forward<U>(v) to val; otherwise direct-non-list-initializes _val_with std::forward<U>(v).
Postconditions: *this contains a value.
Remarks: If any exception is thrown, the result of the expression this->has_value() remains unchanged.
If an exception is thrown during the call to T's constructor, the state of v is determined by the exception safety guarantee of T's constructor.
If an exception is thrown during the call to T's assignment, the states of val and v are determined by the exception safety guarantee of T's assignment.
template<class U> constexpr optional<T>& operator=(const optional<U>& rhs);
Constraints:
- is_constructible_v<T, const U&> is true,
- is_assignable_v<T&, const U&> is true,
- converts-from-any-cvref<T, optional<U>> is false,
- is_assignable_v<T&, optional<U>&> is false,
- is_assignable_v<T&, optional<U>&&> is false,
- is_assignable_v<T&, const optional<U>&> is false, and
- is_assignable_v<T&, const optional<U>&&> is false.
Table 69 — optional::operator=(const optional<U>&) effects [tab:optional.assign.copy.templ]
| 🔗 | *this contains a value | *this does not contain a value |
|---|---|---|
| 🔗rhs contains a value | assigns rhs.operator*() to val | direct-non-list-initializes val with rhs.operator*() |
| 🔗rhs does not contain a value | destroys the contained value by calling val.T::~T() | no effect |
Postconditions: rhs.has_value() == this->has_value().
Remarks: If any exception is thrown, the result of the expression this->has_value() remains unchanged.
If an exception is thrown during the call to T's constructor, the state of rhs.val is determined by the exception safety guarantee of T's constructor.
If an exception is thrown during the call to T's assignment, the states of val and rhs.val are determined by the exception safety guarantee of T's assignment.
template<class U> constexpr optional<T>& operator=(optional<U>&& rhs);
Constraints:
- is_constructible_v<T, U> is true,
- is_assignable_v<T&, U> is true,
- converts-from-any-cvref<T, optional<U>> is false,
- is_assignable_v<T&, optional<U>&> is false,
- is_assignable_v<T&, optional<U>&&> is false,
- is_assignable_v<T&, const optional<U>&> is false, and
- is_assignable_v<T&, const optional<U>&&> is false.
The result of the expression rhs.has_value() remains unchanged.
Table 70 — optional::operator=(optional<U>&&) effects [tab:optional.assign.move.templ]
| 🔗 | *this contains a value | *this does not contain a value |
|---|---|---|
| 🔗rhs contains a value | assigns std::move(rhs).operator*() to val | direct-non-list-initializes val with std::move(rhs).operator*() |
| 🔗rhs does not contain a value | destroys the contained value by calling val.T::~T() | no effect |
Postconditions: rhs.has_value() == this->has_value().
Remarks: If any exception is thrown, the result of the expression this->has_value() remains unchanged.
If an exception is thrown during the call to T's constructor, the state of rhs.val is determined by the exception safety guarantee of T's constructor.
If an exception is thrown during the call to T's assignment, the states of val and rhs.val are determined by the exception safety guarantee of T's assignment.
template<class... Args> constexpr T& emplace(Args&&... args);
Mandates: is_constructible_v<T, Args...> is true.
Effects: Calls *this = nullopt.
Then direct-non-list-initializes _val_with std::forward<Args>(args)....
Postconditions: *this contains a value.
Throws: Any exception thrown by the selected constructor of T.
Remarks: If an exception is thrown during the call to T's constructor,*this does not contain a value, and the previous val (if any) has been destroyed.
template<class U, class... Args> constexpr T& emplace(initializer_list<U> il, Args&&... args);
Constraints: is_constructible_v<T, initializer_list<U>&, Args...> is true.
Effects: Calls *this = nullopt.
Then direct-non-list-initializes val withil, std::forward<Args>(args)....
Postconditions: *this contains a value.
Throws: Any exception thrown by the selected constructor of T.
Remarks: If an exception is thrown during the call to T's constructor,*this does not contain a value, and the previous val (if any) has been destroyed.
22.5.3.5 Swap [optional.swap]
constexpr void swap(optional& rhs) noexcept(_see below_);
Mandates: is_move_constructible_v<T> is true.
Table 71 — optional::swap(optional&) effects [tab:optional.swap]
| 🔗 | *this contains a value | *this does not contain a value |
|---|---|---|
| 🔗rhs contains a value | calls swap(val, rhs.val) | direct-non-list-initializes _val_with std::move(rhs.val), followed by rhs.val.T::~T(); postcondition is that *this contains a value and rhs does not contain a value |
| 🔗rhs does not contain a value | direct-non-list-initializes rhs._val_with std::move(val), followed by val.T::~T(); postcondition is that *this does not contain a value and rhs contains a value | no effect |
Throws: Any exceptions thrown by the operations in the relevant part of Table 71.
Remarks: The exception specification is equivalent to:is_nothrow_move_constructible_v<T> && is_nothrow_swappable_v<T>
If any exception is thrown, the results of the expressions this->has_value() and rhs.has_value() remain unchanged.
If an exception is thrown during the call to function swap, the states of val and rhs.val are determined by the exception safety guarantee of swap for lvalues of T.
If an exception is thrown during the call to T's move constructor, the states of val and rhs.val are determined by the exception safety guarantee of T's move constructor.
22.5.3.6 Iterator support [optional.iterators]
using iterator = _implementation-defined_;using const_iterator = _implementation-defined_;
The reference type is T& for iterator andconst T& for const_iterator.
All requirements on container iterators ([container.reqmts]) apply tooptional::iterator and optional::const_iterator as well.
Any operation that initializes or destroys the contained value of an optional object invalidates all iterators into that object.
constexpr iterator begin() noexcept;constexpr const_iterator begin() const noexcept;
Returns: If has_value() is true, an iterator referring to the contained value.
Otherwise, a past-the-end iterator value.
constexpr iterator end() noexcept;constexpr const_iterator end() const noexcept;
Returns: begin() + has_value().
22.5.3.7 Observers [optional.observe]
constexpr const T* operator->() const noexcept;constexpr T* operator->() noexcept;
Hardened preconditions: has_value() is true.
Remarks: These functions are constexpr functions.
constexpr const T& operator*() const & noexcept;constexpr T& operator*() & noexcept;
Hardened preconditions: has_value() is true.
Remarks: These functions are constexpr functions.
constexpr T&& operator*() && noexcept;constexpr const T&& operator*() const && noexcept;
Hardened preconditions: has_value() is true.
Effects: Equivalent to: return std::move(val);
constexpr explicit operator bool() const noexcept;
Returns: true if and only if *this contains a value.
Remarks: This function is a constexpr function.
constexpr bool has_value() const noexcept;
Returns: true if and only if *this contains a value.
Remarks: This function is a constexpr function.
constexpr const T& value() const &;constexpr T& value() &;
Effects: Equivalent to:return has_value() ? val : throw bad_optional_access();
constexpr T&& value() &&;constexpr const T&& value() const &&;
Effects: Equivalent to:return has_value() ? std::move(val) : throw bad_optional_access();
template<class U = remove_cv_t<T>> constexpr T value_or(U&& v) const &;
Mandates: is_copy_constructible_v<T> && is_convertible_v<U&&, T> is true.
Effects: Equivalent to:return has_value() ? val : static_cast<T>(std::forward<U>(v));
template<class U = remove_cv_t<T>> constexpr T value_or(U&& v) &&;
Mandates: is_move_constructible_v<T> && is_convertible_v<U&&, T> is true.
Effects: Equivalent to:return has_value() ? std::move(val) : static_cast<T>(std::forward<U>(v));
22.5.3.8 Monadic operations [optional.monadic]
template<class F> constexpr auto and_then(F&& f) &;template<class F> constexpr auto and_then(F&& f) const &;
Let U be invoke_result_t<F, decltype((_val_))>.
Mandates: remove_cvref_t<U> is a specialization of optional.
Effects: Equivalent to:if (*this) { return invoke(std::forward<F>(f), val);} else { return remove_cvref_t<U>();}
template<class F> constexpr auto and_then(F&& f) &&;template<class F> constexpr auto and_then(F&& f) const &&;
Let U be invoke_result_t<F, decltype(std::move(_val_))>.
Mandates: remove_cvref_t<U> is a specialization of optional.
Effects: Equivalent to:if (*this) { return invoke(std::forward<F>(f), std::move(val));} else { return remove_cvref_t<U>();}
template<class F> constexpr auto transform(F&& f) &;template<class F> constexpr auto transform(F&& f) const &;
Let U be remove_cv_t<invoke_result_t<F, decltype((_val_))>>.
Mandates: U is a valid contained type for optional.
The declarationU u(invoke(std::forward<F>(f), val));is well-formed for some invented variable u.
Returns: If *this contains a value, an optional<U> object whose contained value is direct-non-list-initialized withinvoke(std::forward<F>(f), val); otherwise, optional<U>().
template<class F> constexpr auto transform(F&& f) &&;template<class F> constexpr auto transform(F&& f) const &&;
Let U beremove_cv_t<invoke_result_t<F, decltype(std::move(_val_))>>.
Mandates: U is a valid contained type for optional.
The declarationU u(invoke(std::forward<F>(f), std::move(val)));is well-formed for some invented variable u.
Returns: If *this contains a value, an optional<U> object whose contained value is direct-non-list-initialized withinvoke(std::forward<F>(f), std::move(val)); otherwise, optional<U>().
template<class F> constexpr optional or_else(F&& f) const &;
Mandates: is_same_v<remove_cvref_t<invoke_result_t<F>>, optional> is true.
Effects: Equivalent to:if (*this) { return *this;} else { return std::forward<F>(f)();}
template<class F> constexpr optional or_else(F&& f) &&;
Mandates: is_same_v<remove_cvref_t<invoke_result_t<F>>, optional> is true.
Effects: Equivalent to:if (*this) { return std::move(*this);} else { return std::forward<F>(f)();}
22.5.3.9 Modifiers [optional.mod]
constexpr void reset() noexcept;
Effects: If *this contains a value, calls val.T::~T() to destroy the contained value; otherwise no effect.
Postconditions: *this does not contain a value.