22 General utilities library [utilities]

22.5 Optional objects [optional]

22.5.3 Class template optional [optional.optional]

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.

Returns: *this.

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constexpr optional<T>& operator=(const optional& rhs);

Effects: See Table 58 .

Table 58: 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 to the contained value	direct-non-list-initializes the contained value with *rhs
🔗	rhs does not contain a value	destroys the contained value by calling val->T::~T()	no effect

Postconditions: rhs.has_value() == this->has_value().

Returns: *this.

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 unless is_copy_constructible_v<T> is true and is_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.

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constexpr optional& operator=(optional&& rhs) noexcept(see below);

Constraints: is_move_constructible_v<T> is true and is_move_assignable_v<T> is true.

Effects: See Table 59 .

The result of the expression rhs.has_value() remains unchanged.

Table 59: 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) to the contained value	direct-non-list-initializes the contained value with std::move(*rhs)
🔗	rhs does not contain a value	destroys the contained value by calling val->T::~T()	no effect

Postconditions: rhs.has_value() == this->has_value().

Returns: *this.

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 state of *val and *rhs.val is 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.

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template<class U = T> constexpr optional<T>& operator=(U&& v);

Constraints: is_same_v<remove_cvref_t, optional> is false, conjunction_v<is_scalar<T>, is_same<T, decay_t>> 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(v) to the contained value; otherwise direct-non-list-initializes the contained value with std::forward(v).

Postconditions: *this contains a value.

Returns: *this.

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 state of *val and v is determined by the exception safety guarantee of T's assignment.

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template<class U> constexpr optional<T>& operator=(const optional<U>& rhs);

Constraints:

(19.1)
is_constructible_v<T, const U&> is true,
(19.2)
is_assignable_v<T&, const U&> is true,
(19.3)
converts-from-any-cvref<T, optional> is false,
(19.4)
is_assignable_v<T&, optional&> is false,
(19.5)
is_assignable_v<T&, optional&&> is false,
(19.6)
is_assignable_v<T&, const optional&> is false, and
(19.7)
is_assignable_v<T&, const optional&&> is false.

Effects: See Table 60 .

Table 60: optional::operator=(const optional&) effects [tab:optional.assign.copy.templ]

🔗		*this contains a value	*this does not contain a value
🔗	rhs contains a value	assigns *rhs to the contained value	direct-non-list-initializes the contained value with *rhs
🔗	rhs does not contain a value	destroys the contained value by calling val->T::~T()	no effect

Postconditions: rhs.has_value() == this->has_value().

Returns: *this.

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 state of *val and *rhs.val is determined by the exception safety guarantee of T's assignment.

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template<class U> constexpr optional<T>& operator=(optional<U>&& rhs);

Constraints:

(24.1)
is_constructible_v<T, U> is true,
(24.2)
is_assignable_v<T&, U> is true,
(24.3)
converts-from-any-cvref<T, optional> is false,
(24.4)
is_assignable_v<T&, optional&> is false,
(24.5)
is_assignable_v<T&, optional&&> is false,
(24.6)
is_assignable_v<T&, const optional&> is false, and
(24.7)
is_assignable_v<T&, const optional&&> is false.

Effects: See Table 61 .

The result of the expression rhs.has_value() remains unchanged.

Table 61: optional::operator=(optional&&) effects [tab:optional.assign.move.templ]

🔗		*this contains a value	*this does not contain a value
🔗	rhs contains a value	assigns std::move(*rhs) to the contained value	direct-non-list-initializes the contained value with std::move(*rhs)
🔗	rhs does not contain a value	destroys the contained value by calling val->T::~T()	no effect

Postconditions: rhs.has_value() == this->has_value().

Returns: *this.

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 state of *val and *rhs.val is determined by the exception safety guarantee of T's assignment.

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

Postconditions: *this contains a value.

Returns: A reference to the new contained 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.

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

Constraints: is_constructible_v<T, initializer_list&, Args...> is true.

Effects: Calls *this = nullopt.

Then direct-non-list-initializes the contained value with il, std::forward<Args>(args)....

Postconditions: *this contains a value.

Returns: A reference to the new contained 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.