20 General utilities library [utilities]

20.6 Optional objects [optional]

20.6.3 Class template optional [optional.optional]

20.6.3.3 Assignment [optional.assign]

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 42 .

Table 42: 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	initializes the contained value as if direct-non-list-initializing an object of type T with *rhs
rhs does not contain a value	destroys the contained value by calling val->T::~T()	no effect

Postconditions: bool(rhs) == bool(*this).

Returns: *this.

Remarks: If any exception is thrown, the result of the expression bool(*this) 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 43 .

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

Table 43: 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	initializes the contained value as if direct-non-list-initializing an object of type T with std::move(*rhs)
rhs does not contain a value	destroys the contained value by calling val->T::~T()	no effect

Postconditions: bool(rhs) == bool(*this).

Returns: *this.

Remarks: The expression inside noexcept 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 bool(*this) 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> 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 initializes the contained value as if direct-non-list-initializing object of type T with std::forward(v).

Postconditions: *this contains a value.

Returns: *this.

Remarks: If any exception is thrown, the result of the expression bool(*this) 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> 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)
is_constructible_v<T, optional&> is false,
(19.4)
is_constructible_v<T, optional&&> is false,
(19.5)
is_constructible_v<T, const optional&> is false,
(19.6)
is_constructible_v<T, const optional&&> is false,
(19.7)
is_convertible_v<optional&, T> is false,
(19.8)
is_convertible_v<optional&&, T> is false,
(19.9)
is_convertible_v<const optional&, T> is false,
(19.10)
is_convertible_v<const optional&&, T> is false,
(19.11)
is_assignable_v<T&, optional&> is false,
(19.12)
is_assignable_v<T&, optional&&> is false,
(19.13)
is_assignable_v<T&, const optional&> is false, and
(19.14)
is_assignable_v<T&, const optional&&> is false.

Effects: See Table 44 .

Table 44: 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	initializes the contained value as if direct-non-list-initializing an object of type T with *rhs
rhs does not contain a value	destroys the contained value by calling val->T::~T()	no effect

Postconditions: bool(rhs) == bool(*this).

Returns: *this.

Remarks: If any exception is thrown, the result of the expression bool(*this) 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> 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)
is_constructible_v<T, optional&> is false,
(24.4)
is_constructible_v<T, optional&&> is false,
(24.5)
is_constructible_v<T, const optional&> is false,
(24.6)
is_constructible_v<T, const optional&&> is false,
(24.7)
is_convertible_v<optional&, T> is false,
(24.8)
is_convertible_v<optional&&, T> is false,
(24.9)
is_convertible_v<const optional&, T> is false,
(24.10)
is_convertible_v<const optional&&, T> is false,
(24.11)
is_assignable_v<T&, optional&> is false,
(24.12)
is_assignable_v<T&, optional&&> is false,
(24.13)
is_assignable_v<T&, const optional&> is false, and
(24.14)
is_assignable_v<T&, const optional&&> is false.

Effects: See Table 45 .

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

Table 45: 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	initializes the contained value as if direct-non-list-initializing an object of type T with std::move(*rhs)
rhs does not contain a value	destroys the contained value by calling val->T::~T()	no effect

Postconditions: bool(rhs) == bool(*this).

Returns: *this.

Remarks: If any exception is thrown, the result of the expression bool(*this) 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> T& emplace(Args&&... args);

Mandates: is_constructible_v<T, Args...> is true.

Effects: Calls *this = nullopt.

Then initializes the contained value as if direct-non-list-initializing an object of type T with the arguments 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> T& emplace(initializer_list<U> il, Args&&... args);

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

Effects: Calls *this = nullopt.

Then initializes the contained value as if direct-non-list-initializing an object of type T with the arguments 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.