20 General utilities library [utilities]

20.6 Optional objects [optional]

20.6.3 Class template optional [optional.optional]

20.6.3.4 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.
constexpr optional<T>& operator=(const optional& rhs);
Effects: See Table 45.
Table 45: 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.
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 46.
The result of the expression rhs.has_­value() remains unchanged.
Table 46: 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.
template<class U = T> constexpr optional<T>& 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 the contained value; otherwise direct-non-list-initializes the contained value with std​::​forward<U>(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.
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,
  • is_­constructible_­v<T, optional<U>&> is false,
  • is_­constructible_­v<T, optional<U>&&> is false,
  • is_­constructible_­v<T, const optional<U>&> is false,
  • is_­constructible_­v<T, const optional<U>&&> is false,
  • is_­convertible_­v<optional<U>&, T> is false,
  • is_­convertible_­v<optional<U>&&, T> is false,
  • is_­convertible_­v<const optional<U>&, T> is false,
  • is_­convertible_­v<const optional<U>&&, T> 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.
Effects: See Table 47.
Table 47: 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 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.
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,
  • is_­constructible_­v<T, optional<U>&> is false,
  • is_­constructible_­v<T, optional<U>&&> is false,
  • is_­constructible_­v<T, const optional<U>&> is false,
  • is_­constructible_­v<T, const optional<U>&&> is false,
  • is_­convertible_­v<optional<U>&, T> is false,
  • is_­convertible_­v<optional<U>&&, T> is false,
  • is_­convertible_­v<const optional<U>&, T> is false,
  • is_­convertible_­v<const optional<U>&&, T> 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.
Effects: See Table 48.
The result of the expression rhs.has_­value() remains unchanged.
Table 48: 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) 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.
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.
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 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.