std::experimental::optional<T>::operator= (3) - Linux Manuals

std::experimental::optional<T>::operator=: std::experimental::optional<T>::operator=

NAME

std::experimental::optional<T>::operator= - std::experimental::optional<T>::operator=

Synopsis

optional& operator=( std::experimental::nullopt_t ) noexcept; (1) (library fundamentals TS)
optional& operator=( const optional& other ); (2) (library fundamentals TS)
optional& operator=( optional&& other ) noexcept(/* see below */); (3) (library fundamentals TS)
template< class U > (4) (library fundamentals TS)
optional& operator=( U&& value );

Replaces contents of *this with the contents of other
1) If *this contains a value before the call, the contained value is destroyed by calling its destructor as if by val->T::~T(). *this does not contain a value after this call.
2-3) Assigns the state of other.

* If both *this and other do not contain a value, the function has no effect.
* If *this contains a value, but other does not, then the contained value is destroyed by calling its destructor. *this does not contain a value after the call.
* If other contains a value, then depending on whether *this contains a value, the contained value is either direct-initialized or assigned from *other (2) or std::move(*other) (3). Note that a moved-from optional still contains a value.

4) Decay-only perfect-forwarded assignment: depending on whether *this contains a value before the call, the contained value is either direct-initialized from std::forward<U>(value) or assigned from std::forward<U>(value). The function does not participate in overload resolution unless std::is_same<std::decay_t<U>, T>::value is true.

Parameters

other - another optional object whose contained value to assign
value - value to assign to the contained value

Type requirements

-
T must meet the requirements of CopyAssignable and CopyConstructible in order to use overload (2).
-
T must meet the requirements of MoveAssignable and MoveConstructible in order to use overload (3).

Return value

*this

Exceptions

2-4) Throws any exception thrown by the constructor or assignment operator of T. If an exception is thrown, the initialization state of *this (and of other in case of (2) ) is unchanged, i.e. if the object contained a value, it still contains a value, and the other way round. The contents of value and the contained values of *this and other depend on the exception safety guarantees of the operation from which the exception originates (copy-constructor, move-assignment, etc.).
(3) has the following noexcept declaration:
noexcept specification:
noexcept(std::is_nothrow_move_assignable<T>::value && std::is_nothrow_move_constructible<T>::value)

Notes

An optional object op may be turned into an empty optional with both op = {}; and op = nullopt;.

Example

// Run this code

  #include <experimental/optional>
  #include <iostream>
  int main()
  {
      std::experimental::optional<const char*> s1 = "abc", s2; // constructor
      s2 = s1; // assignment
      s1 = "def"; // decaying assignment (U = char[4], T = const char*)
      std::cout << *s2 << ' ' << *s1 << '\n';
  }

Output:

  abc def