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# std::experimental::optional::operator=

From cppreference.com
< cpp‎ | experimental‎ | optional

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

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.

### [edit] 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).

### [edit] Return value

*this

### [edit] Exceptions

1)
`noexcept` specification:
noexcept
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)

### [edit] Notes

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

### [edit] 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

### [edit] See also

emplace | constructs the contained value in-place (public member function) [edit]

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