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# std::optional::operator=

From cppreference.com
< cpp‎ | utility‎ | optional

optional& operator=( std::nullopt_t ) noexcept; | (1) | (since C++17)
optional& operator=( const optional& other ); | (2) | (since C++17)
optional& operator=( optional&& other ) noexcept(/* see below */); | (3) | (since C++17)
template< class U = T > optional& operator=( U&& value ); | (4) | (since C++17)
template< class U > optional& operator=( const optional<U>& other ); | (5) | (since C++17)
template< class U > optional& operator=( optional<U>&& other ); | (6) | (since C++17)

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 value().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.
- Overload (2) is defined as deleted unless std::is_copy_constructible_v<T> and std::is_copy_assignable_v<T> are both true.
- Overload (3) does not participate in overload resolution unless std::is_move_constructible_v<T> and std::is_move_assignable_v<T> are both true.

4) 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::decay_t<U> is not std::optional<T>, std::is_constructible_v<T, U> is true, std::is_assignable_v<T&, U> is true, and at least one of the following is true:

- `T` is not a scalar type;
- std::decay_t<U> is not `T`.

5-6) 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 (5) or std::move(*other) (6). Note that a moved-from optional still contains a value.
- These overloads do not participate in overload resolution unless the following conditions are met:
  - `T` is not constructible, convertible, or assignable from any expression of type (possibly const) std::optional<U>, i.e., the following 12 type traits are all false:
    - std::is_constructible_v<T, std::optional<U>&>
    - std::is_constructible_v<T, const std::optional<U>&>
    - std::is_constructible_v<T, std::optional<U>&&>
    - std::is_constructible_v<T, const std::optional<U>&&>
    - std::is_convertible_v<std::optional<U>&, T>
    - std::is_convertible_v<const std::optional<U>&, T>
    - std::is_convertible_v<std::optional<U>&&, T>
    - std::is_convertible_v<const std::optional<U>&&, T>
    - std::is_assignable_v<T&, std::optional<U>&>
    - std::is_assignable_v<T&, const std::optional<U>&>
    - std::is_assignable_v<T&, std::optional<U>&&>
    - std::is_assignable_v<T&, const std::optional<U>&&>.
  - For overload (5), std::is_constructible_v<T, const U&> and std::is_assignable_v<T&, const U&> are both true.
  - For overload (6), std::is_constructible_v<T, U> and std::is_assignable_v<T&, U> are both true.

### [edit] Parameters

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

### [edit] Return value

*this

### [edit] Exceptions

2-6) 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 (2-3) and (5-6) ) 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` 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;. The first expression constructs an empty optional object with {} and assigns it to `op`.

### [edit] Example

Run this code

    #include <optional>
    #include <iostream>
    int main()
    {
        std::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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Category:

- conditionally noexcept