Std::experimental::optional::operator= (3) Linux Manual Page

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

std::experimental::optional::operator= (3) Linux Manual Page