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std::optional::operator=

optional& operator=( std::nullopt_t );	(1)	(since C++17)
optional& operator=( const optional& other );	(2)	(since C++17)
optional& operator=( optional&& other );	(3)	(since C++17)
template< class U > optional& operator=( U&& value );	(4)	(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.

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

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)

Notes

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

Example

#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: