std::optional::operator=

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(value) or assigned from std::forward(value). The function does not participate in overload resolution unless std::decay_t (until C++20)std::remove_cvref_t (since C++20) 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 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, i.e., the following 12 type traits are all false:
 - std::is_constructible_v<T, std::optional&>
 - std::is_constructible_v<T, const std::optional&>
 - std::is_constructible_v<T, std::optional&&>
 - std::is_constructible_v<T, const std::optional&&>
 - std::is_convertible_v<std::optional&, T>
 - std::is_convertible_v<const std::optional&, T>
 - std::is_convertible_v<std::optional&&, T>
 - std::is_convertible_v<const std::optional&&, T>
 - std::is_assignable_v<T&, std::optional&>
 - std::is_assignable_v<T&, const std::optional&>
 - std::is_assignable_v<T&, std::optional&&>
 - std::is_assignable_v<T&, const std::optional&&>.
- 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.

Parameters

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

Return value

*this

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)

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.

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

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