std::optional::operator=(3)

NAME
std::optional::operator= - std::optional::operator=

Synopsis
   optional& operator=( std::nullopt_t ) noexcept; (1) (since C++17)
                        (constexpr since C++20)
   constexpr optional& operator=( const optional& other ); (2) (since C++17)
   constexpr optional& operator=( optional&& other ) (3) (since C++17)
   noexcept(/* see below */);
   template< class U = T > (4) (since C++17)
   optional& operator=( U&& value ); (constexpr since C++20)
   template< class U > (5) (since C++17)
   optional& operator=( const optional<U>& other ); (constexpr since C++20)
   template< class U > (6) (since C++17)
   optional& operator=( optional<U>&& other ); (constexpr since C++20)
   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 deleted when either std::is_copy_constructible_v<T> or
       std::is_copy_assignable_v<T> is false. It is trivial if
       std::is_trivially_copy_constructible_v<T>,
       std::is_trivially_copy_assignable_v<T> and std::is_trivially_destructible_v<T>
       are all true.
     * Overload (3) does not participate in overload resolution when either
       std::is_move_constructible_v<T> or std::is_move_assignable_v<T> is false. It is
       trivial if std::is_trivially_move_constructible_v<T>,
       std::is_trivially_move_assignable_v<T> and std::is_trivially_destructible_v<T>
       are all 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>
   (until C++20)
   std::remove_cvref_t<U>
   (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<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.

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 following
   noexcept specification:
   noexcept(std::is_nothrow_move_assignable_v<T> &&
   std::is_nothrow_move_constructible_v<T>)

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 <iostream>
 #include <optional>
 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
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
     DR Applied to Behavior as published Correct behavior
                      copy/move assignment operator may not be required to propagate
   P0602R4 C++17 trivial triviality
                      even if underlying operations are trivial
                      converting assignment operators were not
   P2231R1 C++20 constexpr made constexpr
                      while the required operations can be in
                      C++20

See also
   emplace constructs the contained value in-place
           (public member function)

Category:
     * conditionally noexcept