std::optional<T>::operator= (3) - Linux Manuals

std::optional<T>::operator=: std::optional<T>::operator=

NAME

std::optional<T>::operator= - std::optional<T>::operator=

Synopsis

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

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. 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 unless std::is_move_constructible_v<T> and std::is_move_assignable_v<T> are both true. 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:

  o 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>&&>.

  o For overload (5), std::is_constructible_v<T, const U&> and std::is_assignable_v<T&, const U&> are both true.
  o 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

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
P0602R4 C++17 copy/move assignment operator may not be trivial even if underlying operations are trivial required to propagate triviality