std::optional<T>::operator=
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 > 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. 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:
-
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
(3) has the following
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 |
See also
constructs the contained value in-place (public member function) |