std::optional::operator=
From cppreference.com
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 val->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.
Contents |
[edit] 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).
|
[edit] Return value
*this
[edit] Exceptions
1)
noexcept specification:
noexcept
2-4) 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) ) 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)
[edit] Notes
An optional object op
may be turned into an empty optional with both op = {}; and op = nullopt;.
[edit] 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
[edit] See also
constructs the contained value in-place (public member function) |