std::experimental::optional<T>::operator=
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< cpp | experimental | optional
| optional& operator=( std::experimental::nullopt_t ) noexcept; |
(1) | (library fundamentals TS) |
| optional& operator=( const optional& other ); |
(2) | (library fundamentals TS) |
| optional& operator=( optional&& other ) noexcept(/* see below */); |
(3) | (library fundamentals TS) |
| template< class U > optional& operator=( U&& value ); |
(4) | (library fundamentals TS) |
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.
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).
| ||
Return value
*this
Exceptions
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)
Notes
An optional object op may be turned into an empty optional with both op = {}; and op = nullopt;.
Example
Run this code
#include <experimental/optional> #include <iostream> int main() { std::experimental::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
See also
| constructs the contained value in-place (public member function) |