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

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    std::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)
    See More

    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 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) [edit]