shared_future& operator=( const shared_future& other );

shared_future& operator=( const shared_future& other ) noexcept;

shared_future& operator=( shared_future&& other ) noexcept;

1) Releases any shared state and assigns the contents of other to *this. After the assignment, this->valid() == other.valid().

2) Releases any shared state and move-assigns the contents of other to *this. After the assignment, other.valid() == false and this->valid() will yield the same value as other.valid() before the assignment.