Transfers the shared state of *this to a std::shared_future object. Multiple std::shared_future objects may reference the same shared state, which is not possible with std::future.
share on a std::future, valid() == false.
The behavior is undefined if valid()== false before the call to this function.
 Return value
A std::shared_future object containing the shared state previously held by *this
|This section is incomplete|
Reason: no example
 See also
| waits for a value (possibly referenced by other futures) that is set asynchronously |