std::future<T>::share (3) - Linux Manuals
std::shared_future<T> share() noexcept;
Transfers the shared state of *this, if any, to a std::shared_future object. Multiple std::shared_future objects may reference the same shared state, which is not possible with std::future.
After calling share on a std::future, valid() == false.
A std::shared_future object containing the shared state previously held by *this, if any, constructed as if by std::shared_future<T>(std:move(*this)).
This section is incomplete
Reason: no example
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior
LWG_2556 C++11 share() requires valid() to be true requirement removed and made noexcept
shared_future waits for a value (possibly referenced by other futures) that is set asynchronously