Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. — Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

If T has a nested type cancellation_slot_type, returns t.get_cancellation_slot(). Otherwise returns s.

static decltype(auto) get(
    const T & t,
    const CancellationSlot & s);