Function lazy_init

extern/boost/boost/thread/future.hpp:2273–2284

        bool future_obtained;

        void lazy_init()
        {
#if defined BOOST_THREAD_PROVIDES_PROMISE_LAZY
#include <boost/thread/detail/atomic_undef_macros.hpp>
          if(!atomic_load(&future_))
            {
                future_ptr blank;
                atomic_compare_exchange(&future_,&blank,future_ptr(new detail::shared_state<R>));
            }
#include <boost/thread/detail/atomic_redef_macros.hpp>
#endif
        }

    public:
        BOOST_THREAD_MOVABLE_ONLY(promise)