Signals that the calling thread has arrived at the synchronization point without blocking. The calling thread is removed from the set of participating threads, and the count of participating threads is thus decreased by one.
Either the calling thread is one of the first N
threads to arrive at
*this since construction, where
N is the value of the
parameter to the constructor, or the calling thread is already
one of the set of participating threads for
Signal that the calling thread has arrived at the flex_barrier.
If this is the last of the participating threads to arrive then
the completion function (if any)
is called on one of the participating threads, and then all threads
are unblocked and the flex_barrier is reset. The calling thread
is removed from the set of participating threads.