Computes a final value from the set of thread-local sub-computations by calling the supplied combine functor.
The type of the function object that will be invoked to combine two thread-local sub-computations.
The functor that is used to combine the sub-computations. Its signature is T (T, T) or T (const T&, const T&), and it must be associative and commutative.
The final result of combining all the thread-private sub-computations.