Computes a final value from the set of thread-local sub-computations by calling the supplied combine functor once per thread-local sub-computation. The final result is accumulated by the function object.
The type of the function object that will be invoked to combine a single thread-local sub-computation.
The functor that is used to combine one sub-computation. Its signature is void (T) or void (const T&), and must be associative and commutative.