It doesn't appear to be necessary to have both a public factory function and a public constructor for derived classes of AccumulatorState, so the interface could be simplified by removing the factory functions altogether and changing any callers to use the constructors instead. 

The create() factory function is currently used in a template function here, and individual accumulators' factory functions may be used in other places as well. All of these callers will need to be updated to use the constructors.