The resulting argument pack is passed as the constructor parameter to all the accumulators in the set. Your accumulator can then extract the coefficients argument from the pack with "args[coefficients]".
My filter depends on the delay buffer I have defined which has the cache_size constructor argument. Since I am specifying the list of coefficients, I want the cache_size to be set to the number of elements in my coefficient list, with out having to specify it, ie without having to write:
array<double,5> coeffs={0.1,0.2,0.1}; accumulator_set<... > acc( .., coefficients = coeffs, cache_size=5...);
Let me see if I understand. You want the delay buffer to use the cache_size parameter if it is specified, and if not, check to see if the coefficients parameter is specified, and if so, use the size of the coefficients array. This is possible.
[cut proposed solution] Thanks Eric, that would probably work (as long as I define the coefficients named paramter seperately from the filter or the delay). Ideally, though, I was looking for a solution that allowed me to reuse the delay buffer without modifying it. That way it could be used as a lower layer for any algorithm that needed recent time history. Maybe I am being over ambitious...