Extract subset from boost dynamic_bitset

Question

I need to extract and decode the bits (idx, idx+1, ... idx+n_bits) from a given boost dynamic_bitset. I have created the following solution:

boost::dynamic_bitset<> mybitset(...);
// build mask 2^{idx+n_bits} - 2^{idx}
const boost::dynamic_bitset<> mask(mybitset.size(), (1 << idx+n_bits) - (1 << idx));
// shift the masked result idx times and get long
unsigned long u = ((mybitset & mask) >> idx ).to_ulong();

It works well, but as this code is critical for the performance of my application, I am curious if there exists a better way to achieve this?