But, in order to add a component requires covarite input, we need to modify statistics_fwd.hpp. (When I described join_histgram component, I need to modify statistics_fwd.hpp and depended other components density.hpp and so on .) I think this is not good. If possible, just add a file which describes a statistical method, and include the file, is better.
I'm not sure I understand. Why did you have to modify statistics_fwd.hpp? You shouldn't have to.
Let us consider the case covariance depend on p_square_cumulative_distribution and p_square_cumulative_distribution_of_variates Attached files are implimentation of the covariance depend on p_square_cumulative_distribution_of_variates. Plz try to compile these file without replacing statistics_fwd.hpp to attached file. In my environment, without modifing statistics_fwd.hpp , I can not compile these files. In order to compile, we need to add the following line in statistics_fwd.hpp namespace impl { template<typename Sample, typename Tag = tag::sample> struct p_square_cumulative_distribution_impl; }