[boost] [accumulators] Help understanding implementation of weighted accumulators

Subject: [boost] [accumulators] Help understanding implementation of weighted accumulators
From: Gabriel Redner (gredner_at_[hidden])
Date: 2013-05-01 16:13:27

Hi folks,

I am trying to implement a weighted accumulator, but I'm having
trouble understanding a few things about the existing ones.

Looking at the implementation of weighted_sum, I see (weighted_sum.hpp:37):

        template<typename Args>
        weighted_sum_impl(Args const &args)
          : weighted_sum_(
                args[parameter::keyword<Tag>::get() | Sample()]
                  * numeric::one<Weight>::value
            )
        {
        }

My questions are:

- It's taking the first sample from the args and multiplying it by a
weight of 1.  Isn't this wrong - ignoring the real weight of this
sample?  Shouldn't this produce incorrect results?

- I've never understood anyways how there can be an initial argument
during construction of the accumulator.  I haven't found any
explanation in the docs beyond "Maybe there is an initial value in the
argument pack".  Maybe?  How?

Probably more questions later.

Thanks,
-Gabe