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Subject: Re: [boost] [accumulators] Help understanding implementation of weighted accumulators
From: Eric Niebler (eniebler_at_[hidden])
Date: 2013-10-02 18:36:09

On 10/2/2013 1:48 PM, Gabriel Redner wrote:
> Sorry to revive an old thread, but I'd still be interested in the
> answers to these questions.


> On Wed, May 1, 2013 at 11:06 PM, Gabriel Redner <gredner_at_[hidden]> wrote:
>> Hi Eric,
>> Thanks very much for your response. That does help, and I have some follow-ups.
>> First, let me make sure I am understanding correctly. If my Sample is
>> some FancyType whose default constructor does not do the right thing
>> (or perhaps does not exist), I should do:
>> accumulator_set<FancyType, features<some_features> > acc(sample =
>> some_initial_fancytype_value);
>> Is that right?


>> If so, I would say that:
>> - This usage is not mentioned in the docs anywhere, and

Well, it's sort of mentioned here:

Look at the comment to the right of the "sum_accumulator" constructor.

But I agree this needs to be dealt with more explicitly in the docs.

>> - It seems strange to overload the meaning of 'sample' in this way.
>> Wouldn't it be clearer and accomplish the same goal to use a different
>> name? "initialValue" perhaps?

That probably makes sense. Although for backward compatibility, the best
I could do would be to provide initialValue as an alias for sample
(though probably something like "default_sample" instead of "initialValue").

>> - Do all accumulators support this feature? I can imagine some that

All accumulators are required to define a constructor that takes an
argument pack. Whether the accumulator avails itself of a default sample
if present is up to the accumulator.

>> might not (indeed I am implementing one). This is not mentioned in
>> the docs either.

The docs should mention this, because it's best-practice if you want
your accumulator to work with types like std::vector and std::valarray.

>> Second, I have a sort-of-question-but-mostly-a-kvetch about defining
>> keywords for accumulators that need initialization parameters (e.g.
>> tail). The library itself uses the macro
>> BOOST_PARAMETER_NESTED_KEYWORD for this, which is not documented.

True. :-(

>> I
>> do not understand what this macro does - why the name, why the alias,
>> etc? My own accumulators need initialization parameters, and I have
>> just been aping what the library itself does - using the macro
>> although it's not documented, and hoping for the best. It seems to be
>> working, but I'd like to understand if this is the right approach,
>> whether I can rely on this undocumented macro, and what it's doing.
>> Can you shed any light?

It exists and with the name that it does because it is very similar to
the BOOST_PARAMETER_KEYWORD [1], except that it lets you define a
keyword nested within an empty struct. Feature tags like tag::density
inherit from these empty structs, so that you can say stuff like
"tag::density::cache_size = 16" for setting parameters.

None of this is documented, sadly. Can you please file a bug to add this
information to the docs. I don't have time to get to it at the moment.



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