[boost] [Review] accumulators

From: Bernhard Mäder (bernhard.maeder_at_[hidden])
Date: 2007-02-07 15:12:59

Hello all

Hope I'm not too late with a small review of the accumulators library:

 • What is your evaluation of the design?

 I very much like the user interface of the library, it's pretty clean
and concise; writing something like

 accumulator_set<double, stats<tag::median(with_p_square_quantile) > > acc;

 is as expressive as it gets. This is just cool.

 On the other side of the library, when defining new features, things
are a bit more complicated. I experienced some ugly compile-time
troubles when trying to implement a group of features that depended on
each other. Writing that code took considerable more time than writing
the same functionality without the framework. I'm quite willing to
take this effort, though, for all the benefits of the accumulators framework.

 Then, I can't really get warm with the automatic feature resolution.
On the one hand, it's a great feature for reducing accumulator set
definition code. On the other hand, the programmer has to know which
features get pulled in anyway (you'll need to include those header
files, right?). Furthermore, it seems a bit dangerous to have such
automatisms in a field, where you should always know what you're doing
anyway (which is again exemplified by the discussion of the various
implementations of the mean and variance features on this list). Then
there is BOOST_ACCUMULATORS_MAX_FEATURES, which has to take into
account even those features that are pulled in automatically. This is
a bit counter intuitive and caused some troubles on my side.

 I'd wish that accumulator_set provided an operator<< to print it to
console or files. We're often using accumulator sets from an
interactive python console, which would further benefit from a
pretty-printed output. Equally, they should be made serializable. The
formerly discussed combine() feature could also come in very handy for
some of our applications.

 Overall, I think the library's complexity is as high as I'm willing
to go for every day's business. It's really nothing for C++ beginners
to start with, as it requires some intermediate level "boost"
thinking.

 • What is your evaluation of the implementation?

 The framework code is inspiring, as most of Eric's work. The stats
are very well separated and pretty easy to read.

 • What is your evaluation of the documentation?

 A bit short at times. Some tools lack detailed description
(depends_on, feature_of etc.), the external<> mechanics are not
clearly described. The overall structure of the user's guide is very
good, though.

 • What is your evaluation of the potential usefulness of the library?

 Well, we're already using it in production code, so I guess it's very
real. There are many other application fields where I could imagine
using it (signal processing, imaging, embedded systems with limited
memory etc.)

 • Did you try to use the library? With what compiler? Did you have
any problems?

 We're using the library on debian sarge with GCC3.4. Apart for some
long and obscure compiler outputs on errors (and that's hardly boost
accumulator's fault), all went fine.

 • How much effort did you put into your evaluation? A glance? A quick
reading? In-depth study?

 I've been using the library as end-user for quite some time now. I
did not study all of the source code nor all of the design decisions,
but come across many aspects of the library during normal usage.

 • Are you knowledgeable about the problem domain?

 I do know our requirements for statistics on current project, and see
where they cause problems, But I'm not really a statistics expert.

 • Do you think the library should be accepted as a Boost library?

 Of course, it's a great enhancement to boost.

 Bernhard