[Boost-users] Having program_options generate a round-trippable config file?

Subject: [Boost-users] Having program_options generate a round-trippable config file?
From: Rhys Ulerich (rhys.ulerich_at_[hidden])
Date: 2011-01-17 13:16:35

Hi all,

I'm using program_options to combine arguments from the command line,
the environment, and one or more input files.  As you'd expect, it
works very well.  However, I have an additional requirement that I'm
unsure how to implement cleanly atop the library:  Given instances of
program_options::options_description and
program_options::variables_map, I'd like to save a
program_options::parse_config_file-friendly file containing the final
values obtained.  Such a config file could be given back to the
program and would identically reproduce the options used.  The need is
for some scientific software where I want a precise record of what a
user has chosen for reasons of scientific repeatability.

I see several ways to go about achieving this need, but all of them
are ugly, brittle hacks:

  1) Accessing the information directly from variables_map seems
difficult due to variables_map's use of type erasure and the fact that
it does not save the tokens from which values are derived.

  2) Obtaining the tokens from the intermediate
program_options::parsed_options instances is a possibility but
requires me to effectively reimplement the logic contained within
program_options::store.  It also fails to account for defaulted
values.

  3) Looping over each program_options::value_semantic found in the
options_description instance and adding a notify() callback again runs
into type erasure troubles.

  4) Adding a notify() to each option_description at its point of
declaration avoids type erasure but makes the code impact O(n) where n
is the number of options.

Has anyone built such a feature atop program_options?  Can anyone lend
some wisdom?

Thanks,
Rhys