boost::Program_options How to support hash character in a value?

Question

I am trying to have the hash sign ('#') in a value of a configuration file.

My use case is a music program in which the values give the tuning of a guitar score. Therefore, supporting '#' in the values is mandatory and supports no workaround, unlike flats, which can be simulated using a 'b'.

I have tried the following syntaxes:

tuning.1=d#
tuning.1=d\#
tuning.1=d##

In all those cases, the key tuning.1 receives the value d, which is of course not the intention.

Is it possible to have a hash sign in the value of a key? I can't seem to find anything about it in the boost documentation or online. Or should I resort to writing a custom parser?

Answer 1

I don't think you can change how boost::program_options parses the values, because the documentation says that The # character introduces a comment that spans until the end of the line.

However, you could transform your hash character into another one when the configuration file is parsed, using a custom filter from boost::iostreams. See the documentation regarding filter usage and input filters. Here is a very basic I wrote that replaces # with @:

#include <boost/iostreams/filtering_stream.hpp>

struct escape_hash_filter: public boost::iostreams::input_filter
{
  template <typename Source>
  int get (Source& src)
  {
    int c = boost::iostreams::get (src);
    if ((c == EOF) or (c == boost::iostreams::WOULD_BLOCK))
      return c;
    return ((c == '#')? '@': c;
  }
};

Example of how to use it:

std::ifstream in {"example.cfg"};
boost::iostreams::filtering_istream escaped_in;
escaped_in.push (escape_hash_filter {});
escaped_in.push (in);

po::store (po::parse_config_file (escaped_in, desc), vm);