Boost property_tree: multiple values per key

Question

Boost property tree seems like an excellent library to use for parsing config files. However, I can't figure out how to handle situations where there are multiple values per key. For example, let's say I was specifying a box like this:

box
{
    x -1 1
    y -1 1
    z -1 1
}

where x, y, and z are the bounds of the box on the x, y, and z axes respectively, specified using property_tree's INFO format. I see mention in the manual of using quotes for values that use spaces, but then I don't see that I could import those values as numbers. I'd have to parse the string into numbers, which seems to defeat the purpose of using property_tree in the first place. I could certainly give each number a key:

box 
{
    xlo -1
    xhi 1
    ylo -1
    yhi 1
    zlo -1
    zhi 1
}    

but that seems cumbersome, and will inflate my config file. I also noted that I could handle this situation in program_options, but I lose the nested config file capabilities (yeah, I know I can use dot notation to "nest", but it's not the same).

Is there a way to import e.g. x as a list of numbers like this?

Answer 1

The standard property_tree handles only one string value per key since it is defined as:

typedef basic_ptree<std::string, std::string> ptree;

So, the only option is to use strings and parse them. I think the best method is to define a new class that stores the low and high values and then create a translator class for the get and set methods. For example:

struct low_high_value
{
  low_high_value() : m_low(0), m_high(0) { }
  low_high_value(double low, double high) : m_low(low), m_high(high) { }
  double m_low;
  double m_high;
};

The translator would be:

struct low_high_value_translator
{
  typedef std::string    internal_type;
  typedef low_high_value external_type;
  // Get a low_high_value from a string
  boost::optional<external_type> get_value(const internal_type& str)
  {
    if (!str.empty())
    {
      low_high_value val;
      std::stringstream s(str);
      s >> val.m_high >> val.m_low;
      return boost::optional<external_type>(val);
    }
    else
      return boost::optional<external_type>(boost::none);
  }

  // Create a string from a low_high_value
  boost::optional<internal_type> put_value(const external_type& b)
  {
    std::stringstream ss;
    ss << b.m_low << " " << b.m_high;
    return boost::optional<internal_type>(ss.str());
  }
};

The previous get_value method is very simple. It should be improved if the file could be written by the user.

This class should be registered using:

namespace boost { namespace property_tree 
{
  template<typename Ch, typename Traits, typename Alloc> 
  struct translator_between<std::basic_string< Ch, Traits, Alloc >, low_high_value>
  {
    typedef low_high_value_translator type;
  };
} }

After you include the previous code, you can use property_tree as:

  pt.get<low_high_value>("box.x")
  pt.put("box.u", low_high_value(-110, 200));