Hi, [My apologies for the long post] I'm trying to port filters and formatters of Boost.Log to Boost.Phoenix and I'm not sure I understand what's the best way to go. Hope someone can help me out. A short introduction of the terms and what is currently available. * An attribute value is a type-erased value, something similar to boost::any but with more advanced interface to allow visitation and extraction of the stored value. * A log record is an object which contains a set of named attribute values. For simplicity, let's say: class attribute_value; typedef map< string, attribute_value > attribute_values_view; class record { public: attribute_values_view const& attribute_values() const; string const& message() const; }; * A filter is a function that accepts attribute values and returns a bool: typedef function< bool (attribute_values_view const&) > filter; The filter is assumed to pass the log record if it returns true. * A formatter is a function that accepts a log record and a stream and outputs strings or attribute values to the stream: typedef function< void (ostream&, record const&) > formatter; * I already have the code that extracts attribute values from log records. What I'm trying to achieve is something along these lines (this is not a finalized interface yet): enum my_severity { info, warning, error }; BOOST_LOG_REGISTER_ATTRIBUTE("Severity", severity, my_severity) filter flt = severity >= warning; formatter fmt = stream << "[" << severity << "] " << message; Here, "stream" and "message" are keywords provided by Boost.Log, the former is a placeholder for the stream argument of the formatter function, and the latter returns the result of the record::message() method. Now, the BOOST_LOG_REGISTER_ATTRIBUTE macro is what I'm struggling on. It should create a new keyword (severity) which should: 1. Be a POD object to be statically initialized. 2. Contain the information on the type (my_severity) and name ("Severity") of the attribute to be able to extract it from the log record. 3. Be a Phoenix object to initiate lazy expression generation.
From what I gathered from the docs and examples, the generated keyword should be a terminal, in terms of Proto. Am I correct? In order to produce the value, during the evaluation the keyword should receive either a log record or a attribute_values_view and I can't quite understand how do I do that. Basically, I don't quite understand how do I declare and implement a terminal so that it works with Phoenix. This question also applies to the "stream" and "message" keywords but I suspect the solution will be similar.
Maybe there is an example of doing this in the docs or elsewhere. I'll be glad if someone provides a link or example of doing this. Thanks in advance.