3) Support for linking multiple error tags in a single expression might be useful and help in standarizing errors throughout an application.

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Could you clarify what you mean?

Lets look at boost::spirit. Lets say that I want to throw a boost::spirit parsing exception. Information available to me includes: 1) the filename 2) line number of error 3) charactar postion of the error Here is how I propose that we throw the error. struct tag_file_name: boost::error_info_value<std::string> { }; struct tag_line_number: boost::error_info_value<std::string> { }; struct tag_charactar_position : boost::error_info_value<std::string> { }; typedef boost::error_info_group< tag_file_name, tag_line_number, tag_charactar_position> spirit_parsing_error_t; int size; std::string path; boost::spirit::parse_info<char const*> info = boost::spirit::parse(def.c_str(), face_grammar(path,size)); if (!info.full) throw my_error() << spirit_parsing_error_t( path, info.line, info.postition); This is much cleaner and gives a standard way of throwing errors that the spirit library generates. Using the "<<" for an insertion operator or the "add_info" member function just makes the throw statement look cluttered. I dont think that they would get used because the just look funny and non-standard. Creating a typedef for each each error grouping is much cleaner. Just for reference, the "<<" insertion operator would look like this: throw my_error() << boost::error_info<tag_file_name>(name) << boost::error_info<tag_line_number>(number) << boost::error_info<tag_charactar_position>(position); This is very verbose, but minimally acceptable!!. Also, Just for reference, the "add_info" insertion operator would look like this: throw my_error().add_info<tag_file_name>(name).add_info< tag_line_number>(number).add_info<tag_charactar_position>(position); This is too funky for me. I'd never use this syntax of "add_info" "boost::error_info_group" would just wrap "boost::error_info" and have defaulted template paramaters, so that if I wanted to create an extended spirit exception, I would just do this: throw my_error() << boost::error_info_group<tag_file_name,tag_line_number, tag_charactar_position, error_number>(name,line,pos,error_no); Simple!! I've already wrote the "error_info_group" wrapper if you want me to send it to you.