<snip> I see. This feature is most valuable. However, so is Tom's suggestion and I don't think both are mutually exclusive (that is if we let go of using the template-id 'error_info' for the latter):
typedef boost::custom_exception<tag:: errno,tag::filename,tag::size> file_error;
Consider this code: void read_file( char const * filename ) { something_unrelated(); .... if( file_failure ) throw file_error() << error_info<tag_filename>(filename); } If I understand correctly, you are concerned with making sure all file_error exceptions pack a file_name. But what if "something_unrelated" throws? Isn't the filename known to our function relevant to that other failure too? It sure is, despite that we have no idea what exceptions "something_unrelated" could throw. So, I'd write that function like this: void read_file( char const * filename ) { try { something_unrelated(); .... if( file_failure ) throw file_error(); } catch( boost::exception & x ) { x << error_info<tag_filename>(filename); throw; } } How would your custom_exception idea fit in this framework? Emil Dotchevski