1. As mentioned above, the ability to iterate through values and/or pass a visitor. <snip> Use case for iterator over values: I may want an automated checker that verifies that exception A contains only somewhere specified values and nothing else. If you don't insist on "and nothing else", what you want can be implemented without adding iteration to Boost Exception.
Let say we disagree here. I see iteration (or less conveniently visitation) as a useful mechanism for not-yet-designed tools that would automatically process, convert or check validity of exceptions.
Perhaps I don't understand exactly what you mean by iteration. I see something like: struct error_info_iterator_value_type { std::type_info const * tag_type; std::type_info const * value_type; void const * value; std::string value_as_string; }; I might be blind, but I can't see how could the iteration give me anything more useful, and I can't think of any useful generic processing that can be done with this (you can dump all this in a string, but that's exactly what boost::exception::what() does.) Emil Dotchevski