I'm not keen on the use of operator<< to add the data to the exceptions.
I do share your concern, but consider that whatever we use for boost exception should support easy composition, because it needs to work when used in a throw expression directly:
throw my_error() << error_info<tag_errno>(errno) << error_info<tag_name>(name);
Instead of << we could use .add:
throw my_error(). add(error_info<tag_errno>(errno)). add(error_info<tag_name>(name));
This isn't bad, but in my opinion the << syntax is better despite my strong dislike for operator overloading.
Maybe a more or less obvious choice will do
throw my_error() = error_info<tag>(whatever), error_info<another_tag>(something_else) // ...
I don't know, I'd rather use .add(), to be honest. Also, in an off-list discussion, Peter Dimov pointed out that instead of layering .add calls directly in the throw expression, we could write: my_error tmp; tmp.add(error_info<tag_errno>(errno)); tmp.add(error_info<tag_name>(name)); throw tmp; I'm not strongly opposed to that either. Anyway, I think that operator<< is a good compromise, but if others feel strongly against it I'll happily change it to .add(). Emil Dotchevski