In xpressive, I'm trying to follow the recommended use of Boost.Exception and use virtual inheritance in my exception class, regex_error. I'm also trying to hew as closely as possible to the standard, which says that regex_error should inherit from std::runtime_error.

Eric Niebler wrote: the standard says that regex_error doesn't not use virtual inheritance.

However, this simple use of Boost.Exception does not compile:

#include <string> #include <exception> #include <boost/throw_exception.hpp> #include <boost/exception/exception.hpp>

#include <stdexcept> is missing

struct regex_error : virtual std::runtime_error , virtual boost::exception { explicit regex_error(std::string const& what) : std::runtime_error(what) {} };

int main() { boost::throw_exception(regex_error("what")); }

On VC9, I get the following:

...

C:\boost\org\trunk\boost/exception/exception.hpp(400) : error C2512: 'std::runtime_error::runtime_error' : no appropriate default constructor available

The trouble is that std::runtime_error does not have a default constructor, and that boost::throw_exception doesn't simply throw the exception passed to it; it throws an object of type of clone_impl<regex_error>. Because of the virtual inheritance, clone_impl needs to call std::runtime_error's directly, but it doesn't. It can't know to do that in general.

What is the recommended practice here?

<aside> It actually doesn't seem to be particularly useful in this case to use virtual inheritance because std::runtime_error inherits from std::exception non-virtually. In order to avoid the pitfall described here <http://tinyurl.com/yb2jpns>, it seems *ever* exception type in the hierarchy needs to use virtual inheritance, and that unfortunately is not the case for the standard exception hierarchy.

gcc says basically the same this make me wonder why the standard exception hierarchy does not use virtual inheritance?

</aside>

BR, Dmitry

Eric Niebler wrote:

struct regex_error : virtual std::runtime_error , virtual boost::exception

There is no need to derive virtually from runtime_error. As you say below, this serves no purpose.