Re: [boost] [exception] virtual inheritance troubles

Subject: Re: [boost] [exception] virtual inheritance troubles
From: Dmitry Goncharov (dgoncharov_at_[hidden])
Date: 2010-02-26 03:19:24

Eric Niebler wrote:
> 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.
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
>
gcc says basically the same
> 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.
this make me wonder why the standard exception hierarchy does not use 
virtual inheritance?
> </aside>
>
BR, Dmitry