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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 <>, 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

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