Re: [Boost-users] exception, filesystem, system_error and consistency

Subject: Re: [Boost-users] exception, filesystem, system_error and consistency
From: Michael Schuerig (michael.lists_at_[hidden])
Date: 2011-06-11 04:06:34

On Saturday 11 June 2011, Emil Dotchevski wrote:
> On Fri, Jun 10, 2011 at 1:09 PM, Michael Schuerig
> 
> <michael.lists_at_[hidden]> wrote:
> > Poring over docs and headers, I noticed that neither
> > boost::system::system_error nor boost::filesystem::filesystem_error
> > are derived from boost::exception. Instead they implement an
> > independent exception hierarchy.
> > 
> > I found this disconcerting. For one thing, I have to catch and
> > handle the respective exceptions separately. Far more importantly,
> > though, I was expecting consistency among boost libraries and now
> > see that this is not the case apparently.
> 
> The reason for this discrepancy is that Boost Exception was added
> later than boost::filesystem and boost::system.
> 
> With the introduction of Boost Exception, we now have a single base
> exception type for Boost libraries to use, but rather than modifying
> all existing Boost libraries, the boost::throw_exception function
> (which most Boost libraries have always been calling to throw
> exceptions) was modified so that the object it throws is guaranteed
> to derive from boost::exception. This is done at the point the
> throw_exception template is instantiated: if you pass it an object of
> type T which doesn't derive from boost::exception, it is wrapped in a
> type that derives both from T and from boost::exception (this
> mechanism is formally documented here:
> http://www.boost.org/doc/libs/release/libs/exception/doc/throw_except
> ion.html)

Yes, I had seen that in the docs, but there's also this bit in 
filesystem/v3/config.hpp:

  //  Exceptions were originally thrown via boost::throw_exception().
  //  As throw_exception() became more complex, it caused user error 
reporting
  //  to be harder to interpret, since the exception reported became 
much more complex.
  //  The immediate fix was to throw directly, wrapped in a macro to 
make any later change
  //  easier.

  #define BOOST_FILESYSTEM_THROW(EX) throw EX

What am I supposed to make of this? The wording of the comment suggests 
that the implementers of boost::filesystem have tried to use 
boost::exception, but as it has proved unwieldy, they are not using it 
anymore.

Now, for me, still being new to boost, that sounds like a strong advice 
to "do as we do". I don't understand all the subtle issues involved, so 
I may better keep away from boost::exception, too. I certainly won't 
just redefine BOOST_FILESYSTEM_THROW to BOOST_THROW_EXCEPTION.

I may be overdramatising just a tiny bit, but the issue stands that 
these real or perceived inconsistencies have a strong impact on my 
impression of boost. I was approaching boost with the expectation of a 
set of well-coordinated libraries, coming with the recommendation "if 
you do it at all, do it like this". For me, this picture has got cracks.

Michael

-- 
Michael Schuerig
mailto:michael_at_[hidden]
http://www.schuerig.de/michael/