Re: boost::throw_exception potential changes

From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2022-02-02 19:34:11

On 2/1/22 07:48, Peter Dimov via Boost wrote:
> Currently, `boost::throw_exception(x)` automatically throws
> an exception derived from `x` that (1) injects a boost::exception
> base class if there's none, and (2) injects a base class that enables
> boost::exception_ptr support (boost::current_exception and
> boost::rethrow_exception.)
> 
> This is convenient, but the downside is the amount of generated
> code, e.g. https://godbolt.org/z/5T8T8GEqP (422 lines) compared
> to https://godbolt.org/z/1zr1odf7n (36 lines.)
> 
> This is not the end of the world as this code is only generated once
> per exception type, not on every call to throw_exception, but it's
> still unpleasant to see.
> 
> It so happens that boost::exception_ptr has recently acquired the
> ability to work under C++11 without the need for the supporting
> base class, by using the standard std::exception_ptr infrastructure.
> So if we also remove the automatic injection of boost::exception
> as a base class, and ask users to derive their exceptions from it if
> they desire having it as a base, it's possible to simplify
> boost::throw_exception considerably.

I would like to ask to not do this, as this is a breaking change and I
have written code (outside Boost) that relies on BOOST_THROW_EXCEPTION
and boost::exception. That is, I expect that an exception thrown by
BOOST_THROW_EXCEPTION triggers `catch (boost::exception& e)` handler,
where I may augment it with additional info before rethrowing. I also
think one could use dynamic_cast to boost::exception to the same effect,
and that would also break after such a change.

If some lightweight throwing mechanism is needed (of which I'm not
convinced), I would prefer that to be a new API.