Re: boost::throw_exception potential changes

From: Emil Dotchevski (emildotchevski_at_[hidden])
Date: 2022-02-02 21:58:19

On Wed, Feb 2, 2022 at 1:49 PM Andrey Semashev via Boost <
boost_at_[hidden]> wrote:
>
> On 2/2/22 22:34, Andrey Semashev wrote:
> > 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.
>
> If we want to optimize code size, I would rather preferred if there was
> an API that allows to move exception construction into a function.

You could move the throw statement itself into a function, so instead of a
throw statement the compiler would generate a call to a function that
throws. But I remain unconvinced that the overhead matters.