Re: boost::throw_exception potential changes

From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2022-02-02 21:49:37

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. That
would save quite some code duplication, especially if there are many
instances of throwing the same exception type. Here is a toy example:

https://godbolt.org/z/hbv56E5z6

Wrapping this into a macro, I think, would require to pass the exception
type as a separate argument, and requires support for variadic macros. E.g.:

  BOOST_THROW(std::runtime_error, "bad things happen");

Note that this would retain compatibility with boost::exception.