On Wed, 18 Jan 2023 at 15:07, Jason McKesson via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> On Wed, Jan 18, 2023 at 4:45 AM Frederick Virchanza Gotham via
> Std-Proposals <std-proposals_at_[hidden]> wrote:
> >
> > I have sent this email to two mailing lists for programming language
> > proposals, the one for C++ and the one for Python. If you reply to
> > this email, please make sure you reply to both. You can see the
> > mailing list archives for this month for each list here:
> >
> > C++ : https://lists.isocpp.org/std-proposals/2023/01/date.php
> > Python :
> https://mail.python.org/archives/list/python-dev_at_[hidden]/2023/1/
> >
> > Both C++ and Python have exception handling, however a C++ program
> > which links with a Python library is unable to handle an exception
> > thrown from Python.
> >
> > Is it conceivable in the future that the C++ Standards Committee and
> > the Python Steering Council could cooperate and furnish each other
> > with an exhaustive list of exceptions thrown from their respective
> > standard libraries, so that both languages can implement compatibility
> > with each other's exceptions?
> >
> > If such a list were to accompany each C++ standard and each Python
> > Language Reference, then for example the C++ standard library could
> > provide classes for handling Python exceptions:
>
> That's not a reasonable thing to have in the standard library. A
> Python library can and should do that, but the C++ standard library
> should not have features that are so tied to a specific language like
> that.
>
> Also, this proposal seems to be missing the biggest issue with
> cross-language exception handling: the fact that you can't throw
> exceptions across languages. The only thing you *can* do is catch
> exceptions on the source language end, convert them into some data
> packet, and throw a different exception on the destination language
> side.
>

You can on most major platforms (they're called "foreign" exceptions; see
previous discussion
https://groups.google.com/a/isocpp.org/g/std-discussion/c/DLQY1iYoe_w).

That said, Python does not use platform exception mechanisms (at least not
on Windows or Itanium), so you can't catch Python exceptions that way
(unless a library e.g. pybind11 has already done the work of translating
Python exceptions to C++).

That is, having some class representing an external exception is not
> the biggest problem in having external exceptions. Any C++ binding for
> a language can provide such exception types; you can just use the
> types the binding system provides. The problem is implementing the
> translation layer uniformly. Which this proposal doesn't address.
>
> Now, C++23 does have a generic tool for transmitting foreign
> exceptions in a uniform way: `std::expected`. Indeed, it would make a
> lot more sense to use that as the interface between the external
> language and C++.
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