Re: Lua 5.3 and C++ exceptions

[Eric Wing <ewmailing@...>]

On 3/13/15, Colin <lua@colin-hirsch.net> wrote:
> Hi,
>
> following the announcement of Lua 5.3 with the greatly appreciated addition
> of native integer support we are planning to update a bunch of C++ projects
> from Lua 5.1 and were wondering how the compatibility with C++ exceptions
> has progressed.
>
>
> The scenario that we are looking at is
>
> C++ application
> calls
> Lua function
> calls
> C++ function
>
> when the inner C++ function throws an exception.
>
>
> Currently we handle this situation at the Lua-to-C++ call point by wrapping
> the C++ function in a try-catch block that logs an exception and calls the
> Lua error function; at the C++-to-Lua transition we detect the Lua error and
> then throw a C++ exception.
>
> (With C++11 we could improve this approach by extending the Lua state by a
> std::exception_ptr, storing the exception caught at the Lua-to-C++
> transition there, and re-throwing it at the C++-to-Lua point.)
>
>
> The question is, can this be simplified?
>
> Ideally the C++ exception would simply "fly through" the Lua interpreter
> from the inner C++ function to the outside C++ application, but as far as I
> understand, that is not, or at least: was not possible because Lua obviously
> can't use RAII and requires an explicit call to its error handling
> facilities to keep everything consistent.
>
> If this "flying through" does not work unmodified, is it possible to somehow
> "patch up" things, i.e. let the exception fly through and then afterwards,
> in a try-catch at the C++-to-Lua point, call some Lua function to put things
> back into a consistent state? (This would allow leaving out the try-catch
> around the inner C++ function and not require adding the exception_ptr to
> the Lua state to recover the original exception.)
>
> Or what else is the recommended way of handling this interoperability
> question, what is the current "best practice"?
>
> Thanks, Colin
>

So I think the expected practice is to compile Lua's ldo.c as C++. It
will define Lua's internal mechanism for pcall to use try/catch
instead of setjmp/longjmp. Then if a C++ exception does occur, pcall
will be aware of it and catch it and then you just do the normal error
handling thing. No need to do anything elaborate.

It looks like this was isolated to just ldo.c in Lua 5.3 (and 5.2?)
whereas it was in luaconf.h in Lua 5.1. Thank you to the Lua team for
this as somebody who had to modify this to work with Obj-C's exception
handling mechanism. I think this will make it much easier to
modify/maintain in the future.

-Eric
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Beginning iPhone Games Development
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