Re: Lua 5.3 and C++ exceptions

[Nagaev Boris <bnagaev@...>]

On Fri, Mar 13, 2015 at 9:23 AM, 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
>
>

Hello, Colin,

LuaJIT [1] can handle C++ exceptions transparently in some setups. C++
exceptions can be caught on the Lua side with pcall(), lua_pcall()
etc. It's safe to throw C++ exceptions across non-protected Lua frames
on the C stack. The contents of the C++ exception object pass through
unmodified. Throwing Lua errors across C++ frames is safe. C++
destructors will be called [2]. LuaJIT is compatible with Lua 5.1.

 [1] http://luajit.org/luajit.html
 [2] http://luajit.org/extensions.html#exceptions



Best regards,
Boris Nagaev