Re: C++ exceptions not really supported?

["David Burgess" <dburgess@...>]

This topic has been talked about in the distant past. I suggest you
search the list. But when using MS VC(6 & 7) on windows and ensuring
that I use the compiler flag(s) that say do not assume that C
functions cannot throw, it works OK for me. It also requires that
ldo.c and lua.cpp be compiled as C++.
I also patched ldo.c (the outline of this is below).
db



#undef luai_jmpbuf
#define luai_jmpbuf	int  /* dummy variable */
#undef LUAI_THROW
#define LUAI_THROW(L,c)	throw lua_exception(errcode)

#if defined(__cplusplus) && defined(ldo_c)
class lua_exception {
  int status;
public:
  lua_exception(int status_) : status(status_) { };
  int get_status() { return status; }
};
#endif


#undef LUAI_TRY
#define LUAI_TRY(L,c,a) \
  try { a }  catch (lua_exception &e) \
    { (c)->status = e.get_status();if ((c)->status == 0) (c)->status = -1; } \
  catch (...) { throw; }

On 10/20/07, Greg Falcon <veloso@verylowsodium.com> wrote:
> On 10/19/07, Colin <share-lua@think42.com> wrote:
> > Hallo,
> >
> > I am currently, for the first time, in the process of adding Lua scripting
> > to a set of C++ applications and have reached the point of digging into
> > error handling; after reading the following statement in the documentation:
> >
> > "You can also choose to use exceptions if you use C++"
>
> What this sentence is referring to is the way Lua implements its error
> handling functions.  Lua can use try/catch rather than setjmp/longjmp
> for unwinding the C++ stack during error handling.  (In fact, this is
> the default behavior; see luaconf.h.)
>
> It helps not to view this as an attempt of Lua to provide a C++ API.
> Lua's API is C through-and-through, and you'll get best results
> thinking of it as such.  In general it's not very sporting to pass
> function pointers to a pure C API when those functions might raise C++
> exceptions.
>
> Lua's use of the C++ exception mechanism does have a point, though.
> The Lua functions pcall() and error() can cause lua_CFunctions in the
> Lua call stack to be exited early as the stack is unwound.  The
> interaction between setjmp/longjmp's and C++ destructors is left
> explicitly undefined in the C++ spec.  By using try/catch instead, Lua
> ensures that the destructors of your C++ locals get called if an error
> causes an early exit.
>
> But as far as I can gather looking at the code, that's all it's for.
> As written, C++ exceptions cannot safely be thrown across a Lua
> boundary [1].  For example, when built in C++, pcall actually gets
> implemented using the scary catch(...).  No C++ exceptions at all will
> ever make it through a pcall().
>
> This actually feels typical and natural to me, but maybe I've spent
> too much of my life writing C++ code to interface with C libraries.
> Lua seems quite well behaved here.  You can rely on C++ idioms -- in
> particular, exceptions and RAII -- in your C++-based implementations
> of Lua fuctions, so long as you promise not to leak any of those
> features to the underlying C library.
>
> Greg F
>
> [1] By that, I mean a C++ function calling a Lua function calling
> another C++ function.
>