Re: Lua error handling and C++

[Sam Roberts <sroberts@...>]

On Mon, Oct 16, 2006 at 05:05:45PM -0400, Glenn Maynard wrote:
> That means every object that I might want to create inside a binding

Not every object.

> needs to be handled in this way.  Unfortunately, "delete p" where p is
> a void* will not call destructors--that would make it easy.  I'm not
> a big fan of the "give everything in the world a base class" paradigm ...

Probably no way to avoid the tedium, but I'm not sure if you understood.

Create a single user-data that contains a pointer to a struct MyArgs {
std::string localA; std::string localB; }, and with a __gc pointing to a
function that calls delete on (struct MyArgs*) when the UD is
collected, the struct members will be destroyed.

Basically, this is a process of taking your stack variables, and putting
them on the heap, wrapped in a UD, so lua can tell you when to destroy
them, longjmp or no longjmp. Example is here:

  http://www.lua.org/pil/29.1.html

but instead of having a DIR object, have a struct MyArgs, and instead of
calling opendir() call new MyArgs, and instead of calling closedir(),
call delete.

This may be tedious, but it is easy to be confident its "correct". C++
takes care of calling the dtors for all struct members, you make sure
that you call delete once in the __gc metamethods correctly, and lua
makes sure that the __gc metamethod is called. Since there is nothing on
the stack that is a leakable resource, there is nothing on the stack
that can be leaked. Also, "local" variables get freed the same way
error() or not, when lua cleans up the the lua->C stack.

You asked for suggestions on "other ways", this is one other way.

Cheers,
Sam

> > Its hard to have the benefits of C++ automatic resource freeing with
> > exceptions, without paying the costs for exceptions!
> 
> Not at all.

What does that mean?

If you have way of getting automatic resource freeing across longjmp
without paying the cost for exceptions, why does this thread exist?