Re: C++ Foreign Function Interface

[Jan Wedekind <jan@wedesoft.de>]

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On Mon, 14 Mar 2016, Chris Vine wrote:
> On Mon, 14 Mar 2016 21:34:24 +0530
> Arun Isaac <theroarofthedragon@gmail.com> wrote:
>> Hans Åberg <haberg-1@telia.com> writes:
>>
>>> When calling C++ from C, you can’t pass a C++ exception through the
>>> C code. So in my example code, there are conversions between C++ and
>>> Guile exceptions.
>>
>> Yeah, this was the discussion in the other thread you linked
>> to. Unfortunately, I don't know anything about C++ exceptions, and
>> hence didn't understand what your code was doing. Can any of this be
>> integrated into guile itself, so that C++ FFI will be easier for the
>> end programmer?
>
> I am not a guile developer but I doubt (as a C++ programmer) that that
> is worth the effort.  If you are calling into a C++ library from any C
> code then you need to consider what exceptions the library might
> throw.  Your 'extern "C"' interface then needs to catch these
> exceptions and turn them into something else.  That might mean
> providing a return value indicating an error condition, or if you are
> programming in guile mode with libguile at that point might mean
> throwing a guile exception.  You can probably ignore std::bad_alloc.
> On most modern systems that exception will not be thrown (you will just
> thrash), and when you are out of memory there is nothing you can do to
> recover anyway as the kernel will take over.  The overall point is that
> you need to ensure that, if you are in guile mode, any C++ exceptions
> are handled locally and do not escape out of a guile dynamic extent nor
> out of a function with C calling convention.
>
> You also need to be aware of the converse, namely that if you throw a
> guile exception out of C++ code, there are no C++ objects with
> non-trivial destructors in scope when the guile exception is thrown, or
> you will get undefined behaviour: most probably the destructors of the
> C++ objects will not be called.  A guile exception is basically a long
> jump up the stack.  But that is almost certainly not an issue if all
> you are doing is calling into a C++ library when in guile mode.  It
> will be an issue if you are yourself constructing your own C++ objects
> when in guile mode.
>
> In most cases this is pretty easy to accomplish once you get the hang
> of it.
>
> Chris
>

It might be worth having a look at Christian Schafmeister's CLASP [1,2]. 
There is no standard C++ binary ABI (application binary interface). CLASP 
basically is a Lisp with an interface to LLVM C++ binaries.
I think in general it will be necessary to parse the C++ headers in order 
to interface with the C++ binaries (e.g. virtual method tables, member 
variables, functions declared in header files).

Jan

[1] https://github.com/drmeister/clasp
[2] https://www.youtube.com/watch?v=8X69_42Mj-g