jextract C++ support

[Rel]

dynamic dispatch

I tried following example [https://github.com/enatai/panamaexperiments/blob/main/libcppexperiments/src/main/public/happy.hpp] and it works fine as long as we generate proper Java bindings.

See test for it [https://github.com/enatai/panamaexperiments/blob/main/cppexperiments/src/test/java/cppexperiments/HappyTests.java#L36]

Please let me know which dynamic dispatch use cases you are concerned with. Because this one seems works fine.

std::string
I totally forgot that such basic type like std::string in C++ is a template.
But it seems possible to call functions which operate with string objects because symbols for them are present:

00000000000013ad T _ZN7unhappy10helloWorldB5cxx11Ev

std::string helloWorld();

I guess it is possible to create/extract layout for std::string using FFM but:
- how to initialize this layout from Java? we cannot just call std::string constructor for it, right?
- this layout may differ between different C++ runtimes (libstdc++ etc). MS C++ may have not same std::string layout as GCC

> Their binding generator adopts the same simple approach as the one I showed in the patch.

I will take a look

------- Original Message -------
On Tuesday, May 23rd, 2023 at 8:58 AM, Maurizio Cimadamore <maurizio.cimadamore at oracle.com> wrote:

> On 23/05/2023 05:11, Rel wrote:
>
>>> What I meant for "robust analysis" was to try and establish how many _real-world_ C++ library can really be tackled in such a direct approach.
>>
>> Ohh I see now, I am affraid we know the answer for this :)
>>
>> Let's imagine if number of C++ libraries which can be covered end-to-end with "simple" approach is 0, does it mean that we should discard it and only focus on shim for binding all kinds of APIs? What about those cases which can be easily extracted using "simple" approach, like Point2d?
>
> Perhaps we should reach out to the Rust community? Their binding generator adopts the same simple approach as the one I showed in the patch. Given how hard it is to support C++ (because the underlying libclang C API is not very solid in that respect), I'd be surprised if they maintained all the necessary code just for stuff like Point2d?
>
>> Because I thought that we would like to do "analysis" of what C++ use cases can/cannot be covered with "simple" approach. For example from your previous message I see that we are not completely sure about exceptions:
>>
>>> * (probably way more stuff, like exceptions, etc.)
>>
>> Similarly for myself I would like to see what are the problems with "dynamic dispatch". I added it to "unhappy" for now, because we expect it not to work, but I plan to test it to see what are the issues there and share here. Similarly with that anyone would be able to reproduce and see same results.
>>
>> I guess my question now is: do we think it may be useful to know exactly how many C++ use cases can be covered with "simple" FFM approach. And if answer is yes, then we can use panamaexperiments as a playground where we can have tests for what is covered. This (possibly?) can give us more confidence in limitations of "simple" approach and how far we can go with it (and this can be easily demonstrated to everyone just by running those tests)
>
> I think it would be useful to know some answer to that question, yes. My intuition tells me that there are probably two kinds of C++ libraries: those who were born that way, and those that moved over from being simpler C libraries. One such example in the latter category is OpenCV [1]. While its "core" header [2] declares an exception, as well as a bunch of classes, eyeballing it, it doesn't seem "too" problematic? Perhaps that would be a good point where to start, and, if a library such as that can be used with some degree of success, perhaps we can expand the search to other similar libraries.
>
> [1] - https://opencv.org/
> [2] - https://github.com/opencv/opencv/blob/4.x/modules/core/include/opencv2/core.hpp
>
>> Ideas?
>>
>> ------- Original Message -------
>> On Monday, May 22nd, 2023 at 9:14 AM, Maurizio Cimadamore [<maurizio.cimadamore at oracle.com>](mailto:maurizio.cimadamore at oracle.com) wrote:
>>
>>> On 22/05/2023 04:12, Rel wrote:
>>>
>>>>> But I believe some more robust
>>>>> analysis should be made to understand exactly how many APIs can be
>>>>> supported in this "simple" fashion.
>>>>
>>>> Yes, I started to gather such analysis here
>>>> https://urldefense.com/v3/__https://github.com/enatai/panamaexperiments__;!!ACWV5N9M2RV99hQ!KOxdK2qmmSzlaaO3kSlSUG2-ifWAVRD6OHlz9NHYvuggmy7NnnNxWvHYcxDm0Vn4gPXlbasjfC-ehIx_KlWNYiU$
>>>> Currently there is only one happy case [
>>>> https://urldefense.com/v3/__https://github.com/enatai/panamaexperiments/blob/main/libcppexperiments/src/main/public/happy.hpp__;!!ACWV5N9M2RV99hQ!KOxdK2qmmSzlaaO3kSlSUG2-ifWAVRD6OHlz9NHYvuggmy7NnnNxWvHYcxDm0Vn4gPXlbasjfC-ehIx_K7Nl50c$
>>>> ] which is Point2d class from your foo.hpp file.
>>>
>>> This is not too surprising - after all the hacky changes I shared were built around that example.
>>>
>>> What I meant for "robust analysis" was to try and establish how many _real-world_ C++ library can really be tackled in such a direct approach. My feeling is "not many" - but I don't have any hard data to back up this claim.
>>>
>>> Maurizio
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