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How to wrap around C++?
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Date: 2010-02-08 (17:44)
From: Guillaume Yziquel <guillaume.yziquel@c...>
Subject: Re: [Caml-list] Re: How to wrap around C++?
Luca de Alfaro a écrit :
> I am trying another approach... it might make more sense for me to embed the
> Ocaml into C++.

This is not the way you'll get the most help out of this list. People 
are more familiar with making C bindings. Making C++ bindings is rather 
close to it.

> I have read the instructions, and it seems feasible, except that I have a
> few questions:
>    - All I need to pass, as arguments, are int, float, string, and arrays of
>    these.  Any example of how to deal with the arrays?

You need to construct them from C side, and it's more a pain than taking 
C structs and wrapping them into OCaml.

The manual describes the structure of OCaml values rather precisely. 
There are also some pages by Richard Jones on his blog which explain 
rather nicely the internals of OCaml values. One advice: stick to the 
macros provided. Do not try to construct manually, say, OCaml strings on 
the C side. Use caml_copy_string and friends.

>    - How can I return arrays, in a way that C or C++ understands?  How can I
>    return tuples, i.e., how can I return multiple values from Ocaml to C?

These are documented in the manual and in Richard Jones' blog.

For couples, you can do

> value couple = caml_alloc(2, 0);
> Store_field(couple, 0, my_ocaml_val);
> Store_field(couple, 1, my_other_ocml_val);

For arrays, you'll have seamless integration by using Bigarrays.

>    - Finally, do I need to worry about the Ocaml garbage collector, if I
>    call Ocaml from C/C++?  Will it run every now and then? How can the garbage
>    collector know whether a value returned by an Ocaml function is still being
>    used in C/C++?  How can I tell it that it is no longer used?

Essentially, the garbage collector will run potentially each time you 
allocate an OCaml value. caml_copy_string? the GC may run.

You have to register values being used on the C side as a GC root. It's 
easier and more documented to do it the other way round by calling C++ 
from OCaml.

> The problem I am trying to solve seems to be a can of worms from whichever
> angle I take it...


The solution I proposed with Swig is very verbose, but it is a clean 
solution if you do it manually.

You have Makefile compilation instructions to compile C++ with OCaml 
(the main issue with C++ and the extern "C" is essentially the name 
mangling of symbols provided by your C++ object files. All the rest is 
pretty similar to C. This is really the *main* point).

You have at the end of my last email and example of how to construct an 
object and feed it back to OCaml. It may not be really clean, it lacks 
finalisers, but these last two points are stuff that you're going to 
have to deal with anyway if you're going with the OCaml/C interface. 
Look up "custom blocks" and finalisation in the OCaml manual section 
concerning the C interface.

> Luca

All the best,

      Guillaume Yziquel