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From: Pascal Brisset <pascal.brisset@cnet.francetelecom.fr>
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Date: Fri, 30 Oct 1998 11:17:33 +0100 (MET)
To: Thierry Bravier <thierry.bravier@dassault-aviation.fr>
CC: caml-list@inria.fr
Subject: Re: problem with ocamlmktop -output-obj
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Thierry Bravier writes:
 > > Are there any clues to help me build this toplevel with C++ code
 > > without -output-obj ?

I've been calling C++ libraries from caml code since ocaml-1.03
without resorting to any particular hack (see example below). I tend
to avoid things like global objects and c++ exceptions (which were
seriously flawed at that time), so this might not cover all of your
problems. Also, it may fail with compilers other than gcc (because gcc
called as a linker resolves c++ symbols).

- Pascal Brisset <pascal.brisset@cnet.francetelecom.fr> +33296051928 -
- France Telecom CNET DTL/MSV | 2 av Pierre Marzin | F-22307 Lannion -

-------------------------- mlcell.C ---------------------------------

#include <stdio.h>

extern "C" {
# include <caml/mlvalues.h>
# include <caml/alloc.h>
}

class Cell {
public:
  Cell(int init) : val(init) { }
  void set(int x) { val = x; }
  int get() { return val; }
private:
  int val;
};

typedef struct {
  final_fun f;
  Cell *c;
} mlcell;

static void free_cell(value mlc) {
  printf("freeing %p\n", mlc);
  delete ((mlcell*)mlc)->c;
}

extern "C" value caml_cell_create(value mlv) {
  mlcell *res = (mlcell*)alloc_final(sizeof(mlcell)/sizeof(value),
				     free_cell, 1, 1000); /* ? */
  res->c = new Cell(Int_val(mlv));
  return (value)res;
}
			  
extern "C" value caml_cell_set(value mlc, value mlv) {
  ((mlcell*)mlc)->c->set(Int_val(mlv));
  return Val_unit;
}

extern "C" value caml_cell_get(value mlc) {
  int v = ((mlcell*)mlc)->c->get();
  return Val_int(v);
}

-------------------------- cell.ml --------------------------------

module Cell = struct
  type t
  external create : int -> t = "caml_cell_create"
  external set : t -> int -> unit = "caml_cell_set"
  external get : t -> int = "caml_cell_get"
end

let _ =
  let c = Cell.create 99 in
  Printf.printf "%d\n" (Cell.get c);
  Cell.set c 42;
  Printf.printf "%d\n" (Cell.get c);
  flush stdout

let _ = Gc.full_major ()

-------------------------- Testing --------------------------------

$ g++ --version
2.8.1
$ g++ -I/usr/local/lib/ocaml -c mlcell.C
$ ocamlc -v -custom mlcell.o cell.ml -o cell.out
The Objective Caml compiler, version 1.07
Standard library directory: /usr/local/lib/ocaml
$ ./cell.out
99
42
freeing a2944

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