From help-octave-request at bevo dot che dot wisc dot edu Thu Aug  8 10:52:26 2002
Subject: Simple oct function
From: "John W. Eaton" <jwe at bevo dot che dot wisc dot edu>
To: Iago Mosqueira <i dot mosqueira at ic dot ac dot uk>
Cc: help-octave at bevo dot che dot wisc dot edu
Date: Thu, 8 Aug 2002 10:52:17 -0500

On  8-Aug-2002, Iago Mosqueira <i dot mosqueira at ic dot ac dot uk> wrote:

| Hello,
| 
| I want to use some fortran subroutines from the GSLIB library in octave, 
| and I am attempting for the first time to write a C++ wrapper. After 
| reading the manual and a few other examples, I have the C++ and F77 
| functions attached at the end. There is some simple clash between int 
| and octave_value_list, when I do
| 
| 	mkoctfile addone.cpp -o addone.oct
| 
| I get
| 
| addone.cpp: In function `class octave_value_list Faddone(const 
| octave_value_list &, int)':
| addone.cpp:13: `retval' cannot be used as a function
| addone.cpp:13: implicit declaration of function `int addone(...)'
| addone.cpp:13: warning: cannot pass objects of type `octave_value' 
| through `...'
| /usr/include/octave-2.1.36/octave/oct-obj.h:173: 
| `octave_value_list::octave_value_list(int)' is private
| addone.cpp:15: within this context
| 
| what is the problem here? I know almost nothing about C++ and F77, but 
| if I could get this simple example running I would understand how to 
| make the real subroutines work.
| 
| Many thanks
| 
| 
| iago
| 
| 
| addone.cpp
| 
| #include <octave/config.h>
| #include <octave/oct.h>
| #include <stdio.h>
| #include <stdlib.h>

You probably want to avoid C headers in C++ files.  Use <cstdio> and
<cstdlib> instead, and then only if actually needed.

| extern "C" int Addone_ (int *, int *);

If this is your Fortran routine, you probably want to use the F77_FUNC
macros to handle possible upcasing and underscoring of the name:

  extern "C" int F77_FUNC (addone, ADDONE) (int*, int*);

But, since your function below accepts scalars and only modifies the
second arg, it might be better to write

  extern "C" int F77_FUNC (addone, ADDONE) (const int&, int&);

Note that the value is not really returned from addone, but passed
back in the second parameter.  The int return type above is used
because that works with most Unixy Fortran compilers, and it's also
the way f2c works.  If you have Fortran functions instead of
subroutines, its much more reliable to wrap them in subroutines
because different compilers use different conventions for returning
values from functions.

| DEFUN_DLD (addone, args, nargout,
|    "Addone")

If you are not using nargout, you can omit it.

| {
|    octave_value_list retval;

If you only have one value to return, you can write

  octave_value retval;

and Octave will automatically convert it to an octave_value_list with
one element.

|    retval(0) = addone(args(0));

You probably want to do something like this:

  int x = args(0).int_value ();

  if (! error_state)
    {
      int result;
      F77_FUNC (addone, ADDONE) (x, result);
      retval = static_cast<double> (result);
    }
  else
    error ("addone: expecting integer argument");

|    return retval;
| }
|
| 
| addone.f
| 
| subroutine addone(x,y)
| 
| integer x
| integer y
| y = 1 + x
| return
| end subroutine

But to be reliable, you will probably also want to check that your
function has been called with the correct number of arguments.
Putting it all together:

#include <octave/config.h>
#include <octave/oct.h>

extern "C" int F77_FUNC (addone, ADDONE) (const int&, int&);

DEFUN_DLD (addone, args, ,
  "y = addone (x)")
{
  octave_value retval;

  int nargin = args.length ();

  if (nargin == 1)
    {
      int x = args(0).int_value ();

      if (! error_state)
        {
          int result;
          F77_FUNC (addone, ADDONE) (x, result);
          retval = static_cast<double> (result);
        }
      else
        error ("addone: expecting integer argument");
    }
  else
    print_usage ("addone");

 return retval;
}

jwe



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