Class Objects Usage

[John W. Eaton]

On  7-Jul-2008, Fred Schuld wrote:

| 
| I have compiled Octave from the latest 3.0.0+ source off HG with MSVC2005.
| Michael G has helped (thanks) with many questions I had with getting a 
successful build with MSVC2005.
| 
| Anyways, I was interested in trying the support for class objects.
| 
| Entering "help class" returns:
| 
|  -- Built-in Function:  class (EXPR)
|  -- Built-in Function:  class (S, ID)
|       Return the class of the expression EXPR, as a string or create a
|       class object from the structure S with name ID.
| 
| 
| 
| Using a simple example I create a file with filename "polynom.m" with the 
@file convention:  current address@hidden
| 
| Script code:
| 
| % Polynom script
| function p = polynom(a)
| 
| % Polynom constructor
| 
| if nargin == 0
|    p.c = [];
|    p = class(p, 'polynom');
| elseif isa(a,'polynom')
|    p = a;
| else
|    p.c = a(:).';
|    p = class(p, 'polynom');
| end
| 
| If the polynom constructor is called with:  p = Polynom([1 0 3])
| 
| 
| I get the following error:
| 
| error: class: invalid call from outside class constructor
| error: evaluating assignment expression near line 11, column 6
| 
| If I use the "methods" octave command:  methods("polynom")
| 
| 
| It returns:
| 
| Methods for class polynom:
| 
| polynom
| 
| Which is what I expect since it only has one method so far: the constructor.
| 
| 
| I looked at the code starting at line 967 in ov-class.cc:
| 
| 
| DEFUN (class, args, ,
|   "-*- texinfo -*-\n\
| @deftypefn {Built-in Function} {} class (@var{expr})\n\
| @deftypefnx {Built-in Function} {} class (@var{s}, @var{id})\n\
| \n\
| Return the class of the expression @var{expr}, as a string or\n\
| 
| create a class object from the structure @var{s} with name @var{id}.\n\
| @end deftypefn")
| {
|   octave_value retval;
| 
|   int nargin = args.length ();
| 
|   if (nargin == 1)
|     retval = args(0).class_name ();
| 
|   else if (nargin == 2)
|     {
|       Octave_map m = args(0).map_value ();
| 
|       if (! error_state)
|     {
|       std::string id = args(1).string_value ();
| 
|       if (! error_state)
|         {
|           octave_function *fcn = octave_call_stack::caller ();
| 
| 
|           if (fcn && fcn->is_class_constructor ())
|         retval = octave_value (new octave_class (m, id));
|           else
|         error ("class: invalid call from outside class constructor");
| 
| ...
| 
| I checked on the "is_class_constructor()" method and it is set to return 
FALSE on line 77 of ov-fcn.h:
| 
| virtual bool is_class_constructor (void) const { return false; }

That's the first definition of the virtual function in the base
class.  Look again at the definition of this function in the
octave_user_function derived class.

| Perhaps the "octave_class::in_class_method" would be better to call?
| 
| In summary it looks like the code is configured to return the output
| 
| "error: class: invalid call from outside class constructor" for all cases.
| 
| 
| Is this expected operation or is there something wrong with my
| example script use of class objects?

I don't know what is causing the problem for you.  Here is what I see
with your function and a copy of Octave built from the current
sources:

  octave:1> x = polynom
  octave:2> class (x)
  ans = polynom
  octave:3> which polynom
  which: `polynom' is the function from the file 
/scratch/jwe/build/octave/@polynom/polynom.m
  octave:4> type polynom
  polynom is the function defined from the file
  /scratch/jwe/build/octave/@polynom/polynom.m:

  % Polynom script
  function p = polynom(a)

  % Polynom constructor

  if nargin == 0
     p.c = [];
     p = class(p, 'polynom');
  elseif isa(a,'polynom')
     p = a;
  else
     p.c = a(:).';
     p = class(p, 'polynom');
  end
  octave:5> methods ("polynom")
  Methods for class polynom:

  polynom


I'm using a Debian system.  If this problem is specific to Windows,
then someone who uses Windows will have to help debug it.

jwe