Re: Class Objects Usage

[David Bateman]

Hi Fred,

Its great that you are using this part of the Octave code. However, this
part of the code is recent and under active development. At this point I
think you'll need to wait till John come back online to answer any
questions about this code..

Regards
David


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; }
>
>
> 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?
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-- 
David Bateman                                address@hidden
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