octave and oo

[Ernst Reissner]

Hi all,
at the moment i work through a tutorial with oo in Matlab
http://www.mathworks.com/help/techdoc/matlab_oop/f3-28024.html
and octave and try to reproduce the example there:
It is about defining a class DocPolynom of polynomials.
The constructor seems to work well,

function obj = DocPolynom(c)
   % Construct a DocPolynom object using the coefficients supplied
   if isa(c,'DocPolynom')
      obj.coef = c.coef;
   else
      obj.coef = c(:).';
   end
end 

but although it is do easy, the double-method does not:

function c = double(obj)
   % DocPolynom/Double Converter
   c = obj.coef;
end 

What i obtain is

 p = DocPolynom([1 0 -2 -5])
p =
{
  coef =

     1   0  -2  -5

}

octave:16> c = double(p)
error: struct type invalid as index value

I am sure that my m-files are evaluated like char.m defined below,
because it produces failures:

octave:16> p = DocPolynom([1 0 -2 -5]);
octave:17> char(p)
error: scalar cannot be indexed with .
error: called from:
error:   /home/ernst/Software/resource/octave/m/@DocPolynom/double.m at line 3, column 3
error: evaluating argument list element number 1
error: evaluating argument list element number 1
error: evaluating argument list element number 1
error: evaluating argument list element number 1
error:   /usr/share/octave/3.2.4/m/general/int2str.m at line 105, column 5
error: evaluating argument list element number 1
error: evaluating argument list element number 1
error:   /usr/share/octave/3.2.4/m/general/int2str.m at line 69, column 11
error: evaluating argument list element number 1
error:   /home/ernst/Software/resource/octave/m/@DocPolynom/char.m at line 36, column 23


what is wrong here??

Greetings,
Ernst