(octave.info)Class Methods

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34.2 Class Methods
==================

There are a number of basic class methods that can (and should) be
defined to allow the contents of the classes to be queried and set.  The
most basic of these is the ‘disp’ method.  The ‘disp’ method is used by
Octave whenever a class should be displayed on the screen.  Usually this
is the result of an Octave expression that doesn’t end with a semicolon.
If this method is not defined, then Octave won’t print anything when
displaying the contents of a class which can be confusing.

An example of a ‘disp’ method for the polynomial class might be

     function disp (p)
     
       a = p.poly;
       first = true;
       for i = 1 : length (a);
         if (a(i) != 0)
           if (first)
             first = false;
           elseif (a(i) > 0 || isnan (a(i)))
             printf (" +");
           endif
           if (a(i) < 0)
             printf (" -");
           endif
           if (i == 1)
             printf (" %.5g", abs (a(i)));
           elseif (abs (a(i)) != 1)
             printf (" %.5g *", abs (a(i)));
           endif
           if (i > 1)
             printf (" X");
           endif
           if (i > 2)
             printf (" ^ %d", i - 1);
           endif
         endif
       endfor
     
       if (first)
         printf (" 0");
       endif
       printf ("\n");
     
     endfunction

   To be consistent with the Octave graphic handle classes, a class
should also define the ‘get’ and ‘set’ methods.  The ‘get’ method
accepts one or two arguments.  The first argument is an object of the
appropriate class.  If no second argument is given then the method
should return a structure with all the properties of the class.  If the
optional second argument is given it should be a property name and the
specified property should be retrieved.

     function val = get (p, prop)
     
       if (nargin < 1 || nargin > 2)
         print_usage ();
       endif
     
       if (nargin == 1)
         val.poly = p.poly;
       else
         if (! ischar (prop))
           error ("@polynomial/get: PROPERTY must be a string");
         endif
     
         switch (prop)
           case "poly"
             val = p.poly;
           otherwise
             error ('@polynomial/get: invalid PROPERTY "%s"', prop);
         endswitch
       endif
     
     endfunction

Similarly, the first argument to the ‘set’ method should be an object
and any additional arguments should be property/value pairs.

     function pout = set (p, varargin)
     
       if (numel (varargin) < 2 || rem (numel (varargin), 2) != 0)
         error ("@polynomial/set: expecting PROPERTY/VALUE pairs");
       endif
     
       pout = p;
       while (numel (varargin) > 1)
         prop = varargin{1};
         val  = varargin{2};
         varargin(1:2) = [];
         if (! ischar (prop) || ! strcmp (prop, "poly"))
           error ("@polynomial/set: invalid PROPERTY for polynomial class");
         elseif (! (isreal (val) && isvector (val)))
           error ("@polynomial/set: VALUE must be a real vector");
         endif
     
         pout.poly = val(:).';  # force row vector
       endwhile
     
     endfunction

Note that Octave does not implement pass by reference; Therefore, to
modify an object requires an assignment statement using the return value
from the ‘set’ method.

     p = set (p, "poly", [1, 0, 0, 0, 1]);

The ‘set’ method makes use of the ‘subsasgn’ method of the class, and
therefore this method must also be defined.  The ‘subsasgn’ method is
discussed more thoroughly in the next section (Note: Indexing
Objects).

   Finally, user classes can be considered to be a special type of a
structure, and they can be saved to a file in the same manner as a
structure.  For example:

     p = polynomial ([1, 0, 1]);
     save userclass.mat p
     clear p
     load userclass.mat

All of the file formats supported by ‘save’ and ‘load’ are supported.
In certain circumstances a user class might contain a field that it
doesn’t make sense to save, or a field that needs to be initialized
before it is saved.  This can be done with the ‘saveobj’ method of the
class.

 -- : B = saveobj (A)
     Method of a class to manipulate an object prior to saving it to a
     file.

     The function ‘saveobj’ is called when the object A is saved using
     the ‘save’ function.  An example of the use of ‘saveobj’ might be
     to remove fields of the object that don’t make sense to be saved or
     it might be used to ensure that certain fields of the object are
     initialized before the object is saved.  For example:

          function b = saveobj (a)
            b = a;
            if (isempty (b.field))
               b.field = initfield (b);
            endif
          endfunction

     See also: Note: loadobj, Note: class.

‘saveobj’ is called just prior to saving the class to a file.
Similarly, the ‘loadobj’ method is called just after a class is loaded
from a file, and can be used to ensure that any removed fields are
reinserted into the user object.

 -- : B = loadobj (A)
     Method of a class to manipulate an object after loading it from a
     file.

     The function ‘loadobj’ is called when the object A is loaded using
     the ‘load’ function.  An example of the use of ‘saveobj’ might be
     to add fields to an object that don’t make sense to be saved.  For
     example:

          function b = loadobj (a)
            b = a;
            b.addmissingfield = addfield (b);
          endfunction

     See also: Note: saveobj, Note: class.