### 34.2 Manipulating Classes

There are a number of basic classes methods that can be defined to allow the contents of the classes to be queried and set. The most basic of these is the `display` method. The `display` method is used by Octave when displaying a class on the screen, due to an expression that is not terminated with a semicolon. If this method is not defined, then Octave will printed nothing when displaying the contents of a class.

Function File: display (a)

  Display the contents of an object. If a is an object of the class `"myclass"`, then `display` is called in a case like

      myclass (…)

  where Octave is required to display the contents of a variable of the type `"myclass"`.

  See also: class, subsref, subsasgn.

An example of a display method for the polynomial class might be

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

Note that in the display method, it makes sense to start the method with the line `fprintf ("%s =", inputname (1))` to be consistent with the rest of Octave and print the variable name to be displayed when displaying the class.

To be consistent with the Octave graphic handle classes, a class should also define the `get` and `set` methods. The `get` method should accept one or two arguments, and given one argument of the appropriate class it should return a structure with all of the properties of the class. For example:

    function s = get (p, f)
      if (nargin == 1)
        s.poly = p.poly;
      elseif (nargin == 2)
        if (ischar (f))
          switch (f)
            case "poly"
              s = p.poly;
            otherwise
              error ("get: invalid property %s", f);
          endswitch
        else
          error ("get: expecting the property to be a string");
        endif
      else
        print_usage ();
      endif
    endfunction

Similarly, the `set` method should taken as its first argument an object to modify, and then take property/value pairs to be modified.

    function s = set (p, varargin)
      s = p;
      if (length (varargin) < 2 || rem (length (varargin), 2) != 0)
        error ("set: expecting property/value pairs");
      endif
      while (length (varargin) > 1)
        prop = varargin{1};
        val = varargin{2};
        varargin(1:2) = [];
        if (ischar (prop) && strcmp (prop, "poly"))
          if (isvector (val) && isreal (val))
            s.poly = val(:).';
          else
            error ("set: expecting the value to be a real vector");
          endif
        else
          error ("set: invalid property of polynomial class");
        endif
      endwhile
    endfunction

Note that as Octave does not implement pass by reference, than the modified object is the return value of the `set` method and it must be called like

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

Also the `set` method makes use of the `subsasgn` method of the class, and this method must be defined. The `subsasgn` method is discussed in the next section.

Finally, user classes can be considered as a special type of a structure, and so 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 either contain a field that it makes no 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

Function File: 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: loadobj, class.

`saveobj` is called just prior to saving the class to a file. Likely, 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.

Function File: 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: saveobj, class.