octave: Class Methods
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 ‘display’ method. The ‘display’ 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.
-- : display (OBJ)
Display the contents of the object OBJ.
The Octave interpreter calls the ‘display’ function whenever it
needs to present a class on-screen. Typically, this would be a
statement which does not end in a semicolon to suppress output.
For example:
myobj = myclass (...)
User-defined classes should overload the ‘display’ method so that
something useful is printed for a class object. Otherwise, Octave
will report only that the object is an instance of its class.
myobj = myclass (...)
⇒ myobj = <class myclass>
DONTPRINTYET See also: 
class XREFclass, subsref XREFsubsref, *noteDONTPRINTYET See also: class XREFclass, subsref XREFsubsref,
subsasgn XREFsubsasgn.
An example of a display method for the polynomial class might be
function display (p)
printf ("%s =", inputname (1));
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
Note that in the display method it makes sense to start the method with
the line ‘printf ("%s =", inputname (1))’ to be consistent with the rest
of Octave which prints the variable name to be displayed followed by the
value.
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 (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: loadobj XREFloadobj, class XREFclass.
‘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: saveobj XREFsaveobj, class XREFclass.
Info Catalog
octave: Creating a Class
octave: Object Oriented Programming
octave: Indexing Objects