Next: Indexing Objects, Previous: Creating a Class, Up: Object Oriented Programming [Contents][Index]
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) 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 (see 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.
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
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.
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
Next: Indexing Objects, Previous: Creating a Class, Up: Object Oriented Programming [Contents][Index]