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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 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>
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 (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]