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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.
Display the contents of an object. If a is an object of the class "myclass", then
displayis called in a case likemyclass (...)where Octave is required to display the contents of a variable of the type "myclass".
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
Method of a class to manipulate an object prior to saving it to a file. The function
saveobjis called when the object a is saved using thesavefunction. An example of the use ofsaveobjmight 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 examplefunction 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. 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.
Method of a class to manipulate an object after loading it from a file. The function
loadobjis called when the object a is loaded using theloadfunction. An example of the use ofsaveobjmight be to add fields to an object that don't make sense to be saved. For examplefunction b = loadobj (a) b = a; b.addmissingfield = addfield (b); endfunction