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Using classes to build new classes is supported by Octave through the use of both inheritance and aggregation.
Class inheritance is provided by Octave using the class
function in the
class constructor. As in the case of the polynomial class, the Octave
programmer will create a structure that contains the data fields required by
the class, and then call the class
function to indicate that an object
is to be created from the structure. Creating a child of an existing object is
done by creating an object of the parent class and providing that object as the
third argument of the class function.
This is most easily demonstrated by example. Suppose the programmer needs a FIR filter, i.e., a filter with a numerator polynomial but a denominator of 1. In traditional Octave programming this would be performed as follows.
>> x = [some data vector]; >> n = [some coefficient vector]; >> y = filter (n, 1, x);
The equivalent behavior can be implemented as a class @FIRfilter
. The
constructor for this class is the file FIRfilter.m in the class
directory @FIRfilter.
## -*- texinfo -*- ## @deftypefn {} {} FIRfilter () ## @deftypefnx {} {} FIRfilter (@var{p}) ## Create a FIR filter with polynomial @var{p} as coefficient vector. ## @end deftypefn function f = FIRfilter (p) if (nargin > 1) print_usage (); endif if (nargin == 0) p = @polynomial ([1]); elseif (! isa (p, "polynomial")) error ("@FIRfilter: P must be a polynomial object"); endif f.polynomial = []; f = class (f, "FIRfilter", p); endfunction
As before, the leading comments provide documentation for the class
constructor. This constructor is very similar to the polynomial class
constructor, except that a polynomial object is passed as the third argument to
the class
function, telling Octave that the FIRfilter
class will
be derived from the polynomial class. The FIR filter class itself does not
have any data fields, but it must provide a struct to the class
function. Given that the @polynomial
constructor will add an element
named polynomial to the object struct, the @FIRfilter
just
initializes a struct with a dummy field polynomial which will later be
overwritten.
Note that the sample code always provides for the case in which no arguments are supplied. This is important because Octave will call a constructor with no arguments when loading objects from saved files in order to determine the inheritance structure.
A class may be a child of more than one class (see class), and inheritance may be nested. There is no limitation to the number of parents or the level of nesting other than memory or other physical issues.
For the FIRfilter
class, more control about the object display is
desired. Therefore, the display
method rather than the disp
method is overloaded (see Class Methods). A simple example might be
function display (f) printf ("%s.polynomial", inputname (1)); disp (f.polynomial); endfunction
Note that the FIRfilter
’s display method relies on the disp
method from the polynomial
class to actually display the filter
coefficients. Furthermore, note that in the display
method it makes
sense to start the method with the line
to be consistent with the
rest of Octave which prints the variable name to be displayed followed by the
value. In general it is not recommended to overload the printf ("%s =", inputname (1))
display
function.
Display the contents of the object obj prepended by its name.
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:
myclass (…)
Or:
myobj = myclass (…)
In general, user-defined classes should overload the disp
method to
avoid the default output:
myobj = myclass (…) ⇒ myobj = <class myclass>
When overloading the display
method instead, one has to take care
of properly displaying the object’s name. This can be done by using the
inputname
function.
Once a constructor and display method exist, it is possible to create an instance of the class. It is also possible to check the class type and examine the underlying structure.
octave:1> f = FIRfilter (polynomial ([1 1 1]/3)) f.polynomial = 0.33333 + 0.33333 * X + 0.33333 * X ^ 2 octave:2> class (f) ans = FIRfilter octave:3> isa (f, "FIRfilter") ans = 1 octave:4> isa (f, "polynomial") ans = 1 octave:5> struct (f) ans = scalar structure containing the fields: polynomial = 0.33333 + 0.33333 * X + 0.33333 * X ^ 2
The only thing remaining to make this class usable is a method for processing
data. But before that, it is usually desirable to also have a way of changing
the data stored in a class. Since the fields in the underlying struct are
private by default, it is necessary to provide a mechanism to access the
fields. The subsref
method may be used for both tasks.
function r = subsref (f, x) switch (x.type) case "()" n = f.polynomial; r = filter (n.poly, 1, x.subs{1}); case "." fld = x.subs; if (! strcmp (fld, "polynomial")) error ('@FIRfilter/subsref: invalid property "%s"', fld); endif r = f.polynomial; otherwise error ("@FIRfilter/subsref: invalid subscript type for FIR filter"); endswitch endfunction
The "()"
case allows us to filter data using the polynomial provided
to the constructor.
octave:2> f = FIRfilter (polynomial ([1 1 1]/3)); octave:3> x = ones (5,1); octave:4> y = f(x) y = 0.33333 0.66667 1.00000 1.00000 1.00000
The "."
case allows us to view the contents of the polynomial field.
octave:1> f = FIRfilter (polynomial ([1 1 1]/3)); octave:2> f.polynomial ans = 0.33333 + 0.33333 * X + 0.33333 * X ^ 2
In order to change the contents of the object a subsasgn
method is
needed. For example, the following code makes the polynomial field publicly
writable
function fout = subsasgn (f, index, val) switch (index.type) case "." fld = index.subs; if (! strcmp (fld, "polynomial")) error ('@FIRfilter/subsasgn: invalid property "%s"', fld); endif fout = f; fout.polynomial = val; otherwise error ("@FIRfilter/subsasgn: Invalid index type") endswitch endfunction
so that
octave:1> f = FIRfilter (); octave:2> f.polynomial = polynomial ([1 2 3]) f.polynomial = 1 + 2 * X + 3 * X ^ 2
Defining the FIRfilter class as a child of the polynomial class implies that a FIRfilter object may be used any place that a polynomial object may be used. This is not a normal use of a filter. It may be a more sensible design approach to use aggregation rather than inheritance. In this case, the polynomial is simply a field in the class structure. A class constructor for the aggregation case might be
## -*- texinfo -*- ## @deftypefn {} {} FIRfilter () ## @deftypefnx {} {} FIRfilter (@var{p}) ## Create a FIR filter with polynomial @var{p} as coefficient vector. ## @end deftypefn function f = FIRfilter (p) if (nargin > 1) print_usage (); endif if (nargin == 0) f.polynomial = @polynomial ([1]); else if (! isa (p, "polynomial")) error ("@FIRfilter: P must be a polynomial object"); endif f.polynomial = p; endif f = class (f, "FIRfilter"); endfunction
For this example only the constructor needs changing, and all other class methods stay the same.
Next: classdef Classes, Previous: Overloading Objects, Up: Object Oriented Programming [Contents][Index]