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### 34.5 Inheritance and Aggregation

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.

    octave:1> x = [some data vector];
    octave:2> n = [some coefficient vector];
    octave:3> 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.

As before, a class requires a `display` method. A simple example might be

    function display (f)
      printf ("%s.polynomial", inputname (1));
      display (f.polynomial);
    endfunction

Note that the FIRfilter’s display method relies on the display method from the polynomial class to actually display the filter coefficients.

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.

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