@c Copyright (C) 2008-2015 David Bateman
@c Copyright (C) 2009 VZLU Prague
@c
@c This file is part of Octave.
@c
@c Octave is free software; you can redistribute it and/or modify it
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@c Free Software Foundation; either version 3 of the License, or (at
@c your option) any later version.
@c
@c Octave is distributed in the hope that it will be useful, but WITHOUT
@c ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
@c FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
@c for more details.
@c
@c You should have received a copy of the GNU General Public License
@c along with Octave; see the file COPYING.  If not, see
@c <http://www.gnu.org/licenses/>.

@c FIXME
@c For now can't include "@" character in the path name, and so name
@c the example directory without the "@"!!

@node Object Oriented Programming
@chapter Object Oriented Programming

Octave has the ability to create user-defined classes---including the
capabilities of operator and function overloading.  Classes can protect
internal properties so that they may not be altered accidentally which
facilitates data encapsulation.  In addition, rules can be created to address
the issue of class precedence in mixed class operations.

This chapter discusses the means of constructing a user class, how to query and
set the properties of a class, and how to overload operators and functions.
Throughout this chapter real code examples are given using a class designed
for polynomials.

@menu
* Creating a Class::
* Class Methods::
* Indexing Objects::
* Overloading Objects::
* Inheritance and Aggregation::
@end menu

@node Creating a Class
@section Creating a Class

This chapter illustrates user-defined classes and object oriented programming
through a custom class designed for polynomials.  This class was chosen for
its simplicity which does not distract unnecessarily from the discussion of
the programming features of Octave.  Even so, a bit of background on the goals
of the polynomial class is necessary before the syntax and techniques of Octave
object oriented programming are introduced.

The polynomial class is used to represent polynomials of the form
@tex
$$
a_0 + a_1 x + a_2 x^2 + \ldots a_n x^n
$$
@end tex
@ifnottex

@example
a0 + a1 * x + a2 * x^2 + @dots{} + an * x^n
@end example

@end ifnottex
@noindent
where
@tex
$a_0$, $a_1$, etc. are elements of $\Re$.
@end tex
@ifnottex
a0, a1, etc.@: are real scalars.
@end ifnottex
Thus the polynomial can be represented by a vector

@example
a = [a0, a1, a2, @dots{}, an];
@end example

This is a sufficient specification to begin writing the constructor for the
polynomial class.  All object oriented classes in Octave must be located in a
directory that is the name of the class prepended with the @samp{@@} symbol.
For example, the polynomial class will have all of its methods defined in the
@file{@@polynomial} directory.

The constructor for the class must be the name of the class itself; in this
example the constructor resides in the file @file{@@polynomial/polynomial.m}.
Ideally, even when the constructor is called with no arguments it should return
a valid object.  A constructor for the polynomial class might look like

@example
@EXAMPLEFILE(@polynomial/polynomial.m)
@end example

Note that the return value of the constructor must be the output of the
@code{class} function.  The first argument to the @code{class} function is a
structure and the second is the name of the class itself.  An example of
calling the class constructor to create an instance is

@example
p = polynomial ([1, 0, 1]);
@end example

Methods are defined by m-files in the class directory and can have embedded
documentation the same as any other m-file.  The help for the constructor can
be obtained by using the constructor name alone, that is, for the polynomial
constructor @code{help polynomial} will return the help string.  Help can be
restricted to a particular class by using the class directory name followed
by the method.  For example, @code{help @@polynomial/polynomial} is another
way of displaying the help string for the polynomial constructor.  This second
means is the only way to obtain help for the overloaded methods and functions
of a class.

The same specification mechanism can be used wherever Octave expects a function
name.  For example @code{type @@polynomial/display} will print the code of the
display method of the polynomial class to the screen, and
@code{dbstop @@polynomial/display} will set a breakpoint at the first
executable line of the display method of the polynomial class.

To check whether a variable belongs to a user class, the @code{isobject} and
@code{isa} functions can be used.  For example:

@example
@group
p = polynomial ([1, 0, 1]);
isobject (p)
  @result{} 1
isa (p, "polynomial")
  @result{} 1
@end group
@end example

@DOCSTRING(isobject)

@noindent
The available methods of a class can be displayed with the @code{methods}
function.

@DOCSTRING(methods)

@noindent
To inquire whether a particular method exists for a user class, the
@code{ismethod} function can be used.

@DOCSTRING(ismethod)

@noindent
For example:

@example
@group
p = polynomial ([1, 0, 1]);
ismethod (p, "roots")
  @result{} 1
@end group
@end example

@node Class Methods
@section Class Methods

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 @code{display} method.  The @code{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.

@DOCSTRING(display)

@noindent
An example of a display method for the polynomial class might be

@example
@EXAMPLEFILE(@polynomial/display.m)
@end example

@noindent
Note that in the display method it makes sense to start the method with the
line @w{@code{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 @code{get} and @code{set} methods.  The @code{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.

@example
@EXAMPLEFILE(@polynomial/get.m)
@end example

@noindent
Similarly, the first argument to the @code{set} method should be an object and
any additional arguments should be property/value pairs.

@example
@EXAMPLEFILE(@polynomial/set.m)
@end example

@noindent
Note that Octave does not implement pass by reference; Therefore, to modify an
object requires an assignment statement using the return value from the
@code{set} method.

@example
p = set (p, "poly", [1, 0, 0, 0, 1]);
@end example

@noindent
The @code{set} method makes use of the @code{subsasgn} method of the class, and
therefore this method must also be defined.  The @code{subsasgn} method is
discussed more thoroughly in the next section (@pxref{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:

@example
@group
p = polynomial ([1, 0, 1]);
save userclass.mat p
clear p
load userclass.mat
@end group
@end example

@noindent
All of the file formats supported by @code{save} and @code{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 @code{saveobj} method of the class.

@DOCSTRING(saveobj)

@noindent
@code{saveobj} is called just prior to saving the class to a file.  Similarly,
the @code{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.

@DOCSTRING(loadobj)

@node Indexing Objects
@section Indexing Objects

@menu
* Defining Indexing And Indexed Assignment::
* Indexed Assignment Optimization::
@end menu

@node Defining Indexing And Indexed Assignment
@subsection Defining Indexing And Indexed Assignment

Objects can be indexed with parentheses or braces, either like
@code{@var{obj}(@var{idx})} or like @code{@var{obj}@{@var{idx}@}}, or even
like @code{@var{obj}(@var{idx}).@var{field}}.  However, it is up to the
programmer to decide what this indexing actually means.  In the case of the
polynomial class @code{@var{p}(@var{n})} might mean either the coefficient of
the @var{n}-th power of the polynomial, or it might be the evaluation of the
polynomial at @var{n}.  The meaning of this subscripted referencing is
determined by the @code{subsref} method.

@DOCSTRING(subsref)

For example, this class uses the convention that indexing with @qcode{"()"}
evaluates the polynomial and indexing with @qcode{"@{@}"} returns the
@var{n}-th coefficient (of the @var{n}-th power).  The code for the
@code{subsref} method looks like

@example
@EXAMPLEFILE(@polynomial/subsref.m)
@end example

The equivalent functionality for subscripted assignments uses the
@code{subsasgn} method.

@DOCSTRING(subsasgn)

@DOCSTRING(optimize_subsasgn_calls)

Note that the @code{subsref} and @code{subsasgn} methods always receive the
whole index chain, while they usually handle only the first element.  It is the
responsibility of these methods to handle the rest of the chain (if needed),
usually by forwarding it again to @code{subsref} or @code{subsasgn}.

If you wish to use the @code{end} keyword in subscripted expressions of an
object, then there must be an @code{end} method defined.  For example, the
@code{end} method for the polynomial class might look like

@example
@group
@EXAMPLEFILE(@polynomial/end.m)
@end group
@end example

@noindent
which is a fairly generic @code{end} method that has a behavior similar to the
@code{end} keyword for Octave Array classes.  An example using the polynomial
class is then

@example
@group
p = polynomial ([1,2,3,4]);
p@{end-1@}
  @result{} 3
@end group
@end example

Objects can also be used themselves as the index in a subscripted expression
and this is controlled by the @code{subsindex} function.

@DOCSTRING(subsindex)

Finally, objects can be used like ranges by providing a @code{colon} method.

@DOCSTRING(colon)

@node Indexed Assignment Optimization
@subsection Indexed Assignment Optimization

Octave's ubiquitous lazily-copied pass-by-value semantics implies a problem for
performance of user-defined @code{subsasgn} methods.  Imagine the following
call to @code{subsasgn}

@example
@group
ss = substruct ("()", @{1@});
x = subsasgn (x, ss, 1);
@end group
@end example

@noindent
where the corresponding method looking like this:

@example
@group
function x = subsasgn (x, ss, val)
  @dots{}
  x.myfield (ss.subs@{1@}) = val;
endfunction
@end group
@end example

The problem is that on entry to the @code{subsasgn} method, @code{x} is still
referenced from the caller's scope, which means that the method will first need
to unshare (copy) @code{x} and @code{x.myfield} before performing the
assignment.  Upon completing the call, unless an error occurs, the result is
immediately assigned to @code{x} in the caller's scope, so that the previous
value of @code{x.myfield} is forgotten.  Hence, the Octave language implies a
copy of N elements (N being the size of @code{x.myfield}), where modifying just
a single element would actually suffice.  In other words, a constant-time
operation is degraded to linear-time one.  This may be a real problem for user
classes that intrinsically store large arrays.

To partially solve the problem Octave uses a special optimization for
user-defined @code{subsasgn} methods coded as m-files.  When the method gets
called as a result of the built-in assignment syntax (not a direct
@code{subsasgn} call as shown above), i.e., @w{@code{x(1) = 1}},  @b{AND} if
the @code{subsasgn} method is declared with identical input and output
arguments, as in the example above, then Octave will ignore the copy of
@code{x} inside the caller's scope; therefore, any changes made to @code{x}
during the method execution will directly affect the caller's copy as well.
This allows, for instance, defining a polynomial class where modifying a single
element takes constant time.

It is important to understand the implications that this optimization brings.
Since no extra copy of @code{x} will exist in the caller's scope, it is
@emph{solely} the callee's responsibility to not leave @code{x} in an invalid
state if an error occurs during the execution.  Also, if the method partially
changes @code{x} and then errors out, the changes @emph{will} affect @code{x}
in the caller's scope.  Deleting or completely replacing @code{x} inside
subsasgn will not do anything, however, only indexed assignments matter.

Since this optimization may change the way code works (especially if badly
written), a built-in variable @code{optimize_subsasgn_calls} is provided to
control it.  It is on by default.  Another way to avoid the optimization is to
declare subsasgn methods with different output and input arguments like this:

@example
@group
function y = subsasgn (x, ss, val)
  @dots{}
endfunction
@end group
@end example

@node Overloading Objects
@section Overloading Objects

@menu
* Function Overloading::
* Operator Overloading::
* Precedence of Objects::
@end menu

@node Function Overloading
@subsection Function Overloading

Any Octave function can be overloaded, and this allows an object-specific
version of a function to be called as needed.  A pertinent example for the
polynomial class might be to overload the @code{polyval} function.

@example
@group
@EXAMPLEFILE(@polynomial/polyval.m)
@end group
@end example

This function just hands off the work to the normal Octave @code{polyval}
function.  Another interesting example of an overloaded function for the
polynomial class is the @code{plot} function.

@example
@group
@EXAMPLEFILE(@polynomial/plot.m)
@end group
@end example

@noindent
which allows polynomials to be plotted in the domain near the region of the
roots of the polynomial.

Functions that are of particular interest for overloading are the class
conversion functions such as @code{double}.  Overloading these functions allows
the @code{cast} function to work with a user class.  It can also can aid in the
use of a class object with methods and functions from other classes since the
object can be transformed to the requisite input form for the new function.
An example @code{double} function for the polynomial class might look like

@example
@group
@EXAMPLEFILE(@polynomial/double.m)
@end group
@end example

@node Operator Overloading
@subsection Operator Overloading
@cindex addition
@cindex and operator
@cindex arithmetic operators
@cindex boolean expressions
@cindex boolean operators
@cindex comparison expressions
@cindex complex-conjugate transpose
@cindex division
@cindex equality operator
@cindex equality, tests for
@cindex exponentiation
@cindex expressions, boolean
@cindex expressions, comparison
@cindex expressions, logical
@cindex greater than operator
@cindex Hermitian operator
@cindex less than operator
@cindex logical expressions
@cindex logical operators
@cindex matrix multiplication
@cindex multiplication
@cindex negation
@cindex not operator
@cindex operators, arithmetic
@cindex operators, boolean
@cindex operators, logical
@cindex operators, relational
@cindex or operator
@cindex quotient
@cindex relational operators
@cindex subtraction
@cindex tests for equality
@cindex transpose
@cindex transpose, complex-conjugate
@cindex unary minus

@c Need at least one plaintext sentence here between the @node and @float
@c table below or the two will overlap due to a bug in Texinfo.
@c This is not our fault; this *is* a ridiculous kluge.
The following table shows, for each built-in numerical operation, the
corresponding function name to use when providing an overloaded method for a
user class.

@float Table,tab:overload_ops
@opindex +
@opindex -
@opindex .*
@opindex *
@opindex ./
@opindex /
@opindex .\
@opindex \
@opindex .^
@opindex ^
@opindex <
@opindex <=
@opindex >
@opindex >=
@opindex ==
@opindex !=
@opindex ~=
@opindex &
@opindex |
@opindex !
@opindex '
@opindex .'
@opindex :
@opindex <

@tex
\vskip 6pt
{\hbox to \hsize {\hfill\vbox{\offinterlineskip \tabskip=0pt
\halign{
\vrule height2.0ex depth1.ex width 0.6pt #\tabskip=0.3em &
# \hfil & \vrule # & # \hfil & \vrule # & # \hfil & # \vrule
width 0.6pt \tabskip=0pt\cr
\noalign{\hrule height 0.6pt}
& Operation && Method && Description &\cr
\noalign{\hrule}
& $a + b$ && plus (a, b) && Binary addition operator&\cr
& $a - b$ && minus (a, b) && Binary subtraction operator&\cr
& $+ a$ && uplus (a) && Unary addition operator&\cr
& $- a$ && uminus (a) && Unary subtraction operator&\cr
& $a .* b$ && times (a, b) && Element-wise multiplication operator&\cr
& $a * b$ && mtimes (a, b) && Matrix multiplication operator&\cr
& $a ./ b$ && rdivide (a, b) && Element-wise right division operator&\cr
& $a / b$ && mrdivide (a, b) && Matrix right division operator&\cr
& $a .\backslash b$ && ldivide (a, b) && Element-wise left division operator&\cr
& $a \backslash b$ && mldivide (a, b) && Matrix left division operator&\cr
& $a .\hat b$ && power (a, b) && Element-wise power operator&\cr
& $a \hat b$ && mpower (a, b) && Matrix power operator&\cr
& $a < b$ && lt (a, b) && Less than operator&\cr
& $a <= b$ && le (a, b) && Less than or equal to operator&\cr
& $a > b$ && gt (a, b) && Greater than operator&\cr
& $a >= b$ && ge (a, b) && Greater than or equal to operator&\cr
& $a == b$ && eq (a, b) && Equal to operator&\cr
& $a != b$ && ne (a, b) && Not equal to operator&\cr
& $a \& b$ && and (a, b) && Logical and operator&\cr
& $a | b$ && or (a, b) && Logical or operator&\cr
& $! b$ && not (a) && Logical not operator&\cr
& $a'$ && ctranspose (a) && Complex conjugate transpose operator &\cr
& $a.'$ && transpose (a) && Transpose operator &\cr
& $a : b$ && colon (a, b) && Two element range operator &\cr
& $a : b : c$ && colon (a, b, c) && Three element range operator &\cr
& $[a, b]$ && horzcat (a, b) && Horizontal concatenation operator &\cr
& $[a; b]$ && vertcat (a, b) && Vertical concatenation operator &\cr
& $a(s_1, \ldots, s_n)$ && subsref (a, s) && Subscripted reference &\cr
& $a(s_1, \ldots, s_n) = b$ && subsasgn (a, s, b) && Subscripted assignment &\cr
& $b (a)$ && subsindex (a) && Convert to zero-based index &\cr
& {\it display} && display (a) && Commandline display function &\cr
\noalign{\hrule height 0.6pt}
}}\hfill}}
@end tex
@ifnottex
@multitable @columnfractions .1 .20 .20 .40 .1
@headitem @tab Operation @tab Method @tab Description @tab
@item @tab a + b @tab plus (a, b) @tab Binary addition @tab
@item @tab a - b @tab minus (a, b) @tab Binary subtraction operator @tab
@item @tab + a @tab uplus (a) @tab Unary addition operator @tab
@item @tab - a @tab uminus (a) @tab Unary subtraction operator @tab
@item @tab a .* b @tab times (a, b) @tab Element-wise multiplication operator @tab
@item @tab a * b @tab mtimes (a, b) @tab Matrix multiplication operator @tab
@item @tab a ./ b @tab rdivide (a, b) @tab Element-wise right division operator @tab
@item @tab a / b @tab mrdivide (a, b) @tab Matrix right division operator @tab
@item @tab a .\ b @tab ldivide (a, b) @tab Element-wise left division operator @tab
@item @tab a \ b @tab mldivide (a, b) @tab Matrix left division operator @tab
@item @tab a .^ b @tab power (a, b) @tab Element-wise power operator @tab
@item @tab a ^ b @tab mpower (a, b) @tab Matrix power operator @tab
@item @tab a < b @tab lt (a, b) @tab Less than operator @tab
@item @tab a <= b @tab le (a, b) @tab Less than or equal to operator @tab
@item @tab a > b @tab gt (a, b) @tab Greater than operator @tab
@item @tab a >= b @tab ge (a, b) @tab Greater than or equal to operator @tab
@item @tab a == b @tab eq (a, b) @tab Equal to operator @tab
@item @tab a != b @tab ne (a, b) @tab Not equal to operator @tab
@item @tab a & b @tab and (a, b) @tab Logical and operator @tab
@item @tab a | b @tab or (a, b) @tab Logical or operator @tab
@item @tab ! b @tab not (a) @tab Logical not operator @tab
@item @tab a' @tab ctranspose (a) @tab Complex conjugate transpose operator @tab
@item @tab a.' @tab transpose (a) @tab Transpose operator @tab
@item @tab a : b @tab colon (a, b) @tab Two element range operator @tab
@item @tab a : b : c @tab colon (a, b, c) @tab Three element range operator @tab
@item @tab [a, b] @tab horzcat (a, b) @tab Horizontal concatenation operator @tab
@item @tab [a; b] @tab vertcat (a, b) @tab Vertical concatenation operator @tab
@item @tab a(s_1, @dots{}, s_n) @tab subsref (a, s) @tab Subscripted reference @tab
@item @tab a(s_1, @dots{}, s_n) = b @tab subsasgn (a, s, b) @tab Subscripted assignment @tab
@item @tab b (a) @tab subsindex (a) @tab Convert to zero-based index @tab
@item @tab  @dfn{display} @tab display (a) @tab Commandline display function @tab
@end multitable
@end ifnottex
@caption{Available overloaded operators and their corresponding class method}
@end float

An example @code{mtimes} method for the polynomial class might look like

@example
@group
@EXAMPLEFILE(@polynomial/mtimes.m)
@end group
@end example

@node Precedence of Objects
@subsection Precedence of Objects

Many functions and operators take two or more arguments and the situation can
easily arise where these functions are called with objects of different
classes.  It is therefore necessary to determine the precedence of which method
from which class to call when there are mixed objects given to a function or
operator.  To do this the @code{superiorto} and @code{inferiorto} functions can
be used

@DOCSTRING(superiorto)

@DOCSTRING(inferiorto)

With the polynomial class, consider the case

@example
2 * polynomial ([1, 0, 1]);
@end example

@noindent
that mixes an object of the class @qcode{"double"} with an object of the class
@qcode{"polynomial"}.  In this case the return type should be
@qcode{"polynomial"} and so the @code{superiorto} function is used in the class
constructor.  In particular the polynomial class constructor would be modified
to

@example
@EXAMPLEFILE(@polynomial/polynomial_superiorto.m)
@end example

Note that user classes @emph{always} have higher precedence than built-in
Octave types.  Thus, marking the polynomial class higher than the
@qcode{"double"} class is not actually necessary.

When confronted with two objects of equal precedence, Octave will use the
method of the object that appears first in the list of arguments.

@node Inheritance and Aggregation
@section 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 @code{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 @code{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.

@example
@group
octave:1> x = [some data vector];
octave:2> n = [some coefficient vector];
octave:3> y = filter (n, 1, x);
@end group
@end example

The equivalent behavior can be implemented as a class @@FIRfilter.  The
constructor for this class is the file @file{FIRfilter.m} in the class
directory @file{@@FIRfilter}.

@example
@EXAMPLEFILE(@FIRfilter/FIRfilter.m)
@end example

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 @code{class} function, telling Octave that the @w{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 @code{class} function.
Given that the @@polynomial constructor will add an element named
@var{polynomial} to the object struct, the @@FIRfilter just initializes a
struct with a dummy field @var{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 (@pxref{XREFclass,,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 @code{display} method.  A simple example might be

@example
@group
@EXAMPLEFILE(@FIRfilter/display.m)
@end group
@end example

Note that the @w{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.

@example
@group
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
@end group
@end example

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 @code{subsref} method may be used for both tasks.

@example
@EXAMPLEFILE(@FIRfilter/subsref.m)
@end example

The @qcode{"()"} case allows us to filter data using the polynomial provided
to the constructor.

@example
@group
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
@end group
@end example

The @qcode{"."} case allows us to view the contents of the polynomial field.

@example
@group
octave:1> f = FIRfilter (polynomial ([1 1 1]/3));
octave:2> f.polynomial
ans = 0.33333 + 0.33333 * X + 0.33333 * X ^ 2
@end group
@end example

In order to change the contents of the object a @code{subsasgn} method is
needed.  For example, the following code makes the polynomial field publicly
writable

@example
@group
@EXAMPLEFILE(@FIRfilter/subsasgn.m)
@end group
@end example

@noindent
so that

@example
@group
octave:1> f = FIRfilter ();
octave:2> f.polynomial = polynomial ([1 2 3])
f.polynomial = 1 + 2 * X + 3 * X ^ 2
@end group
@end example

Defining the @w{FIRfilter} class as a child of the polynomial class implies
that a @w{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

@example
@EXAMPLEFILE(@FIRfilter/FIRfilter_aggregation.m)
@end example

For this example only the constructor needs changing, and all other class
methods stay the same.