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<a name="Creating-a-Class"></a>
<div class="header">
<p>
Next: <a href="Manipulating-Classes.html#Manipulating-Classes" accesskey="n" rel="next">Manipulating Classes</a>, Up: <a href="Object-Oriented-Programming.html#Object-Oriented-Programming" accesskey="u" rel="up">Object Oriented Programming</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Creating-a-Class-1"></a>
<h3 class="section">34.1 Creating a Class</h3>

<p>We use in the following text a polynomial class to demonstrate the use
of object oriented programming within Octave.  This class was chosen as
it is simple, and so doesn&rsquo;t distract unnecessarily from the
discussion of the programming features of Octave.  However, even still
a small understand of the polynomial class itself is necessary to
fully grasp the techniques described.
</p>
<p>The polynomial class is used to represent polynomials of the form
</p>
<div class="example">
<pre class="example">a0 + a1 * x + a2 * x^2 + &hellip; + an * x^n
</pre></div>

<p>where
a0, a1, etc. are real scalars.
Thus the polynomial can be represented by a vector
</p>
<div class="example">
<pre class="example">a = [a0, a1, a2, &hellip;, an];
</pre></div>

<p>We therefore now have sufficient information about the requirements of
the class constructor for our polynomial class to write it.  All object
oriented classes in Octave, must be contained with a directory taking
the name of the class, prepended with the @ symbol.  For example, with
our polynomial class, we would place the methods defining the class in
the @polynomial directory.
</p>
<p>The constructor of the class, must have the name of the class itself
and so in our example the constructor with have the name
<samp>@polynomial/polynomial.m</samp>.  Also ideally when the constructor is
called with no arguments to should return a value object.  So for example
our polynomial might look like
</p>
<div class="example">
<pre class="verbatim">## -*- texinfo -*-
## @deftypefn  {Function File} {} polynomial ()
## @deftypefnx {Function File} {} polynomial (@var{a})
## Create a polynomial object representing the polynomial
##
## @example
## a0 + a1 * x + a2 * x^2 + @dots{} + an * x^n
## @end example
##
## @noindent
## from a vector of coefficients [a0 a1 a2 @dots{} an].
## @end deftypefn

function p = polynomial (a)
  if (nargin == 0)
    p.poly = [0];
    p = class (p, &quot;polynomial&quot;);
  elseif (nargin == 1)
    if (strcmp (class (a), &quot;polynomial&quot;))
      p = a;
    elseif (isvector (a) &amp;&amp; isreal (a))
      p.poly = a(:).';
      p = class (p, &quot;polynomial&quot;);
    else
      error (&quot;polynomial: expecting real vector&quot;);
    endif
  else
    print_usage ();
  endif
endfunction
</pre><pre class="example">
</pre></div>

<p>Note that the return value of the constructor must be the output of
the <code>class</code> function called with the first argument being a
structure and the second argument being the class name.  An example of
the call to this constructor function is then
</p>
<div class="example">
<pre class="example">p = polynomial ([1, 0, 1]);
</pre></div>

<p>Note that methods of a class can be documented.  The help for the
constructor itself can be obtained with the constructor name, that is
for the polynomial constructor <code>help polynomial</code> will return the
help string.  Also the help can be obtained by restricting the search
for the help to a particular class, for example <code>help
@polynomial/polynomial</code>.  This second method is the only means of
getting help for the overloaded methods and functions of the class.
</p>
<p>The same is true for other Octave functions that take a function name
as an argument.  For example <code>type @polynomial/display</code> will
print the code of the display method of the polynomial class to the
screen, and <code>dbstop @polynomial/display</code> will set a breakpoint
at the first executable line of the display method of the polynomial
class.
</p>
<p>To check where a variable is a user class, the <code>isobject</code> and
<code>isa</code> functions can be used.  For example:
</p>
<div class="example">
<pre class="example">p = polynomial ([1, 0, 1]);
isobject (p)
  &rArr; 1
isa (p, &quot;polynomial&quot;)
  &rArr; 1
</pre></div>

<a name="XREFisobject"></a><dl>
<dt><a name="index-isobject"></a>Built-in Function: <em></em> <strong>isobject</strong> <em>(<var>x</var>)</em></dt>
<dd><p>Return true if <var>x</var> is a class object.
</p>
<p><strong>See also:</strong> <a href="Built_002din-Data-Types.html#XREFclass">class</a>, <a href="Data-Types.html#XREFtypeinfo">typeinfo</a>, <a href="Built_002din-Data-Types.html#XREFisa">isa</a>, <a href="#XREFismethod">ismethod</a>.
</p></dd></dl>


<p>The available methods of a class can be displayed with the
<code>methods</code> function.
</p>
<a name="XREFmethods"></a><dl>
<dt><a name="index-methods"></a>Function File: <em></em> <strong>methods</strong> <em>(<var>obj</var>)</em></dt>
<dt><a name="index-methods-1"></a>Function File: <em></em> <strong>methods</strong> <em>(&quot;<var>classname</var>&quot;)</em></dt>
<dt><a name="index-methods-2"></a>Function File: <em><var>mtds</var> =</em> <strong>methods</strong> <em>(&hellip;)</em></dt>
<dd>
<p>Return a cell array containing the names of the methods for the
object <var>obj</var> or the named class <var>classname</var>.
<var>obj</var> may be an Octave class object or a Java object.
</p>

<p><strong>See also:</strong> <a href="Manipulating-Structures.html#XREFfieldnames">fieldnames</a>.
</p></dd></dl>


<p>To inquire whether a particular method is available to a user class, the
<code>ismethod</code> function can be used.
</p>
<a name="XREFismethod"></a><dl>
<dt><a name="index-ismethod"></a>Built-in Function: <em></em> <strong>ismethod</strong> <em>(<var>x</var>, <var>method</var>)</em></dt>
<dd><p>Return true if <var>x</var> is a class object and the string <var>method</var>
is a method of this class.
</p>
<p><strong>See also:</strong> <a href="Introduction-to-Graphics-Structures.html#XREFisprop">isprop</a>, <a href="#XREFisobject">isobject</a>.
</p></dd></dl>


<p>For example:
</p>
<div class="example">
<pre class="example">p = polynomial ([1, 0, 1]);
ismethod (p, &quot;roots&quot;)
  &rArr; 1
</pre></div>

<hr>
<div class="header">
<p>
Next: <a href="Manipulating-Classes.html#Manipulating-Classes" accesskey="n" rel="next">Manipulating Classes</a>, Up: <a href="Object-Oriented-Programming.html#Object-Oriented-Programming" accesskey="u" rel="up">Object Oriented Programming</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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