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<a name="Creating-a-Class-1"></a>
<h3 class="section">34.1 Creating a Class</h3>

<p>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.
</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>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 &lsquo;<samp>@</samp>&rsquo; symbol.
For example, the polynomial class will have all of its methods defined in the
<samp>@polynomial</samp> directory.
</p>
<p>The constructor for the class must be the name of the class itself; in this
example the constructor resides in the file <samp>@polynomial/polynomial.m</samp>.
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
</p>
<div class="example">
<pre class="verbatim">## -*- texinfo -*-
## @deftypefn  {} {} polynomial ()
## @deftypefnx {} {} 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 &gt; 1)
    print_usage ();
  endif

  if (nargin == 0)
    p.poly = [0];
    p = class (p, &quot;polynomial&quot;);
  else
    if (strcmp (class (a), &quot;polynomial&quot;))
      p = a;
    elseif (isreal (a) &amp;&amp; isvector (a))
      p.poly = a(:).';  # force row vector
      p = class (p, &quot;polynomial&quot;);
    else
      error (&quot;@polynomial: A must be a real vector&quot;);
    endif
  endif

endfunction
</pre></div>

<p>Note that the return value of the constructor must be the output of the
<code>class</code> function.  The first argument to the <code>class</code> 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
</p>
<div class="example">
<pre class="example">p = polynomial ([1, 0, 1]);
</pre></div>

<p>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</code> 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</code> 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.
</p>
<p>The same specification mechanism can be used wherever Octave expects a function
name.  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 whether a variable belongs to 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>: <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>, <a href="Introduction-to-Graphics-Structures.html#XREFisprop">isprop</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>: <em></em> <strong>methods</strong> <em>(<var>obj</var>)</em></dt>
<dt><a name="index-methods-1"></a>: <em></em> <strong>methods</strong> <em>(&quot;<var>classname</var>&quot;)</em></dt>
<dt><a name="index-methods-2"></a>: <em><var>mtds</var> =</em> <strong>methods</strong> <em>(&hellip;)</em></dt>
<dd><p>List the names of the public methods for the object <var>obj</var> or the
named class <var>classname</var>.
</p>
<p><var>obj</var> may be an Octave class object or a Java object.
<var>classname</var> may be the name of an Octave class or a Java class.
</p>
<p>When called with no output arguments, <code>methods</code> prints the list of
method names to the screen.  Otherwise, the output argument <var>mtds</var>
contains the list in a cell array of strings.
</p>
<p><strong>See also:</strong> <a href="Manipulating-Structures.html#XREFfieldnames">fieldnames</a>.
</p></dd></dl>


<p>To inquire whether a particular method exists for a user class, the
<code>ismethod</code> function can be used.
</p>
<a name="XREFismethod"></a><dl>
<dt><a name="index-ismethod"></a>: <em></em> <strong>ismethod</strong> <em>(<var>obj</var>, <var>method</var>)</em></dt>
<dt><a name="index-ismethod-1"></a>: <em></em> <strong>ismethod</strong> <em>(<var>clsname</var>, <var>method</var>)</em></dt>
<dd><p>Return true if the string <var>method</var> is a valid method of the object
<var>obj</var> or of the class <var>clsname</var>.
</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>

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Next: <a href="Class-Methods.html#Class-Methods" accesskey="n" rel="next">Class Methods</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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