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<p>
Next: <a href="Class-Methods.html" accesskey="n" rel="next">Class Methods</a>, Up: <a href="Object-Oriented-Programming.html" accesskey="u" rel="up">Object Oriented Programming</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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<hr>
<h3 class="section" id="Creating-a-Class-1"><span>34.1 Creating a Class<a class="copiable-link" href="#Creating-a-Class-1"> ¶</a></span></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-preformatted">a0 + a1 * x + a2 * x^2 + ... + 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-preformatted">a = [a0, a1, a2, ..., an];
</pre></div>
<a class="index-entry-id" id="index-_0040-class-methods"></a>
<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 ‘<samp class="samp">@</samp>’ symbol.
For example, the polynomial class will have all of its methods defined in the
<samp class="file">@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 class="file">@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 == 0)
p.poly = 0;
p = class (p, "polynomial");
else
if (isa (a, "polynomial"))
p = a;
elseif (isreal (a) && isvector (a))
p.poly = a(:).'; # force row vector
p = class (p, "polynomial");
else
error ("@polynomial: A must be a real vector");
endif
endif
endfunction
</pre></div>
<p>Note that the return value of the constructor must be the output of the
<code class="code">class</code> function. The first argument to the <code class="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-preformatted">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 class="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 class="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 class="code">type @polynomial/disp</code> will print the code of the
<code class="code">disp</code> method of the polynomial class to the screen, and
<code class="code">dbstop @polynomial/disp</code> will set a breakpoint at the first executable
line of the <code class="code">disp</code> method of the polynomial class.
</p>
<p>To check whether a variable belongs to a user class, the <code class="code">isobject</code> and
<code class="code">isa</code> functions can be used. For example:
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">p = polynomial ([1, 0, 1]);
isobject (p)
⇒ 1
isa (p, "polynomial")
⇒ 1
</pre></div></div>
<a class="anchor" id="XREFisobject"></a><span style="display:block; margin-top:-4.5ex;"> </span>
<dl class="first-deftypefn">
<dt class="deftypefn" id="index-isobject"><span><code class="def-type"><var class="var">tf</var> =</code> <strong class="def-name">isobject</strong> <code class="def-code-arguments">(<var class="var">x</var>)</code><a class="copiable-link" href="#index-isobject"> ¶</a></span></dt>
<dd><p>Return true if <var class="var">x</var> is a class object.
</p>
<p><strong class="strong">See also:</strong> <a class="ref" href="Built_002din-Data-Types.html#XREFclass">class</a>, <a class="ref" href="Data-Types.html#XREFtypeinfo">typeinfo</a>, <a class="ref" href="Built_002din-Data-Types.html#XREFisa">isa</a>, <a class="ref" href="#XREFismethod">ismethod</a>, <a class="ref" 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 class="code">methods</code>
function.
</p>
<a class="anchor" id="XREFmethods"></a><span style="display:block; margin-top:-4.5ex;"> </span>
<dl class="first-deftypefn">
<dt class="deftypefn" id="index-methods"><span><strong class="def-name">methods</strong> <code class="def-code-arguments">(<var class="var">obj</var>)</code><a class="copiable-link" href="#index-methods"> ¶</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-methods-1"><span><strong class="def-name">methods</strong> <code class="def-code-arguments">("<var class="var">classname</var>")</code><a class="copiable-link" href="#index-methods-1"> ¶</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-methods-2"><span><strong class="def-name">methods</strong> <code class="def-code-arguments">(…, "-full")</code><a class="copiable-link" href="#index-methods-2"> ¶</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-methods-3"><span><code class="def-type"><var class="var">mtds</var> =</code> <strong class="def-name">methods</strong> <code class="def-code-arguments">(…)</code><a class="copiable-link" href="#index-methods-3"> ¶</a></span></dt>
<dd><p>List the names of the public methods for the object <var class="var">obj</var> or the
named class <var class="var">classname</var>.
</p>
<p><var class="var">obj</var> may be an Octave class object or a Java object.
<var class="var">classname</var> may be the name of an Octave class or a Java class.
</p>
<p>If the optional argument <code class="code">"-full"</code> is given then Octave returns
full method signatures which include output type, name of method,
and the number and type of inputs.
</p>
<p>When called with no output arguments, <code class="code">methods</code> prints the list of
method names to the screen. Otherwise, the output argument <var class="var">mtds</var>
contains the list in a cell array of strings.
</p>
<p><strong class="strong">See also:</strong> <a class="ref" href="#XREFismethod">ismethod</a>, <a class="ref" href="Properties.html#XREFproperties">properties</a>, <a class="ref" href="Manipulating-Structures.html#XREFfieldnames">fieldnames</a>.
</p></dd></dl>
<p>To inquire whether a particular method exists for a user class, the
<code class="code">ismethod</code> function can be used.
</p>
<a class="anchor" id="XREFismethod"></a><span style="display:block; margin-top:-4.5ex;"> </span>
<dl class="first-deftypefn">
<dt class="deftypefn" id="index-ismethod"><span><code class="def-type"><var class="var">tf</var> =</code> <strong class="def-name">ismethod</strong> <code class="def-code-arguments">(<var class="var">obj</var>, <var class="var">method</var>)</code><a class="copiable-link" href="#index-ismethod"> ¶</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-ismethod-1"><span><code class="def-type"><var class="var">tf</var> =</code> <strong class="def-name">ismethod</strong> <code class="def-code-arguments">(<var class="var">class_name</var>, <var class="var">method</var>)</code><a class="copiable-link" href="#index-ismethod-1"> ¶</a></span></dt>
<dd><p>Return true if the string <var class="var">method</var> is a valid method of the object
<var class="var">obj</var> or of the class <var class="var">clsname</var>.
</p>
<p><strong class="strong">See also:</strong> <a class="ref" href="Introduction-to-Graphics-Structures.html#XREFisprop">isprop</a>, <a class="ref" href="#XREFisobject">isobject</a>, <a class="ref" href="Java-Interface-Functions.html#XREFisjava">isjava</a>, <a class="ref" href="#XREFmethods">methods</a>.
</p></dd></dl>
<p>For a polynomial class it makes sense to have a method to compute its roots.
</p>
<div class="example">
<div class="group"><pre class="verbatim">function r = roots (p)
r = roots (fliplr (p.poly));
endfunction
</pre></div></div>
<p>We can check for the existence of the <code class="code">roots</code>-method by calling:
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">p = polynomial ([1, 0, 1]);
ismethod (p, "roots")
⇒ 1
</pre></div></div>
</div>
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