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<a name="Class-Methods"></a>
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Next: <a href="Indexing-Objects.html#Indexing-Objects" accesskey="n" rel="next">Indexing Objects</a>, Previous: <a href="Creating-a-Class.html#Creating-a-Class" accesskey="p" rel="prev">Creating a Class</a>, Up: <a href="Object-Oriented-Programming.html#Object-Oriented-Programming" 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#Concept-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="Class-Methods-1"></a>
<h3 class="section">34.2 Class Methods</h3>
<p>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>disp</code> method. The <code>disp</code> 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.
</p>
<p>An example of a <code>disp</code> method for the polynomial class might be
</p>
<div class="example">
<pre class="verbatim">function disp (p)
a = p.poly;
first = true;
for i = 1 : length (a);
if (a(i) != 0)
if (first)
first = false;
elseif (a(i) > 0 || isnan (a(i)))
printf (" +");
endif
if (a(i) < 0)
printf (" -");
endif
if (i == 1)
printf (" %.5g", abs (a(i)));
elseif (abs (a(i)) != 1)
printf (" %.5g *", abs (a(i)));
endif
if (i > 1)
printf (" X");
endif
if (i > 2)
printf (" ^ %d", i - 1);
endif
endif
endfor
if (first)
printf (" 0");
endif
printf ("\n");
endfunction
</pre></div>
<p>To be consistent with the Octave graphic handle classes, a class should also
define the <code>get</code> and <code>set</code> methods. The <code>get</code> 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.
</p>
<div class="example">
<pre class="verbatim">function val = get (p, prop)
if (nargin < 1 || nargin > 2)
print_usage ();
endif
if (nargin == 1)
val.poly = p.poly;
else
if (! ischar (prop))
error ("@polynomial/get: PROPERTY must be a string");
endif
switch (prop)
case "poly"
val = p.poly;
otherwise
error ('@polynomial/get: invalid PROPERTY "%s"', prop);
endswitch
endif
endfunction
</pre></div>
<p>Similarly, the first argument to the <code>set</code> method should be an object and
any additional arguments should be property/value pairs.
</p>
<div class="example">
<pre class="verbatim">function pout = set (p, varargin)
if (numel (varargin) < 2 || rem (numel (varargin), 2) != 0)
error ("@polynomial/set: expecting PROPERTY/VALUE pairs");
endif
pout = p;
while (numel (varargin) > 1)
prop = varargin{1};
val = varargin{2};
varargin(1:2) = [];
if (! ischar (prop) || ! strcmp (prop, "poly"))
error ("@polynomial/set: invalid PROPERTY for polynomial class");
elseif (! (isreal (val) && isvector (val)))
error ("@polynomial/set: VALUE must be a real vector");
endif
pout.poly = val(:).'; # force row vector
endwhile
endfunction
</pre></div>
<p>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</code> method.
</p>
<div class="example">
<pre class="example">p = set (p, "poly", [1, 0, 0, 0, 1]);
</pre></div>
<p>The <code>set</code> method makes use of the <code>subsasgn</code> method of the class, and
therefore this method must also be defined. The <code>subsasgn</code> method is
discussed more thoroughly in the next section (see <a href="Indexing-Objects.html#Indexing-Objects">Indexing Objects</a>).
</p>
<p>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:
</p>
<div class="example">
<pre class="example">p = polynomial ([1, 0, 1]);
save userclass.mat p
clear p
load userclass.mat
</pre></div>
<p>All of the file formats supported by <code>save</code> and <code>load</code> 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</code> method of the class.
</p>
<a name="XREFsaveobj"></a><dl>
<dt><a name="index-saveobj"></a><em><var>b</var> =</em> <strong>saveobj</strong> <em>(<var>a</var>)</em></dt>
<dd><p>Method of a class to manipulate an object prior to saving it to a file.
</p>
<p>The function <code>saveobj</code> is called when the object <var>a</var> is saved
using the <code>save</code> function. An example of the use of <code>saveobj</code>
might be to remove fields of the object that don’t make sense to be saved
or it might be used to ensure that certain fields of the object are
initialized before the object is saved. For example:
</p>
<div class="example">
<pre class="example">function b = saveobj (a)
b = a;
if (isempty (b.field))
b.field = initfield (b);
endif
endfunction
</pre></div>
<p><strong>See also:</strong> <a href="#XREFloadobj">loadobj</a>, <a href="Built_002din-Data-Types.html#XREFclass">class</a>.
</p></dd></dl>
<p><code>saveobj</code> is called just prior to saving the class to a file. Similarly,
the <code>loadobj</code> 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.
</p>
<a name="XREFloadobj"></a><dl>
<dt><a name="index-loadobj"></a><em><var>b</var> =</em> <strong>loadobj</strong> <em>(<var>a</var>)</em></dt>
<dd><p>Method of a class to manipulate an object after loading it from a file.
</p>
<p>The function <code>loadobj</code> is called when the object <var>a</var> is loaded
using the <code>load</code> function. An example of the use of <code>saveobj</code>
might be to add fields to an object that don’t make sense to be saved.
For example:
</p>
<div class="example">
<pre class="example">function b = loadobj (a)
b = a;
b.addmissingfield = addfield (b);
endfunction
</pre></div>
<p><strong>See also:</strong> <a href="#XREFsaveobj">saveobj</a>, <a href="Built_002din-Data-Types.html#XREFclass">class</a>.
</p></dd></dl>
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Next: <a href="Indexing-Objects.html#Indexing-Objects" accesskey="n" rel="next">Indexing Objects</a>, Previous: <a href="Creating-a-Class.html#Creating-a-Class" accesskey="p" rel="prev">Creating a Class</a>, Up: <a href="Object-Oriented-Programming.html#Object-Oriented-Programming" 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#Concept-Index" title="Index" rel="index">Index</a>]</p>
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