<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> <!-- Created by GNU Texinfo 6.5, http://www.gnu.org/software/texinfo/ --> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> <title>Class Methods (GNU Octave (version 5.1.0))</title> <meta name="description" content="Class Methods (GNU Octave (version 5.1.0))"> <meta name="keywords" content="Class Methods (GNU Octave (version 5.1.0))"> <meta name="resource-type" content="document"> <meta name="distribution" content="global"> <meta name="Generator" content="makeinfo"> <link href="index.html#Top" rel="start" title="Top"> <link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index"> <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> <link href="Object-Oriented-Programming.html#Object-Oriented-Programming" rel="up" title="Object Oriented Programming"> <link href="Indexing-Objects.html#Indexing-Objects" rel="next" title="Indexing Objects"> <link href="Creating-a-Class.html#Creating-a-Class" rel="prev" title="Creating a Class"> <style type="text/css"> <!-- a.summary-letter {text-decoration: none} blockquote.indentedblock {margin-right: 0em} blockquote.smallindentedblock {margin-right: 0em; font-size: smaller} blockquote.smallquotation {font-size: smaller} div.display {margin-left: 3.2em} div.example {margin-left: 3.2em} div.lisp {margin-left: 3.2em} div.smalldisplay {margin-left: 3.2em} div.smallexample {margin-left: 3.2em} div.smalllisp {margin-left: 3.2em} kbd {font-style: oblique} pre.display {font-family: inherit} pre.format {font-family: inherit} pre.menu-comment {font-family: serif} pre.menu-preformatted {font-family: serif} pre.smalldisplay {font-family: inherit; font-size: smaller} pre.smallexample {font-size: smaller} pre.smallformat {font-family: inherit; font-size: smaller} pre.smalllisp {font-size: smaller} span.nolinebreak {white-space: nowrap} span.roman {font-family: initial; font-weight: normal} span.sansserif {font-family: sans-serif; font-weight: normal} ul.no-bullet {list-style: none} --> </style> <link rel="stylesheet" type="text/css" href="octave.css"> </head> <body lang="en"> <a name="Class-Methods"></a> <div class="header"> <p> 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> </div> <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> <hr> <div class="header"> <p> 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> </div> </body> </html>