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<H1><A NAME="sec:7"><SPAN class="sec-nr">7</SPAN> <SPAN class="sec-title">Defining 
classes</SPAN></A></H1>

<A NAME="sec:udc"></A>

<P>The user defined class interface provides a natural way to define new
<font size=-1>XPCE</font> classes. It is both used to create higher 
level libraries that have the same interface as the built-in <font size=-1>XPCE</font> 
classes as to define entire applications. Many of the library modules 
and <font size=-1>XPCE/P</font>rolog demo programs are implemented as 
user-defined classes. The PceDraw demo is an elaborate example defined 
entirely in user-defined classes.

<P>A user defined class lives in <font size=-1>XPCE</font>, just as any 
other <font size=-1>XPCE</font> class. There is no difference. Both use 
dynamic resolution of messages to method objects and then execute the 
method object. Both use the same object-management and storage 
facilities.

<P><font size=-1>XPCE/P</font>rolog user-defined classes have their 
methods implemented in Prolog. This provides a neat and transparent 
interface between the two systems.<SUP class="fn">8<SPAN class="fn-text"><font size=-1>XPCE</font> 
defines four implementation techniques for methods. <EM>C-function 
pointers</EM> are used for almost all the built-in behaviour. <EM>C++-function 
pointers</EM> are used when classes are defined in C++ (<CITE><A class="cite" href="Bibliography.html#XPCE:cpp">Wielemaker &amp; 
Anjewierden, 1994</A></CITE>) . Instances of <A class="" href="summary.html#class:c_pointer">c_pointer</A> 
are left to the host object for interpretation and finally, <A class="" href="summary.html#class:code">code</A> 
objects are executed.</SPAN></SUP>

<P>User defined classes are defined using Prolog syntax, where some 
operators have special meaning. The definition of an <font size=-1>XPCE/P</font>rolog 
class is enclosed in

<PRE class="code">
:- pce_begin_class(&lt;Class&gt;, &lt;Super&gt; [, &lt;Comment&gt;]).
&lt;Class definition&gt;
:- pce_end_class.
</PRE>

<P>Multiple classes may be defined in the same Prolog source file, but 
class definitions may not be nested.

<P>
<HR>
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<H2>Section Index</H2>

</CENTER>
<HR>
<DIV class="toc">
<DIV class="toc-h2"><A class="sec" href="sec-7.1.html"><SPAN class="sec-nr">7.1</SPAN> <SPAN class="sec-title">The 
class definition skeleton</SPAN></A></DIV>
<DIV class="toc-h3"><A class="sec" href="sec-7.1.html#sec:7.1.1"><SPAN class="sec-nr">7.1.1</SPAN> <SPAN class="sec-title">Definition 
of the template elements</SPAN></A></DIV>
<DIV class="toc-h4"><A class="sec" href="sec-7.1.html#sec:7.1.1.1"><SPAN class="sec-nr">7.1.1.1</SPAN> <SPAN class="sec-title">Syntax 
details</SPAN></A></DIV>
<DIV class="toc-h2"><A class="sec" href="slots.html"><SPAN class="sec-nr">7.2</SPAN> <SPAN class="sec-title">Accessing 
instance variables (slots)</SPAN></A></DIV>
<DIV class="toc-h2"><A class="sec" href="sec-7.3.html"><SPAN class="sec-nr">7.3</SPAN> <SPAN class="sec-title">Refining 
and redefining methods</SPAN></A></DIV>
<DIV class="toc-h3"><A class="sec" href="sec-7.3.html#sec:7.3.1"><SPAN class="sec-nr">7.3.1</SPAN> <SPAN class="sec-title">General 
redefinitions</SPAN></A></DIV>
<DIV class="toc-h3"><A class="sec" href="sec-7.3.html#sec:7.3.2"><SPAN class="sec-nr">7.3.2</SPAN> <SPAN class="sec-title">Redefinition 
in graphical classes</SPAN></A></DIV>
<DIV class="toc-h2"><A class="sec" href="udcpreds.html"><SPAN class="sec-nr">7.4</SPAN> <SPAN class="sec-title">Handling 
default arguments</SPAN></A></DIV>
<DIV class="toc-h2"><A class="sec" href="sec-7.5.html"><SPAN class="sec-nr">7.5</SPAN> <SPAN class="sec-title">Advanced 
topics</SPAN></A></DIV>
<DIV class="toc-h3"><A class="sec" href="sec-7.5.html#sec:7.5.1"><SPAN class="sec-nr">7.5.1</SPAN> <SPAN class="sec-title">More 
on type declarations</SPAN></A></DIV>
<DIV class="toc-h3"><A class="sec" href="sec-7.5.html#sec:7.5.2"><SPAN class="sec-nr">7.5.2</SPAN> <SPAN class="sec-title">Methods 
with variable number of arguments</SPAN></A></DIV>
<DIV class="toc-h4"><A class="sec" href="sec-7.5.html#sec:7.5.2.1"><SPAN class="sec-nr">7.5.2.1</SPAN> <SPAN class="sec-title">Using 
class templates</SPAN></A></DIV>
<DIV class="toc-h3"><A class="sec" href="sec-7.5.html#sec:7.5.3"><SPAN class="sec-nr">7.5.3</SPAN> <SPAN class="sec-title">Implementation 
notes</SPAN></A></DIV>
</DIV>
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