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<H2><A NAME="sec:C.2"><SPAN class="sec-nr">C.2</SPAN> <SPAN class="sec-title"><font size=-1>XPCE</font>'s
objects</SPAN></A></H2>
<P>More concretely, a <font size=-1>XPCE</font> object is a set of <EM>values</EM>
of <EM>instance variables</EM> bundled into a single entity which is
referred to by its <EM>object reference</EM>. An object is an
instantiation of a <EM>class</EM>. A class holds the key to decoding the
information of its instances:<SUP class="fn">16<SPAN class="fn-text">We
will mix the terms <EM>instance</EM> and <EM>object</EM> freely in this
document. They are considered synonyms.</SPAN></SUP> the instance
variables. The class also serves as a placeholder for storing the
methods understood by its instances. <A class="fig" href="sec-C.2.html#fig:arch1">Figure
41</A> illustrates this.
<P><A NAME="fig:arch1"></A>
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<TABLE ALIGN=center WIDTH="75%"><TR><TD>
<B>Figure 41 : </B>Classes and Objects in <font size=-1>XPCE</font></TABLE>
<H3><A NAME="sec:C.2.1"><SPAN class="sec-nr">C.2.1</SPAN> <SPAN class="sec-title">Classes</SPAN></A></H3>
<P>As explained above, a <font size=-1>XPCE</font> class describes the
storage-layout and the methods of its instances. In <font size=-1>XPCE</font>
a class is a normal object. It is an instance of class <EM>class</EM>.<SUP class="fn">17<SPAN class="fn-text">Class
class is an instance of itself. In other systems (SmallTalk, <CITE><A class="cite" href="Bibliography.html#Goldberg:83a">Goldberg &
Robson, 1983</A></CITE>), classes are instances of a <EM>meta-class</EM>.
Yet in other systems, classes have a completely different status (for
example widgets in the X11 Intrinsics)</SPAN></SUP>
<A NAME="idx:inheritanceofclasses:643"></A>As in most OO systems <font size=-1>XPCE</font>
classes may inherit from a <EM>super-class</EM>. <font size=-1>XPCE</font>
classes are organised in a single-inheritance hierarchy.<SUP class="fn">18<SPAN class="fn-text">Multiple
inheritance introduces various technical and conceptual problems. <font size=-1>XPCE</font>
uses delegation and templates to achieve similar results. This is
explained in <A class="sec" href="delegation.html">section C.4</A> and <A class="sec" href="sec-7.5.html">section
7.5.2.1</A>.</SPAN></SUP> The root of this hierarchy is class <EM>object</EM>.
Class object is the only class without a super-class. <A class="fig" href="sec-C.2.html#fig:pceclasshierarchy">Figure
42</A> gives the complete hierarchy of <font size=-1>XPCE</font>
built-in classes.
<P><A NAME="fig:pceclasshierarchy"></A>
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<TABLE ALIGN=center WIDTH="75%"><TR><TD>
<B>Figure 42 : </B><font size=-1>XPCE</font>'s Class hierarchy</TABLE>
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