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<H2><A NAME="sec:C.1"><SPAN class="sec-nr">C.1</SPAN> <SPAN class="sec-title">What 
is ``Object-Oriented''?</SPAN></A></H2>

<P><font size=-1>XPCE</font> is an object-oriented system. This implies 
that the basic entity in <font size=-1>XPCE</font>'s world is an object, 
an entity with state capable of performing actions. Such an action is 
activated by sending the object a <EM>message</EM>.

<P>So far, most object oriented systems agree. Starting from these 
notions however one can find object oriented environments that take 
widely different approaches for representing objects, actions on objects 
and sending messages.

<P>Rather than specifying operations on each individual object most OO 
environments define some way of sharing the operation definitions 
(called <EM>methods</EM>). There are two ways to share methods. One is 
to create objects as a copy of other objects and then modify them (by 
attaching and deleting slots and methods) to fit the particular need. If 
a series of similar objects is needed, one first creates an object that 
satisfies the common functionality and then creates multiple copies of 
this object. This approach is followed by SELF
<CITE><A class="cite" href="Bibliography.html#chambers:89">Chambers <EM>et 
al.</EM>, 1989</A></CITE>. The other ---more traditional--- approach is 
to define a <EM>class</EM>. A class is an entity in the object oriented 
environment that defines the constituents of the persistent state and 
the methods for each of its <EM>instantiations</EM>.

<P><font size=-1>XPCE</font> takes the latter approach, but adds some 
notions of the object-copying approach because GUI's often contain 
unique objects and because object modification is more dynamic and 
therefore more suitable for rapid prototyping.

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