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<H2><A NAME="sec:C.7"><SPAN class="sec-nr">C.7</SPAN> <SPAN class="sec-title">Summary</SPAN></A></H2>
<P>This section explained the basic object-oriented notions used in
<font size=-1>XPCE</font>. <font size=-1>XPCE</font>'s data is organised
in <EM>objects</EM> and integers. An object represents a state. An
object is an instance of a class. A class describes the constituents of
the state represented in its instances and the methods (actions)
understood by its instances.
<P>A class is a normal object, as are all the other constituents of <font size=-1>XPCE</font>'s
programming world: methods, instance variables, messages, expressions,
etc. This uniform representation allows for inspecting and changing <font size=-1>XPCE</font>
using the four basic interface predicates from Prolog.
<P>The basic interface predicates pass control from Prolog to <font size=-1>XPCE</font>.
As control is to be passed from <font size=-1>XPCE</font> to Prolog (for
example if the user presses a button), a message is send to <A NAME="idx:prolog:651"></A><B>@prolog</B>,
the only instance of class host. This object will create a goal from the
message and pass this goal to the Prolog environment.
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