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<H2><A NAME="sec:C.4"><SPAN class="sec-nr">C.4</SPAN> <SPAN class="sec-title">Delegation</SPAN></A></H2>

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

<P><A NAME="idx:delegation:647"></A><A NAME="idx:multipleinheritance:648"></A><A NAME="idx:inheritancemultiple:649"></A><font size=-1>XPCE</font> 
does not offer multiple inheritance. Sharing functionality from multiple 
classes is generally dealt with using <EM>delegation</EM>. Delegation 
implies that messages not understood by a principal object are forwarded 
to an object that is associated to it.

<P>For example, <font size=-1>XPCE</font> defines class <A class="" href="summary.html#class:editor">editor</A> 
to be a graphical object capable of editing text. Most applications 
require a <A class="" href="summary.html#class:window">window</A> 
capable of editing text. This is implemented by <font size=-1>XPCE</font>'s 
class
<A class="" href="summary.html#class:view">view</A>, which is not a 
subclass of both editor and window, but just of window. The window 
displays an instance of class <A class="" href="summary.html#class:editor">editor</A> 
and constrains the size of the editor to occupy the entire visible area 
of the window. Any message arriving on the view that is not defined on 
class <A class="" href="summary.html#class:view">view</A> (or class <A class="" href="summary.html#class:window">window</A>) 
will be forwarded to the associated editor object.

<P>The dynamic nature of delegation makes this mechanism more flexible 
than multiple inheritance. For example, <font size=-1>XPCE</font> 
defines class <A class="" href="summary.html#class:node">node</A>. This 
class defines the communication to a <A class="" href="summary.html#class:tree">tree</A> 
to automate the layout of hierarchies. A node can manipulate any 
graphical object. Using multiple inheritance would require a class <EM>box_node</EM>,
<EM>circle_node</EM>, etc.

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swi-prolog-doc 5.6.59-2 / usr / share / doc / swi-prolog-doc / UserGuide / delegation.html
