<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<HTML>
<HEAD>
<TITLE>Programming in XPCE/Prolog: Section C.4</TITLE><LINK REL=home HREF="index.html">
<LINK REL=contents HREF="Contents.html">
<LINK REL=index HREF="DocIndex.html">
<LINK REL=summary HREF="summary.html">
<LINK REL=previous HREF="sec-C.3.html">
<LINK REL=next HREF="sec-C.5.html">
<STYLE type="text/css">
/* Style sheet for SWI-Prolog latex2html
*/
dd.defbody
{ margin-bottom: 1em;
}
dt.pubdef
{ background-color: #c5e1ff;
}
pre.code
{ margin-left: 1.5em;
margin-right: 1.5em;
border: 1px dotted;
padding-top: 5px;
padding-left: 5px;
padding-bottom: 5px;
background-color: #f8f8f8;
}
div.navigate
{ text-align: center;
background-color: #f0f0f0;
border: 1px dotted;
padding: 5px;
}
div.title
{ text-align: center;
padding-bottom: 1em;
font-size: 200%;
font-weight: bold;
}
div.author
{ text-align: center;
font-style: italic;
}
div.abstract
{ margin-top: 2em;
background-color: #f0f0f0;
border: 1px dotted;
padding: 5px;
margin-left: 10%; margin-right:10%;
}
div.abstract-title
{ text-align: center;
padding: 5px;
font-size: 120%;
font-weight: bold;
}
div.toc-h1
{ font-size: 200%;
font-weight: bold;
}
div.toc-h2
{ font-size: 120%;
font-weight: bold;
margin-left: 2em;
}
div.toc-h3
{ font-size: 100%;
font-weight: bold;
margin-left: 4em;
}
div.toc-h4
{ font-size: 100%;
margin-left: 6em;
}
span.sec-nr
{
}
span.sec-title
{
}
span.pred-ext
{ font-weight: bold;
}
/* Footnotes */
sup.fn { color: blue; text-decoration: underline; }
span.fn-text: { display: none; }
sup.fn span {display: none;}
sup:hover span
{ display: block !important;
position: absolute; top: auto; left: auto; width: 80%;
color: #000; background: white;
border: 2px solid;
padding: 5px; margin: 10px; z-index: 100;
font-size: smaller;
}
</STYLE>
</HEAD>
<BODY BGCOLOR="white">
<DIV class="navigate"><A class="nav" href="index.html"><IMG SRC="home.gif" BORDER=0 ALT="Home"></A>
<A class="nav" href="Contents.html"><IMG SRC="index.gif" BORDER=0 ALT="Contents"></A>
<A class="nav" href="DocIndex.html"><IMG SRC="yellow_pages.gif" BORDER=0 ALT="Index"></A>
<A class="nav" href="summary.html"><IMG SRC="info.gif" BORDER=0 ALT="Summary"></A>
<A class="nav" href="sec-C.3.html"><IMG SRC="prev.gif" BORDER=0 ALT="Previous"></A>
<A class="nav" href="sec-C.5.html"><IMG SRC="next.gif" BORDER=0 ALT="Next"></A>
</DIV>
<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.
<P></BODY></HTML>
