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<HTML><HEAD><TITLE>The Anatomy Of A Widget</TITLE><METANAME="GENERATOR"CONTENT="Modular DocBook HTML Stylesheet Version 1.49"><LINKREL="HOME"TITLE="GTK+ 1.2 Tutorial"HREF="gtk-tut.html"><LINKREL="UP"TITLE="Writing Your Own Widgets"HREF="ch-writingyourownwidgets.html"><LINKREL="PREVIOUS"TITLE="Writing Your Own Widgets"HREF="ch-writingyourownwidgets.html"><LINKREL="NEXT"TITLE="Creating a Composite widget"HREF="sec-creatingacompositewidget.html"></HEAD></head><body bgcolor="#FFFFFF" marginheight=0 marginwidth=0 width="100%"><table cellspacing=6 border=0 cellpadding=0 width="100%">  <tr>    <td bgcolor="#FFFFFF" valign=top nowrap>    <centeR><A HREF="/"><img src="/images/gtk-logo-rgb.gif" width=107 height=140 border=0></A>      <font face="helvetica,lucidia" color="#000000"><BR><BR><b>GTK+<BR>The GIMP Toolkit</b></center><BR><table width=100% cellspacing=0 cellpadding=2 border=0><tr><td bgcolor="#000000"><table width=100% cellspacing=0 cellpadding=4 border=0><tr><td bgcolor="#AAAAEE" nowrap><B>General</B></td></tr><tr><td bgcolor="#FFFFFF" nowrap><A HREF="/">Introduction</A><BR><A HREF="/screenshots/">Screenshots</A><br><A HREF="/download/">Download</A><br><A HREF="/mailinglists.html">Mailing Lists</A><BR><A HREF="/bindings.html">Language Bindings</A><BR><A HREF="http://gtk.themes.org/">Themes</A><BR><A HREF="/bugs.html">Bug Tracker</A><BR></td></tr></table></td></tr></table><BR>  <table width=100% cellspacing=0 cellpadding=2 border=0><tr><td bgcolor="#000000"><table width=100% cellspacing=0 cellpadding=4 border=0><tr><td bgcolor="#AAAAEE" nowrap><B>Documentation</B></td></tr><tr><td bgcolor="#FFFFFF" nowrap><A HREF="/faq/">FAQ</A><br><A HREF="/tutorial/">Tutorial</A><BR><A HREF="/api/">API Reference</A><br><A HREF="/books.html">Published Books</A><BR></td></tr></table></td></tr></table><BR>  <table width=100% cellspacing=0 cellpadding=2 border=0><tr><td bgcolor="#000000"><table width=100% cellspacing=0 cellpadding=4 border=0><tr><td bgcolor="#AAAAEE" nowrap><B>Projects</B></td></tr><tr><td bgcolor="#FFFFFF" nowrap><A HREF="http://www.pango.org/">Pango</A><BR><A HREF="http://sources.redhat.com/inti/">Inti</A><BR><A HREF="http://www.gnome.org/">GNOME</A><BR><A HREF="http://user.sgic.fi/~tml/gimp/win32/">GTK+ for Win32</A><br><A HREF="http://people.redhat.com/sopwith/gtkfb/">GtkFB (Framebuffer)</A><br><A HREF="http://www.directfb.org/gtk.xml">GTK+ on DirectFB</A><BR><A HREF="/beos/">GTK+ for BeOS</A></td></tr></table></td></tr></table><BR>  <table width=100% cellspacing=0 cellpadding=2 border=0><tr><td bgcolor="#000000"><table width=100% cellspacing=0 cellpadding=4 border=0><tr><td bgcolor="#AAAAEE" nowrap><B><B>Applications</B></B></td></tr><tr><td bgcolor="#FFFFFF" nowrap><A HREF="http://www.gimp.org/">GIMP</A><BR><A HREF="http://www.abiword.org/">Abiword</A><BR><A HREF="http://www.lysator.liu.se/~alla/dia/dia.html">Dia</A><BR><A HREF="http://glade.pn.org/">Glade</A><BR><A HREF="http://www.gnucash.org/">GnuCash</A><BR><A HREF="http://www.gnome.org/projects/gnumeric/">Gnumeric</A><BR><BR><A HREF="http://www.gnome.org/applist/">GNOME Software Map</A><br></td></tr></table></td></tr></table><BR>      </td>  <td bgcolor="#ffffff" valign=top width="99%"><font face="lucida,helvetica"><BODYCLASS="SECT1"BGCOLOR="#FFFFFF"TEXT="#000000"LINK="#0000FF"VLINK="#840084"ALINK="#0000FF"><DIVCLASS="NAVHEADER"><TABLEWIDTH="100%"BORDER="0"CELLPADDING="0"CELLSPACING="0"><TR><THCOLSPAN="3"ALIGN="center">GTK+ 1.2 Tutorial</TH></TR><TR><TDWIDTH="10%"ALIGN="left"VALIGN="bottom"><AHREF="ch-writingyourownwidgets.html">&#60;&#60;&#60; Previous</A></TD><TDWIDTH="80%"ALIGN="center"VALIGN="bottom">Chapter 24. Writing Your Own Widgets</TD><TDWIDTH="10%"ALIGN="right"VALIGN="bottom"><AHREF="sec-creatingacompositewidget.html">Next &#62;&#62;&#62;</A></TD></TR></TABLE><HRALIGN="LEFT"WIDTH="100%"></DIV><DIVCLASS="SECT1"><H1CLASS="SECT1"><ANAME="SEC-THEANATOMYOFAWIDGET">24.2. The Anatomy Of A Widget</A></H1><P>In order to create a new widget, it is important to have anunderstanding of how GTK objects work. This section is just meant as abrief overview. See the reference documentation for the details. </P><P>GTK widgets are implemented in an object oriented fashion. However,they are implemented in standard C. This greatly improves portabilityand stability over using current generation C++ compilers; however,it does mean that the widget writer has to pay attention to some ofthe implementation details. The information common to all instances ofone class of widgets (e.g., to all Button widgets) is stored in the <ICLASS="EMPHASIS">class structure</I>. There is only one copy of this inwhich is stored information about the class's signals(which act like virtual functions in C). To support inheritance, thefirst field in the class structure must be a copy of the parent'sclass structure. The declaration of the class structure of GtkButttonlooks like:</P><TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="100%"><TR><TD><PRECLASS="PROGRAMLISTING">struct _GtkButtonClass{  GtkContainerClass parent_class;  void (* pressed)  (GtkButton *button);  void (* released) (GtkButton *button);  void (* clicked)  (GtkButton *button);  void (* enter)    (GtkButton *button);  void (* leave)    (GtkButton *button);};</PRE></TD></TR></TABLE><P>When a button is treated as a container (for instance, when it isresized), its class structure can be cast to GtkContainerClass, andthe relevant fields used to handle the signals.</P><P>There is also a structure for each widget that is created on aper-instance basis. This structure has fields to store information thatis different for each instance of the widget. We'll call thisstructure the <ICLASS="EMPHASIS">object structure</I>. For the Button class, it lookslike:</P><TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="100%"><TR><TD><PRECLASS="PROGRAMLISTING">struct _GtkButton{  GtkContainer container;  GtkWidget *child;  guint in_button : 1;  guint button_down : 1;};</PRE></TD></TR></TABLE><P>Note that, similar to the class structure, the first field is theobject structure of the parent class, so that this structure can becast to the parent class' object structure as needed.</P></DIV><DIVCLASS="NAVFOOTER"><HRALIGN="LEFT"WIDTH="100%"><TABLEWIDTH="100%"BORDER="0"CELLPADDING="0"CELLSPACING="0"><TR><TDWIDTH="33%"ALIGN="left"VALIGN="top"><AHREF="ch-writingyourownwidgets.html">&#60;&#60;&#60; Previous</A></TD><TDWIDTH="34%"ALIGN="center"VALIGN="top"><AHREF="gtk-tut.html">Home</A></TD><TDWIDTH="33%"ALIGN="right"VALIGN="top"><AHREF="sec-creatingacompositewidget.html">Next &#62;&#62;&#62;</A></TD></TR><TR><TDWIDTH="33%"ALIGN="left"VALIGN="top">Writing Your Own Widgets</TD><TDWIDTH="34%"ALIGN="center"VALIGN="top"><AHREF="ch-writingyourownwidgets.html">Up</A></TD><TDWIDTH="33%"ALIGN="right"VALIGN="top">Creating a Composite widget</TD></TR></TABLE></DIV>        </td>    </tr></table>  </td>  </tr></table></body></BODY></HTML>
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