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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"><html> <head> <title> The GTK+ Object and Type System </title> <meta name="GENERATOR" content= "Modular DocBook HTML Stylesheet Version 1.45"> <link rel="HOME" title="GTK+ / Gnome Application Development" href="ggad.html"> <link rel="UP" title="Advanced GTK+/Gnome Techniques" href= "advanced.html"> <link rel="PREVIOUS" title="Advanced GTK+/Gnome Techniques" href="advanced.html"> <link rel="NEXT" title="Type Checking and New Types" href= "z105.html"> </head> <body bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink= "#840084" alink="#0000FF"> <div class="NAVHEADER"> <table width="100%" border="0" bgcolor="#ffffff" cellpadding= "1" cellspacing="0"> <tr> <th colspan="4" align="center"> <font color="#000000" size="2">GTK+ / Gnome Application Development</font> </th> </tr> <tr> <td width="25%" bgcolor="#ffffff" align="left"> <a href="advanced.html"><font color="#0000ff" size="2"> <b><<< Previous</b></font></a> </td> <td width="25%" colspan="2" bgcolor="#ffffff" align= "center"> <font color="#0000ff" size="2"><b><a href="ggad.html"> <font color="#0000ff" size="2"><b> Home</b></font></a></b></font> </td> <td width="25%" bgcolor="#ffffff" align="right"> <a href="z105.html"><font color="#0000ff" size="2"><b> Next >>></b></font></a> </td> </tr> </table> </div> <div class="CHAPTER"> <h1> <a name="CHA-OBJECTS">The GTK+ Object and Type System</a> </h1> <div class="TOC"> <dl> <dt> <b>Table of Contents</b> </dt> <dt> <a href="cha-objects.html#SEC-OBJECTSTRUCTS">Object and Class Structures</a> </dt> <dt> <a href="z105.html">Type Checking and New Types</a> </dt> <dt> <a href="sec-classinit.html">Initializing a New Class</a> </dt> <dt> <a href="sec-gtkarg.html"><span class="STRUCTNAME"> GtkArg</span> and the Type System</a> </dt> <dt> <a href="hc-objectargs.html">Object Arguments</a> </dt> <dt> <a href="z109.html">Signals</a> </dt> <dt> <a href="sec-finalization.html">Object Finalization</a> </dt> <dt> <a href="sec-objectdata.html">Attaching Data to Objects</a> </dt> </dl> </div> <p> People often ask why GTK+ was written in C rather than an object-oriented language. The answer is that C is more portable and standardly available than any other language. However, although C lacks syntactic sugar for object-oriented programming, it in no way prohibits an object-oriented approach. </p> <p> GTK+ implements its own custom object system, which offers standard object-oriented features such as inheritance and virtual functions. In the tradition of languages such as Lisp, Smalltalk, and Java, the GTK+ object system is more runtime-centric than that of C++, allowing interpreted language bindings and GUI builders to interact with it in powerful ways. </p> <p> You may recall from <a href="cha-gtk.html">the chapter called <i>GTK+ Basics</i></a> that widgets are a special type of <span class="STRUCTNAME">GtkObject</span>; any object with <tt class="CLASSNAME">GtkWidget</tt> in its ancestry is a widget. Widgets represent a region on the screen---most of them are user interface elements, such as buttons or menus. There's nothing GUI-specific about <span class="STRUCTNAME">GtkObject</span>; the object system can be used in non-graphical programs. </p> <p> This chapter dives right into the details of GTK+'s object system, giving you an idea what's happening "behind the scenes" in any GTK+ program. Sooner or later you'll need this information: to write your own objects, debug existing objects, or just understand GTK+ code on a conceptual level. </p> <div class="SECT1"> <h1 class="SECT1"> <a name="SEC-OBJECTSTRUCTS">Object and Class Structures</a> </h1> <p> Each <span class="STRUCTNAME">GtkObject</span> has two essential components: a struct representing an <i class= "EMPHASIS">instance</i> of the object, and a struct representing the <i class="EMPHASIS">class</i>. In general, the instance struct contains the data members for each instance, and the class struct contains class function pointers (which can be overridden by subclasses). The class struct can also contain class data members---however, it's more typical to use static variables in the <span class="STRUCTNAME">.c</span> file implementing the object. If you're familiar with C++, the class struct is equivalent to a vtable, only the class struct is written by hand. It stores virtual functions for an object type. </p> <p> Here are the structs used in <tt class="CLASSNAME"> GtkButton</tt>: </p> <table border="0" bgcolor="#E0E0E0" width="100%"> <tr> <td><pre class="PROGRAMLISTING"> typedef struct _GtkButton GtkButton;typedef struct _GtkButtonClass GtkButtonClass;struct _GtkButton{ GtkBin bin; GtkWidget *child; guint in_button : 1; guint button_down : 1; guint relief : 2;};struct _GtkButtonClass{ GtkBinClass 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> Notice that the first member of <span class="STRUCTNAME"> struct _GtkButton</span> is <tt class="CLASSNAME"> GtkBin</tt>---that's because <tt class="CLASSNAME"> GtkButton</tt> is a subclass of <tt class="CLASSNAME"> GtkBin</tt>. (<tt class="CLASSNAME">GtkBin</tt> is a <tt class="CLASSNAME">GtkContainer</tt> that can hold one child.) Since <tt class="CLASSNAME">GtkBin</tt> is the first member, we can safely cast a <tt class="CLASSNAME"> GtkButton</tt> to <tt class="CLASSNAME">GtkBin</tt>. In <span class="STRUCTNAME">struct _GtkButtonClass</span>, the same principle applies, and <span class="STRUCTNAME"> GtkBinClass</span> is the first member. </p> </div> </div> <div class="NAVFOOTER"> <br> <br> <table width="100%" border="0" bgcolor="#ffffff" cellpadding= "1" cellspacing="0"> <tr> <td width="25%" bgcolor="#ffffff" align="left"> <a href="advanced.html"><font color="#0000ff" size="2"> <b><<< Previous</b></font></a> </td> <td width="25%" colspan="2" bgcolor="#ffffff" align= "center"> <font color="#0000ff" size="2"><b><a href="ggad.html"> <font color="#0000ff" size="2"><b> Home</b></font></a></b></font> </td> <td width="25%" bgcolor="#ffffff" align="right"> <a href="z105.html"><font color="#0000ff" size="2"><b> Next >>></b></font></a> </td> </tr> <tr> <td colspan="2" align="left"> <font color="#000000" size="2"><b>Advanced GTK+/Gnome Techniques</b></font> </td> <td colspan="2" align="right"> <font color="#000000" size="2"><b>Type Checking and New Types</b></font> </td> </tr> </table> </div> </body></html>⌨️ 快捷键说明
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