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✏️ Editing: CH01.xml

<chapter id='Intrinsics_and_Widgets'>
<title>Intrinsics and Widgets</title>
<para>
The Intrinsics are a programming library tailored to the special requirements
of user interface construction within a network window system,
specifically the X Window System.
The Intrinsics and a widget set make up an X Toolkit.
</para>
<sect1 id="Intrinsics">
<title>Intrinsics</title>
<para>
The Intrinsics provide the base mechanism necessary to build
a wide variety of interoperating widget sets and application environments.
The Intrinsics are a layer on top of Xlib, the
C Library X Interface.  They extend the
fundamental abstractions provided by the X Window System while still
remaining independent of any particular user interface policy or
style.
</para>

<para>
The Intrinsics use object-oriented programming techniques to supply a
consistent architecture for constructing and composing user interface
components, known as widgets.  This
allows programmers to extend a widget set in new ways, either by
deriving new widgets from existing ones (subclassing) or by writing
entirely new widgets following the established conventions.
</para>

<para>
When the Intrinsics were first conceived, the root of the object
hierarchy was a widget class named
Core.
In Release 4 of the
Intrinsics, three nonwidget superclasses were added above Core.
These superclasses are described in <xref linkend='Nonwidget_Objects' />.
The name of the class
now at the root of the Intrinsics class hierarchy is
Object.
The remainder of this
specification refers uniformly to <emphasis remap='I'>widgets</emphasis> and <emphasis remap='I'>Core</emphasis>
as if they were the
base class for all Intrinsics operations.  The argument descriptions
for each Intrinsics procedure and <xref linkend='Nonwidget_Objects' />
describe which operations
are defined for the nonwidget superclasses of Core.  The reader may
determine by context whether a specific reference to <emphasis remap='I'>widget</emphasis>
actually means ``widget'' or ``object.''
</para>
</sect1>

<sect1 id="Languages">
<title>Languages</title>
<para>
The Intrinsics are intended to be used for two programming purposes.
Programmers writing widgets will be using most of the facilities
provided by the
Intrinsics to construct user interface components from the simple, such
as buttons and scrollbars, to the complex, such as control panels and
property sheets.  Application programmers will use a much smaller subset of
the Intrinsics procedures in combination with one or more sets of widgets to
construct and present complete user interfaces on an X display.  The
Intrinsics
programming interfaces primarily
intended for application use are designed to be callable from most
procedural programming languages.  Therefore, most arguments are passed by
reference rather than by value.  The interfaces primarily
intended for widget programmers are expected to be used principally
from the C language.  In these cases, the usual C programming
conventions apply.  In this specification, the term <emphasis remap='I'>client</emphasis> refers to
any module, widget, or application that calls an Intrinsics procedure.
</para>

<para>
Applications that use the Intrinsics mechanisms
must include the header files
<function>&lt;X11/Intrinsic.h&gt;</function>
and
<function>&lt;X11/StringDefs.h&gt;</function>,
or their equivalent,
and they may also include
<function>&lt;X11/Xatoms.h&gt;</function>
and
<function>&lt;X11/Shell.h&gt;</function>.
In addition, widget implementations should include
<function>&lt;X11/IntrinsicP.h&gt;</function>
instead of
<function>&lt;X11/Intrinsic.h&gt;</function>.
</para>

<para>
The applications must also include the additional header files for
each widget class that they are to use (for example,
<function>&lt;X11/Xaw/Label.h&gt;</function>
or
<function>&lt;X11/Xaw/Scrollbar.h&gt;).</function>
On a POSIX-based system,
the Intrinsics object library file is named
<function>libXt.a</function>
and is usually referenced as \-lXt when linking the application.
</para>
</sect1>

<sect1 id="Procedures_and_Macros">
<title>Procedures and Macros</title>
<para>
All functions defined in this specification except those specified below
may be implemented as C macros with arguments.  C applications may use
``#undef'' to remove a macro definition and ensure that the actual function
is referenced.  Any such macro will expand to a single expression that
has the same precedence as a function call and that evaluates each
of its arguments exactly once, fully protected by parentheses, so that
arbitrary expressions may be used as arguments.
</para>

<para>
The following symbols are macros that do not have function
equivalents and that may expand their arguments in a manner other
than that described above:
<xref linkend='XtCheckSubclass' xrefstyle='select: title'/>,
<xref linkend='XtNew' xrefstyle='select: title'/>,
<xref linkend='XtNumber' xrefstyle='select: title'/>,
<xref linkend='XtOffsetOf' xrefstyle='select: title'/>,
<xref linkend='XtOffset' xrefstyle='select: title'/>,
and
<xref linkend='XtSetArg' xrefstyle='select: title'/>.
</para>
</sect1>

<sect1 id="Widgets">
<title>Widgets</title>
<para>
The fundamental abstraction and data type of the X Toolkit is the widget,
which is a combination of an X window and its associated
input and display semantics
and which is dynamically allocated and contains state information.
Some widgets display information (for example, text or graphics),
and others are merely containers for other widgets (for example, a menu box).
Some widgets are output-only and do not react to pointer or keyboard input,
and others change their display in response to input
and can invoke functions that an application has attached to them.
</para>

<para>
Every widget belongs to exactly one widget class, which is statically
allocated and initialized and which contains the operations allowable on
widgets of that class.
Logically, a widget class is the procedures and data associated
with all widgets belonging to that class.
These procedures and data can be inherited by
subclasses.
Physically, a widget class is a pointer to a structure.
The contents of this structure are constant for all widgets of the widget
class but will vary from class to class.
(Here, ``constant'' means the class structure is initialized at compile time
and never changed, except for a one-time class initialization
and in-place compilation of resource lists,
which takes place when the first widget of the class or subclass is created.)
For further information,
see <xref linkend='Creating_Widgets' />
</para>

<para>
The distribution of the declarations and code for a new widget class
among a public .h file for application programmer use, a private .h file
for widget programmer use,
and the implementation .c file is described in <xref linkend='Widget_Classing' />
The predefined widget classes adhere to these conventions.
</para>

<para>
A widget instance is composed of two parts:
</para>
<itemizedlist spacing='compact'>
  <listitem>
    <para>
A data structure which contains instance-specific values.
    </para>
  </listitem>
  <listitem>
    <para>
A class structure which contains information that is applicable to
all widgets of that class.
    </para>
  </listitem>
</itemizedlist>
<para>
Much of the input/output of a widget (for example, fonts, colors, sizes,
or border widths) is customizable by users.
</para>

<para>
This chapter discusses the base widget classes,
Core, Composite, and Constraint, and
ends with a discussion of widget classing.
</para>
<sect2 id="Core_Widgets">
<title>Core Widgets</title>
<para>
The
Core
widget class contains the definitions of fields common to all widgets.
All widgets classes are subclasses of the
Core class,
which is defined by the
<function>CoreClassPart</function>
and
<function>CorePart</function>
structures.
</para>
<sect3 id="CoreClassPart_Structure">
<title>CoreClassPart Structure</title>
<para>
All widget classes contain the fields defined in the
<function>CoreClassPart</function>
structure.
</para>
<literallayout>
typedef struct {
     WidgetClass superclass;    See <xref linkend="Widget_Classing" xrefstyle='select: label' />
     String class_name;         See <xref linkend="Resource_Management" xrefstyle='select: label' />
     Cardinal widget_size;      See <xref linkend="Widget_Classing" xrefstyle='select: label' />
     XtProc class_initialize;   See <xref linkend="Widget_Classing" xrefstyle='select: label' />
     XtWidgetClassProc class_part_initialize;  See <xref linkend="Widget_Classing" xrefstyle='select: label' />
     XtEnum class_inited;       See <xref linkend="Widget_Classing" xrefstyle='select: label' />
     XtInitProc initialize;     See <xref linkend='Creating_Widgets' xrefstyle='select: label' />
     XtArgsProc initialize_hook;  See <xref linkend='Creating_Widgets' xrefstyle='select: label' />
     XtRealizeProc realize;     See <xref linkend='Realizing_Widgets' xrefstyle='select: label' />
     XtActionList actions;      See <xref linkend='Translation_Management' xrefstyle='select: label' />
     Cardinal num_actions;      See <xref linkend='Translation_Management' xrefstyle='select: label' />
     XtResourceList resources;  See <xref linkend="Resource_Management" xrefstyle='select: label' />
     Cardinal num_resources;    See <xref linkend="Resource_Management" xrefstyle='select: label' />
     XrmClass xrm_class;     Private to resource manager
     Boolean compress_motion;      See <xref linkend='X_Event_Filters' xrefstyle='select: label' />
     XtEnum compress_exposure;     See <xref linkend='X_Event_Filters' xrefstyle='select: label' />
     Boolean compress_enterleave;  See <xref linkend='X_Event_Filters' xrefstyle='select: label' />
     Boolean visible_interest;     See <xref linkend='Widget_Exposure_and_Visibility' xrefstyle='select: label' />
     XtWidgetProc destroy;         See <xref linkend='Destroying_Widgets' xrefstyle='select: label' />
     XtWidgetProc resize;          See <xref linkend='Geometry_Management' xrefstyle='select: label' /> 
     XtExposeProc expose;          See <xref linkend='Widget_Exposure_and_Visibility' xrefstyle='select: label' />
     XtSetValuesFunc set_values;   See <xref linkend='Reading_and_Writing_Widget_State' xrefstyle='select: label' />
     XtArgsFunc set_values_hook;   See <xref linkend='Reading_and_Writing_Widget_State' xrefstyle='select: label' />
     XtAlmostProc set_values_almost;  See <xref linkend='Reading_and_Writing_Widget_State' xrefstyle='select: label' />
     XtArgsProc get_values_hook;   See <xref linkend='Reading_and_Writing_Widget_State' xrefstyle='select: label' />
     XtAcceptFocusProc accept_focus;  See <xref linkend='Focusing_Events_on_a_Child' xrefstyle='select: label' />
     XtVersionType version;        See <xref linkend="Widget_Classing" xrefstyle='select: label' />
     XtPointer callback_private;  Private to callbacks
     String tm_table;              See <xref linkend='Translation_Management' xrefstyle='select: label' />
     XtGeometryHandler query_geometry;  See <xref linkend ='Geometry_Management' xrefstyle='select: label' />
     XtStringProc display_accelerator;  See <xref linkend='Translation_Management' xrefstyle='select: label' />
     XtPointer extension;          See <xref linkend="Widget_Classing" xrefstyle='select: label' />
} CoreClassPart;
</literallayout>
<para>
All widget classes have the Core class fields as their first component.
The prototypical
<function>WidgetClass</function>
and
<function>CoreWidgetClass</function>
are defined with only this set of fields.
</para>
<literallayout>
typedef struct {
     CoreClassPart core_class;
} WidgetClassRec, *WidgetClass, CoreClassRec, *CoreWidgetClass;
</literallayout>
<para>
Various routines can cast widget class pointers, as needed,
to specific widget class types.
</para>

<para>
The single occurrences of the class record and pointer for
creating instances of Core are
</para>

<para>
In
<function>IntrinsicP.h</function>:
</para>
<literallayout>
extern WidgetClassRec widgetClassRec;
#define coreClassRec widgetClassRec
</literallayout>
<para>
In
<function>Intrinsic.h</function>:
</para>
<literallayout>
extern WidgetClass widgetClass, coreWidgetClass;
</literallayout>
<para>
The opaque types
<function>Widget</function>
and
<function>WidgetClass</function>
and the opaque variable
<function>widgetClass</function>
are defined for generic actions on widgets.
In order to make these types opaque and ensure that the compiler
does not allow applications to access private data, the Intrinsics use
incomplete structure definitions in
<function>Intrinsic.h</function>:
</para>
<literallayout>
typedef struct _WidgetClassRec *WidgetClass, *CoreWidgetClass;
</literallayout>
</sect3>
<sect3 id="CorePart_Structure">
<title>CorePart Structure</title>
<para>
All widget instances contain the fields defined in the
<function>CorePart</function>
structure.
</para>
<literallayout>
typedef struct _CorePart {
     Widget self;     Described below
     WidgetClass widget_class;  See <xref linkend='Widget_Classing' xrefstyle='select: label' />
     Widget parent;             See <xref linkend='Creating_Widgets' xrefstyle='select: label' />
     Boolean being_destroyed;   See <xref linkend='Destroying_Widgets' xrefstyle='select: label' />
     XtCallbackList destroy_callbacks;  <xref linkend='Destroying_Widgets' xrefstyle='select: label' />
     XtPointer constraints;     See <xref linkend='Constrained_Composite_Widgets' xrefstyle='select: label' />
     Position x;                See <xref linkend='Geometry_Management' xrefstyle='select: label' />
     Position y;                See <xref linkend='Geometry_Management' xrefstyle='select: label' />
     Dimension width;           See <xref linkend='Geometry_Management' xrefstyle='select: label' />
     Dimension height;          See <xref linkend='Geometry_Management' xrefstyle='select: label' />
     Dimension border_width;    See <xref linkend='Geometry_Management' xrefstyle='select: label' />
     Boolean managed;           See <xref linkend='Composite_Widgets_and_Their_Children' xrefstyle='select: label' />
     Boolean sensitive;         See <xref linkend='Setting_and_Checking_the_Sensitivity_State_of_a_Widget' xrefstyle='select: label' />
     Boolean ancestor_sensitive;  See <xref linkend='Setting_and_Checking_the_Sensitivity_State_of_a_Widget' xrefstyle='select: label' />
     XtTranslations accelerators; See <xref linkend='Translation_Management' xrefstyle='select: label' />
     Pixel border_pixel;        See <xref linkend='Realizing_Widgets' xrefstyle='select: label' />
     Pixmap border_pixmap;      See <xref linkend='Realizing_Widgets' xrefstyle='select: label' />
     WidgetList popup_list;     See <xref linkend='Pop_Up_Widgets' xrefstyle='select: label' />
     Cardinal num_popups;       See <xref linkend='Pop_Up_Widgets' xrefstyle='select: label' />
     String name;               See <xref linkend='Resource_Management' xrefstyle='select: label' />
     Screen *screen;            See <xref linkend='Realizing_Widgets' xrefstyle='select: label' />
     Colormap colormap;         See <xref linkend='Realizing_Widgets' xrefstyle='select: label' />
     Window window;             See <xref linkend='Realizing_Widgets' xrefstyle='select: label' />
     Cardinal depth;            See <xref linkend='Realizing_Widgets' xrefstyle='select: label' />
     Pixel background_pixel;    See <xref linkend='Realizing_Widgets' xrefstyle='select: label' />
     Pixmap background_pixmap;  See <xref linkend='Realizing_Widgets' xrefstyle='select: label' />
     Boolean visible;           See <xref linkend='Widget_Exposure_and_Visibility' xrefstyle='select: label' />
     Boolean mapped_when_managed;  See <xref linkend='Composite_Widgets_and_Their_Children' xrefstyle='select: label' />
} CorePart;
</literallayout>
<para>
All widget instances have the Core fields as their first component.
The prototypical type
<function>Widget</function>
is defined with only this set of fields.
</para>
<literallayout>
typedef struct {
     CorePart core;
} WidgetRec, *Widget, CoreRec, *CoreWidget;
</literallayout>
<para>
Various routines can cast widget pointers, as needed,
to specific widget types.
</para>

<para>
In order to make these types opaque and ensure that the compiler
does not allow applications to access private data, the Intrinsics use
incomplete structure definitions in
<function>Intrinsic.h</function>.
</para>
<literallayout>
typedef struct _WidgetRec *Widget, *CoreWidget;
</literallayout>
</sect3>
<sect3 id="Core_Resources">
<title>Core Resources</title>
<para>
The resource names, classes, and representation types specified in the
<function>coreClassRec</function>
resource list are
</para>

<informaltable frame='topbot'>
  <?dbfo keep-together="always" ?>
  <tgroup cols='3' align='left' colsep='0' rowsep='0'>
  <colspec colname='c1' colwidth='1.0*'/>
  <colspec colname='c2' colwidth='1.0*'/>
  <colspec colname='c3' colwidth='1.0*'/>
  <thead>
    <row rowsep='1'>
      <entry>Name</entry>
      <entry>Class</entry>
      <entry>Representation</entry>
    </row>
  </thead>
  <tbody>
    <row>
      <entry>XtNaccelerators</entry>
      <entry>XtCAccelerators</entry>
      <entry>XtRAcceleratorTable</entry>
    </row>
    <row>
      <entry>XtNbackground</entry>
      <entry>XtCBackground</entry>
      <entry>XtRPixel</entry>
    </row>
    <row>
      <entry>XtNbackgroundPixmap</entry>
      <entry>XtCPixmap</entry>
      <entry>XtRPixmap</entry>
    </row>
    <row>
      <entry>XtNborderColor</entry>
      <entry>XtCBorderColor</entry>
      <entry>XtRPixel</entry>
    </row>
    <row>
      <entry>XtNborderPixmap</entry>
      <entry>XtCPixmap</entry>
      <entry>XtRPixmap</entry>
    </row>
    <row>
      <entry>XtNcolormap</entry>
      <entry>XtCColormap</entry>
      <entry>XtRColormap</entry>
    </row>
    <row>
      <entry>XtNdepth</entry>
      <entry>XtCDepth</entry>
      <entry>XtRInt</entry>
    </row>
    <row>
      <entry>XtNmappedWhenManaged</entry>
      <entry>XtCMappedWhenManaged</entry>
      <entry>XtRBoolean</entry>
    </row>
    <row>
      <entry>XtNscreen</entry>
      <entry>XtCScreen</entry>
      <entry>XtRScreen</entry>
    </row>
    <row>
      <entry>XtNtranslations</entry>
      <entry>XtCTranslations</entry>
      <entry>XtRTranslationTable</entry>
    </row>
  </tbody>
  </tgroup>
</informaltable>
<para>
Additional resources are defined for all widgets via the
<function>objectClassRec</function>
and
<function>rectObjClassRec</function>
resource lists; see <xref linkend='Object_Objects' /> and <xref linkend='Rectangle_Objects' /> for details.
</para>
</sect3>
<sect3 id="CorePart_Default_Values">
<title>CorePart Default Values</title>
<para>
The default values for the Core fields, which are filled in by the Intrinsics,
from the resource lists, and by the initialize procedures, are
</para>
<informaltable frame='topbot'>
  <?dbfo keep-together="always" ?>
  <tgroup cols='2' align='left' colsep='0' rowsep='0'>
  <colspec colname='c1' colwidth='0.4*'/>
  <colspec colname='c2' colwidth='1.0*'/>
  <thead>
    <row rowsep='1'>
      <entry>Field</entry>
      <entry>Default Value</entry>
    </row>
  </thead>
  <tbody>
    <row>
      <entry>self</entry>
      <entry>Address of the widget structure (may not be changed).</entry>
    </row>
    <row>
      <entry>widget_class</entry>
      <entry><emphasis remap='I'>widget_class</emphasis> argument to
      <xref linkend='XtCreateWidget' xrefstyle='select: title'/>
      (may not be changed).</entry>
    </row>
    <row>
      <entry>parent</entry>
      <entry><emphasis remap='I'>parent</emphasis> argument to
      <xref linkend='XtCreateWidget' xrefstyle='select: title'/>
      (may not be changed).</entry>
    </row>
    <row>
      <entry>being_destroyed</entry>
      <entry>Parent's <emphasis remap='I'>being_destroyed</emphasis> value.</entry>
    </row>
    <row>
      <entry>destroy_callbacks</entry>
      <entry>NULL</entry>
    </row>
    <row>
      <entry>constraints</entry>
      <entry>NULL</entry>
    </row>
    <row>
      <entry>x</entry>
      <entry>0</entry>
    </row>
    <row>
      <entry>y</entry>
      <entry>0</entry>
    </row>
    <row>
      <entry>width</entry>
      <entry>0</entry>
    </row>
    <row>
      <entry>height</entry>
      <entry>0</entry>
    </row>
    <row>
      <entry>border_width</entry>
      <entry>1</entry>
    </row>
    <row>
      <entry>managed</entry>
      <entry><function>False</function></entry>
    </row>
    <row>
      <entry>sensitive</entry>
      <entry><function>True</function></entry>
    </row>
    <row>
      <entry>ancestor_sensitive</entry>
      <entry>logical AND of parent's <emphasis remap='I'>sensitive</emphasis> and
      <emphasis remap='I'>ancestor_sensitive</emphasis> values.</entry>
    </row>
    <row>
      <entry>accelerators</entry>
      <entry>NULL</entry>
    </row>
    <row>
      <entry>border_pixel</entry>
      <entry><function>XtDefaultForeground</function></entry>
    </row>
    <row>
      <entry>border_pixmap</entry>
      <entry><function>XtUnspecifiedPixmap</function></entry>
    </row>
    <row>
      <entry>popup_list</entry>
      <entry>NULL</entry>
    </row>
    <row>
      <entry>num_popups</entry>
      <entry>0</entry>
    </row>
    <row>
      <entry>name</entry>
      <entry><emphasis remap='I'>name</emphasis> argument to
      <xref linkend='XtCreateWidget' xrefstyle='select: title'/>
      (may not be changed).</entry>
    </row>
    <row>
      <entry>screen</entry>
      <entry>Parent's <emphasis remap='I'>screen</emphasis>; top-level widget gets screen from display specifier
      (may not be changed).</entry>
    </row>
    <row>
      <entry>colormap</entry>
      <entry>Parent's <emphasis remap='I'>colormap</emphasis> value.</entry>
    </row>
    <row>
      <entry>window</entry>
      <entry>NULL</entry>
    </row>
    <row>
      <entry>depth</entry>
      <entry>Parent's <emphasis remap='I'>depth</emphasis>; top-level widget gets root window depth.</entry>
    </row>
    <row>
      <entry>background_pixel</entry>
      <entry><function>XtDefaultBackground</function></entry>
    </row>
    <row>
      <entry>background_pixmap</entry>
      <entry><function>XtUnspecifiedPixmap</function></entry>
    </row>
    <row>
      <entry>visible</entry>
      <entry><function>True</function></entry>
    </row>
    <row>
      <entry>mapped_when_managed</entry>
      <entry><function>True</function></entry>
    </row>
  </tbody>
  </tgroup>
</informaltable>
<para>
<function>XtUnspecifiedPixmap</function>
is a symbolic constant guaranteed to be unequal to
any valid Pixmap id,
<function>None</function>,
and
<function>ParentRelative</function>.
</para>
</sect3>
</sect2>

<sect2 id="Composite_Widgets">
<title>Composite Widgets</title>
<para>
The Composite
widget class is a subclass of the
Core
widget class (see <xref linkend='Composite_Widgets_and_Their_Children' />).
Composite widgets are intended to be containers for other widgets.
The additional data used by composite widgets are defined by the
<function>CompositeClassPart</function>
and
<function>CompositePart</function>
structures.
</para>
<sect3 id="CompositeClassPart_Structure">
<title>CompositeClassPart Structure</title>
<para>
In addition to the
Core
class fields,
widgets of the Composite class have the following class fields.
</para>
<literallayout>
typedef struct {
     XtGeometryHandler geometry_manager;  See <xref linkend='Geometry_Management' xrefstyle='select: label' />
     XtWidgetProc change_managed;   See <xref linkend='Composite_Widgets_and_Their_Children' xrefstyle='select: label' />
     XtWidgetProc insert_child;     See <xref linkend='Composite_Widgets_and_Their_Children' xrefstyle='select: label' />
     XtWidgetProc delete_child;     See <xref linkend='Composite_Widgets_and_Their_Children' xrefstyle='select: label' />
     XtPointer extension;           See <xref linkend='Widget_Classing' xrefstyle='select: label' />
} CompositeClassPart;
</literallayout>
<para>
The extension record defined for
<function>CompositeClassPart</function>
with <emphasis remap='I'>record_type</emphasis>
equal to
<emphasis role='strong'>NULLQUARK</emphasis>
is
<function>CompositeClassExtensionRec</function>.
</para>
<literallayout>
typedef struct {
     XtPointer next_extension;  See <xref linkend='Class_Extension_Records' xrefstyle='select: label' />
     XrmQuark record_type;      See <xref linkend='Class_Extension_Records' xrefstyle='select: label' />
     long version;              See <xref linkend='Class_Extension_Records' xrefstyle='select: label' />
     Cardinal record_size;      See <xref linkend='Class_Extension_Records' xrefstyle='select: label' />
     Boolean accepts_objects;   See <xref linkend='Creating_a_Widget_Instance' xrefstyle='select: label' />
     Boolean allows_change_managed_set;  See <xref linkend='Bundling_Changes_to_the_Managed_Set' xrefstyle='select: label' />
} CompositeClassExtensionRec, *CompositeClassExtension;
</literallayout>
<para>
Composite
classes have the Composite class fields immediately following the
Core class fields.
</para>
<literallayout>
typedef struct {
     CoreClassPart core_class;
     CompositeClassPart composite_class;
} CompositeClassRec, *CompositeWidgetClass;
</literallayout>
<para>
The single occurrences of the class record and pointer for creating
instances of Composite are
</para>

<para>
In
<function>IntrinsicP.h</function>:
</para>
<literallayout>
extern CompositeClassRec compositeClassRec;
</literallayout>
<para>
In
<function>Intrinsic.h</function>:
</para>
<literallayout>
extern WidgetClass compositeWidgetClass;
</literallayout>
<para>
The opaque types
<function>CompositeWidget</function>
and
<function>CompositeWidgetClass</function>
and the opaque variable
<function>compositeWidgetClass</function>
are defined for generic operations on widgets whose class
is Composite or a subclass of Composite.
The symbolic constant for the
<function>CompositeClassExtension</function>
version identifier is
<function>XtCompositeExtensionVersion</function>
(see <xref linkend='Class_Extension_Records' />).
<function>Intrinsic.h</function>
uses an incomplete structure
definition to ensure that the compiler catches attempts to access
private data.
</para>
<literallayout>
typedef struct _CompositeClassRec *CompositeWidgetClass;
</literallayout>
</sect3>
<sect3 id="CompositePart_Structure">
<title>CompositePart Structure</title>
<para>
In addition to the
Core instance
fields,
widgets of the Composite class have the following
instance fields defined in the
<function>CompositePart</function>
structure.
</para>
<literallayout>
typedef struct {
     WidgetList children;   See <xref linkend='Composite_Widgets_and_Their_Children' xrefstyle='select: label' />
     Cardinal num_children; See <xref linkend='Composite_Widgets_and_Their_Children' xrefstyle='select: label' />
     Cardinal num_slots;    See <xref linkend='Composite_Widgets_and_Their_Children' xrefstyle='select: label' />
     XtOrderProc insert_position;  See <xref linkend='Insertion_Order_of_Children_The_insert_position_Procedure' xrefstyle='select: label' />
} CompositePart;
</literallayout>
<para>
Composite
widgets have the Composite instance fields immediately following the Core
instance fields.
</para>
<literallayout>
typedef struct {
     CorePart core;
     CompositePart composite;
} CompositeRec, *CompositeWidget;
</literallayout>
<para>
<function>Intrinsic.h</function>
uses an incomplete structure definition to ensure that the
compiler catches attempts to access private data.
</para>
<literallayout>
typedef struct _CompositeRec *CompositeWidget;
</literallayout>
</sect3>
<sect3 id="Composite_Resources">
<title>Composite Resources</title>
<para>
The resource names, classes, and representation types
that are specified in
the
<function>compositeClassRec</function>
resource list are
</para>

</sect3>
<sect3 id="CompositePart_Default_Values">
<title>CompositePart Default Values</title>
<para>
The default values for the Composite fields,
which are filled in from the
Composite
resource list and by the
Composite
initialize procedure, are
</para>

<informaltable frame='topbot'>
  <?dbfo keep-together="always" ?>
  <?dbfo table-width="50%" ?>
  <tgroup cols='2' align='left' colsep='0' rowsep='0'>
  <colspec colname='c1' colwidth='1.0*' colsep='0'/>
  <colspec colname='c2' colwidth='1.0*' colsep='0'/>
  <thead>
    <row rowsep='1'>
      <entry>Field</entry>
      <entry>Default Value</entry>
    </row>
  </thead>
  <tbody>
    <row>
      <entry>children</entry><entry>NULL</entry>
    </row>
    <row>
      <entry>num_children</entry><entry>0</entry>
    </row>
    <row>
      <entry>num_slots</entry><entry>0</entry>
    </row>
    <row>
      <entry>insert_position</entry><entry>Internal function to insert at end</entry>
    </row>
  </tbody>
  </tgroup>
</informaltable>

<para>
The <emphasis remap='I'>children</emphasis>, <emphasis remap='I'>num_children</emphasis>,
and <emphasis remap='I'>insert_position</emphasis> fields are declared
as resources;
XtNinsertPosition
is a settable resource,
XtNchildren
and
XtNnumChildren
may be read by any client but should only be modified by the composite
widget class procedures.
</para>
</sect3>
</sect2>

<sect2 id="Constraint_Widgets">
<title>Constraint Widgets</title>
<para>
The Constraint
widget class is a subclass of the
Composite
widget class (see <xref linkend='Constrained_Composite_Widgets' />).  Constraint
widgets maintain additional state
data for each child; for example, client-defined constraints on the child's
geometry.
The additional data used by constraint widgets are defined by the
<function>ConstraintClassPart</function>
and
<function>ConstraintPart</function>
structures.
</para>
<sect3 id="ConstraintClassPart_Structure">
<title>ConstraintClassPart Structure</title>
<para>
In addition to the
Core
and
Composite
class fields,
widgets of the Constraint class
have the following class fields.
</para>
<literallayout>
typedef struct {
     XtResourceList resources;   See <xref linkend='Resource_Management' xrefstyle='select: label' />
     Cardinal num_resources;     See <xref linkend='Resource_Management' xrefstyle='select: label' />
     Cardinal constraint_size;   See <xref linkend='Constrained_Composite_Widgets' xrefstyle='select: label' />
     XtInitProc initialize;      See <xref linkend='Constrained_Composite_Widgets' xrefstyle='select: label' />
     XtWidgetProc destroy;       See <xref linkend='Constrained_Composite_Widgets' xrefstyle='select: label' />
     XtSetValuesFunc set_values; See <xref linkend='Setting_Widget_State' xrefstyle='select: label' />
     XtPointer extension;        See <xref linkend='Widget_Classing' xrefstyle='select: label' />
} ConstraintClassPart;
</literallayout>
<para>
The extension record defined for
<function>ConstraintClassPart</function>
with <emphasis remap='I'>record_type</emphasis> equal to
<emphasis role='strong'>NULLQUARK</emphasis>
is
<function>ConstraintClassExtensionRec</function>.
</para>
<literallayout>
typedef struct {
     XtPointer next_extension;  See <xref linkend='Class_Extension_Records' xrefstyle='select: label' />
     XrmQuark record_type;      See <xref linkend='Class_Extension_Records' xrefstyle='select: label' />
     long version;              See <xref linkend='Class_Extension_Records' xrefstyle='select: label' />
     Cardinal record_size;      See <xref linkend='Class_Extension_Records' xrefstyle='select: label' />
     XtArgsProc get_values_hook;  See <xref linkend='Obtaining_Widget_State' xrefstyle='select: label' />
} ConstraintClassExtensionRec, *ConstraintClassExtension;
</literallayout>
<para>
Constraint
classes have the Constraint class fields immediately following the
Composite class fields.
</para>
<literallayout>
typedef struct _ConstraintClassRec {
     CoreClassPart core_class;
     CompositeClassPart composite_class;
     ConstraintClassPart constraint_class;
} ConstraintClassRec, *ConstraintWidgetClass;
</literallayout>
<para>
The single occurrences of the class record and pointer for creating
instances of Constraint are
</para>

<para>
In
<function>IntrinsicP.h</function>:
</para>
<literallayout>
extern ConstraintClassRec constraintClassRec;
</literallayout>
<para>
In
<function>Intrinsic.h</function>:
</para>
<literallayout>
extern WidgetClass constraintWidgetClass;
</literallayout>
<para>
The opaque types
<function>ConstraintWidget</function>
and
<function>ConstraintWidgetClass</function>
and the opaque variable
<function>constraintWidgetClass</function>
are defined for generic operations on widgets
whose class is Constraint or a subclass
of Constraint.
The symbolic constant for the
<function>ConstraintClassExtension</function>
version identifier is
<function>XtConstraintExtensionVersion</function>
(see <xref linkend='Class_Extension_Records' />).
<function>Intrinsic.h</function>
uses an incomplete structure definition to ensure that the
compiler catches attempts to access private data.
</para>
<literallayout>
typedef struct _ConstraintClassRec *ConstraintWidgetClass;
</literallayout>
</sect3>
<sect3 id="ConstraintPart_Structure">
<title>ConstraintPart Structure</title>
<para>
In addition to the
Core
and
Composite instance
fields,
widgets of the Constraint class have the following unused
instance fields defined in the
<function>ConstraintPart</function>
structure
</para>
<literallayout>
typedef struct {
        int empty;
} ConstraintPart;
</literallayout>
<para>
Constraint
widgets have the Constraint instance fields immediately following the
Composite instance fields.
</para>
<literallayout>
typedef struct {
     CorePart core;
     CompositePart composite;
     ConstraintPart constraint;
} ConstraintRec, *ConstraintWidget;
</literallayout>
<para>
<function>Intrinsic.h</function>
uses an incomplete structure definition to ensure that the
compiler catches attempts to access private data.
</para>
<literallayout>
typedef struct _ConstraintRec *ConstraintWidget;
</literallayout>
</sect3>
<sect3 id="Constraint_Resources">
<title>Constraint Resources</title>
<para>
The
<function>constraintClassRec</function>
<emphasis remap='I'>core_class</emphasis> and <emphasis remap='I'>constraint_class resources</emphasis> fields are NULL,
and the <emphasis remap='I'>num_resources</emphasis> fields are zero;
no additional resources beyond those declared by
the superclasses
are defined for
Constraint.
</para>
</sect3>
</sect2>
</sect1>

<sect1 id="Implementation_Specific_Types">
<title>Implementation-Specific Types</title>
<para>
To increase the portability of widget and application source code
between different system environments, the Intrinsics define several
types whose precise representation is explicitly dependent upon,
and chosen by, each individual implementation of the Intrinsics.
</para>

<para>
These implementation-defined types are
</para>
<variablelist>
  <varlistentry>
    <term>
     <emphasis role='strong'>Boolean</emphasis>
    </term>
    <listitem>
      <para>
A datum that contains a zero or nonzero value.
Unless explicitly stated, clients should not assume
that the nonzero value is equal to the symbolic
value
<function>True</function>.
      </para>
    </listitem>
  </varlistentry>

<varlistentry>
    <term>
     <emphasis role='strong'>Cardinal</emphasis>
    </term>
    <listitem>
      <para>
An unsigned integer datum with a minimum range of [0..2^16-1].
      </para>
    </listitem>
  </varlistentry>

<varlistentry>
    <term>
      <emphasis role='strong'>Dimension</emphasis>
    </term>
    <listitem>
      <para>
An unsigned integer datum with a minimum range of [0..2^16-1].
      </para>
    </listitem>
  </varlistentry>

<varlistentry>
    <term>
     <emphasis role='strong'>Position</emphasis>
    </term>
    <listitem>
      <para>
A signed integer datum with a minimum range of [-2^15..2^15-1].
      </para>
    </listitem>
  </varlistentry>

<varlistentry>
    <term>
      <emphasis role='strong'>XtPointer</emphasis>
    </term>
    <listitem>
      <para>
A datum large enough to contain the largest of a char*, int*, function
pointer, structure pointer, or long value.  A pointer
to any type or function, or a long value may be converted
to an
<function>XtPointer</function>
and back again and the result will
compare equal to the original value.  In ANSI C
environments it is expected that
<function>XtPointer</function>
will be
defined as void*.
      </para>
    </listitem>
  </varlistentry>

<varlistentry>
    <term>
      <emphasis role='strong'>XtArgVal</emphasis>
    </term>
    <listitem>
      <para>
A datum large enough to contain an
<function>XtPointer</function>,
<function>Cardinal</function>,
<function>Dimension</function>,
or
<function>Position</function>
value.
      </para>
    </listitem>
  </varlistentry>

<varlistentry>
    <term>
      <emphasis role='strong'>XtEnum</emphasis>
    </term>
    <listitem>
      <para>
An integer datum large enough to encode at least 128 distinct
values, two of which are the symbolic values
<function>True</function>
and
<function>False</function>.
The symbolic values
<emphasis role='strong'>TRUE</emphasis>
and
<emphasis role='strong'>FALSE</emphasis>
are
also defined to be equal to
<function>True</function>
and
<function>False</function>,
respectively.
      </para>
    </listitem>
  </varlistentry>
</variablelist>

<para>
In addition to these specific types, the precise order of the
fields within the structure declarations for any of the instance
part records
<function>ObjectPart</function>,
<function>RectObjPart</function>,
<function>CorePart</function>,
<function>CompositePart</function>,
<function>ShellPart</function>,
<function>WMShellPart</function>,
<function>TopLevelShellPart</function>,
and
<function>ApplicationShellPart</function>
is implementation-defined.  These
structures may also have additional private
fields internal to the implementation.
The
<function>ObjectPart</function>,
<function>RectObjPart</function>,
and
<function>CorePart</function>
structures must be defined so that any member with the same name
appears at the same offset in
<function>ObjectRec</function>,
<function>RectObjRec</function>,
and
<function>CoreRec</function>
<function>( WidgetRec ).</function>
No other relations between the offsets of any two
fields may be assumed.
</para>
</sect1>

<sect1 id="Widget_Classing">
<title>Widget Classing</title>
<para>
The <emphasis remap='I'>widget_class</emphasis> field of a widget points to its widget class structure,
which contains information that is constant across all widgets of that class.
As a consequence,
widgets usually do not implement directly callable procedures;
rather, they implement procedures, called methods, that are available through
their widget class structure.
These methods are invoked by generic procedures that envelop common actions
around the methods implemented by the widget class.
Such procedures are applicable to all widgets
of that class and also to widgets whose classes are subclasses of that class.
</para>

<para>
All widget classes are a subclass of
Core
and can be subclassed further.
Subclassing reduces the amount of code and declarations
necessary to make a
new widget class that is similar to an existing class.
For example, you do not have to describe every resource your widget uses in an
<function>XtResourceList</function>.
Instead, you describe only the resources your widget has
that its superclass does not.
Subclasses usually inherit many of their superclasses' procedures
(for example, the expose procedure or geometry handler).
</para>

<para>
Subclassing, however, can be taken too far.
If you create a subclass that inherits none of the procedures of its
superclass,
you should consider whether you have chosen the most
appropriate superclass.
</para>

<para>
To make good use of subclassing,
widget declarations and naming conventions are highly stylized.
A widget consists of three files:
</para>
<itemizedlist spacing='compact'>
  <listitem>
    <para>
A public .h file, used by client widgets or applications.
    </para>
  </listitem>
  <listitem>
    <para>
A private .h file, used by widgets whose classes
are subclasses of the widget class.
    </para>
  </listitem>
  <listitem>
    <para>
A .c file, which implements the widget.
    </para>
  </listitem>
</itemizedlist>
<sect2 id="Widget_Naming_Conventions">
<title>Widget Naming Conventions</title>
<para>
The Intrinsics provide a vehicle by which programmers can create
new widgets and organize a collection of widgets into an application.
To ensure that applications need not deal with as many styles of capitalization
and spelling as the number of widget classes it uses,
the following guidelines should be followed when writing new widgets:
</para>
<itemizedlist spacing='compact'>
  <listitem>
    <para>
Use the X library naming conventions that are applicable.
For example, a record component name is all lowercase
and uses underscores (_) for compound words (for example, background_pixmap).
Type and procedure names start with uppercase and use capitalization for
compound words (for example,
<function>ArgList</function>
or
<function>XtSetValues ).</function>
    </para>
  </listitem>
  <listitem>
    <para>
A resource name is spelled identically to the field name
except that compound names use capitalization rather than underscore.
To let the compiler catch spelling errors,
each resource name should have a symbolic identifier prefixed with
``XtN''.
For example,
the <emphasis remap='I'>background_pixmap</emphasis> field has the corresponding identifier
XtNbackgroundPixmap,
which is defined as the string ``backgroundPixmap''.
Many predefined names are listed in
<function>&lt;X11/StringDefs.h&gt;</function>.
Before you invent a new name,
you should make sure there is not already a name that you can use.
    </para>
  </listitem>
  <listitem>
    <para>
A resource class string starts with a capital letter
and uses capitalization for compound names (for example,``BorderWidth'').
Each resource class string should have a symbolic identifier prefixed with
``XtC''
(for example, XtCBorderWidth).
Many predefined classes are listed in
<function>&lt;X11/StringDefs.h&gt;</function>.
    </para>
  </listitem>
  <listitem>
    <para>
A resource representation string is spelled identically to the type name
(for example, ``TranslationTable'').
Each representation string should have a symbolic identifier prefixed with
``XtR''
(for example, XtRTranslationTable).
Many predefined representation types are listed in
<function>&lt;X11/StringDefs.h&gt;</function>.
    </para>
  </listitem>
  <listitem>
    <para>
New widget classes start with a capital and use uppercase for compound
words.
Given a new class name AbcXyz, you should derive several names:
    </para>
  </listitem>
  <listitem>
    <itemizedlist spacing='compact'>
      <listitem>
        <para>
Additional widget instance structure part name AbcXyzPart.
        </para>
      </listitem>
      <listitem>
        <para>
Complete widget instance structure names AbcXyzRec and _AbcXyzRec.
        </para>
      </listitem>
      <listitem>
        <para>
Widget instance structure pointer type name AbcXyzWidget.
        </para>
      </listitem>
      <listitem>
        <para>
Additional class structure part name AbcXyzClassPart.
        </para>
      </listitem>
      <listitem>
        <para>
Complete class structure names AbcXyzClassRec and _AbcXyzClassRec.
        </para>
      </listitem>
      <listitem>
        <para>
Class structure pointer type name AbcXyzWidgetClass.
        </para>
      </listitem>
      <listitem>
        <para>
Class structure variable abcXyzClassRec.
        </para>
      </listitem>
      <listitem>
        <para>
Class structure pointer variable abcXyzWidgetClass.
        </para>
      </listitem>
    </itemizedlist>
  </listitem>
  <listitem>
    <para>
Action procedures available to translation specifications should follow the
same naming conventions as procedures.
That is,
they start with a capital letter, and compound names use uppercase
(for example, ``Highlight'' and ``NotifyClient'').
    </para>
  </listitem>
</itemizedlist>
<para>
The symbolic identifiers XtN..., XtC..., and XtR...
may be implemented
as macros, as global symbols, or as a mixture of the two.  The
(implicit) type of the identifier is
<function>String</function>.
The pointer value itself
is not significant; clients must not assume that inequality of two
identifiers implies inequality of the resource name, class, or
representation string.  Clients should also note that although global
symbols permit savings in literal storage in some environments, they
also introduce the possibility of multiple definition conflicts when
applications attempt to use independently developed widgets
simultaneously.
</para>
</sect2>

<sect2 id="Widget_Subclassing_in_Public_h_Files">
<title>Widget Subclassing in Public .h Files</title>
<para>
The public .h file for a widget class is imported by clients
and contains
</para>
<itemizedlist spacing='compact'>
  <listitem>
    <para>
A reference to the public .h file for the superclass.
    </para>
  </listitem>
  <listitem>
    <para>
Symbolic identifiers for
the names and classes of the new resources that this widget adds
to its superclass.
The definitions should
have a single space between the definition name and the value and no
trailing space or comment in order to reduce the possibility of
compiler warnings from similar declarations in multiple classes.
    </para>
  </listitem>
  <listitem>
    <para>
Type declarations for any new resource data types defined by the class.
    </para>
  </listitem>
  <listitem>
    <para>
The class record pointer variable used to create widget instances.
    </para>
  </listitem>
  <listitem>
    <para>
The C type that corresponds to widget instances of this class.
    </para>
  </listitem>
  <listitem>
    <para>
Entry points for new class methods.
    </para>
  </listitem>
</itemizedlist>
<para>
For example, the following is the public .h file for a possible
implementation of a Label widget:
</para>
<literallayout>
#ifndef LABEL_H
#define LABEL_H
/* New resources */
#define XtNjustify "justify"
#define XtNforeground "foreground"
#define XtNlabel "label"
#define XtNfont "font"
#define XtNinternalWidth "internalWidth"
#define XtNinternalHeight "internalHeight"
/* Class record pointer */
extern WidgetClass labelWidgetClass;
/* C Widget type definition */
typedef struct _LabelRec      *LabelWidget;
/* New class method entry points */
extern void LabelSetText();
     /* Widget w */
     /* String text */
extern String LabelGetText();
     /* Widget w */
#endif LABEL_H
</literallayout>
<para>
The conditional inclusion of the text allows the application
to include header files for different widgets without being concerned
that they already may be included as a superclass of another widget.
</para>

<para>
To accommodate operating systems with file name length restrictions,
the name of the public .h file is the first ten characters of the
widget class.
For example,
the public .h file for the
Constraint
widget class is
<function>Constraint.h</function>.
</para>
</sect2>

<sect2 id="Widget_Subclassing_in_Private_h_Files">
<title>Widget Subclassing in Private .h Files</title>
<para>
The private .h file for a widget is imported by widget classes that are
subclasses of the widget and contains
</para>
<itemizedlist spacing='compact'>
  <listitem>
    <para>
A reference to the public .h file for the class.
    </para>
  </listitem>
  <listitem>
    <para>
A reference to the private .h file for the superclass.
    </para>
  </listitem>
  <listitem>
    <para>
Symbolic identifiers for any new resource representation types defined
by the class.  The definitions should have a single space between the
definition name and the value and no trailing space or comment.
    </para>
  </listitem>
  <listitem>
    <para>
A structure part definition for
the new fields that the widget instance adds to its superclass's
widget structure.
    </para>
  </listitem>
  <listitem>
    <para>
The complete widget instance structure definition for this widget.
    </para>
  </listitem>
  <listitem>
    <para>
A structure part definition for
the new fields that this widget class adds to its superclass's
constraint
structure if the widget class is a subclass of
Constraint.
    </para>
  </listitem>
  <listitem>
    <para>
The complete
constraint
structure definition if the widget class is a subclass of
Constraint.
    </para>
  </listitem>
  <listitem>
    <para>
Type definitions for any new procedure types used by class methods
declared in the widget class part.
    </para>
  </listitem>
  <listitem>
    <para>
A structure part definition for
the new fields that this widget class adds to its superclass's widget class
structure.
    </para>
  </listitem>
  <listitem>
    <para>
The complete widget class structure definition for this widget.
    </para>
  </listitem>
  <listitem>
    <para>
The complete widget class extension structure definition
for this widget, if any.
    </para>
  </listitem>
  <listitem>
    <para>
The symbolic constant identifying the class extension version, if any.
    </para>
  </listitem>
  <listitem>
    <para>
The name of the global class structure variable containing the generic
class structure for this class.
    </para>
  </listitem>
  <listitem>
    <para>
An inherit constant for each new procedure in the widget class part structure.
    </para>
  </listitem>
</itemizedlist>
<para>
For example, the following is the private .h file for a possible Label widget:
</para>
<literallayout>
#ifndef LABELP_H
#define LABELP_H
#include &lt;X11/Label.h&gt;
/* New representation types used by the Label widget */
#define XtRJustify "Justify"
/* New fields for the Label widget record */
typedef struct {
/* Settable resources */
     Pixel foreground;
     XFontStruct *font;
     String label;     /* text to display */
     XtJustify justify;
     Dimension internal_width;     /* # pixels horizontal border */
     Dimension internal_height;     /* # pixels vertical border */
/* Data derived from resources */
     GC normal_GC;
     GC gray_GC;
     Pixmap gray_pixmap;
     Position label_x;
     Position label_y;
     Dimension label_width;
     Dimension label_height;
     Cardinal label_len;
     Boolean display_sensitive;
} LabelPart;
</literallayout>
<literallayout>
/* Full instance record declaration */
typedef struct _LabelRec {
     CorePart core;
     LabelPart label;
} LabelRec;
/* Types for Label class methods */
typedef void (*LabelSetTextProc)();
     /* Widget w */
     /* String text */
typedef String (*LabelGetTextProc)();
     /* Widget w */
/* New fields for the Label widget class record */
typedef struct {
     LabelSetTextProc set_text;
     LabelGetTextProc get_text;
     XtPointer extension;
} LabelClassPart;
/* Full class record declaration */
typedef struct _LabelClassRec {
     CoreClassPart core_class;
     LabelClassPart label_class;
} LabelClassRec;
/* Class record variable */
extern LabelClassRec labelClassRec;
#define LabelInheritSetText((LabelSetTextProc)_XtInherit)
#define LabelInheritGetText((LabelGetTextProc)_XtInherit)
#endif LABELP_H
</literallayout>
<para>
To accommodate operating systems with file name length restrictions,
the name of the private .h file is the first nine characters of the
widget class followed by a capital P.
For example,
the private .h file for the
Constraint
widget class is
<function>ConstrainP.h</function>.
</para>
</sect2>

<sect2 id="Widget_Subclassing_in_c_Files">
<title>Widget Subclassing in .c Files</title>
<para>
The .c file for a widget contains the structure initializer
for the class record variable,
which contains the following parts:
</para>
<itemizedlist spacing='compact'>
  <listitem>
    <para>
Class information (for example, <emphasis remap='I'>superclass</emphasis>, <emphasis remap='I'>class_name</emphasis>,
<emphasis remap='I'>widget_size</emphasis>,
<emphasis remap='I'>class_initialize</emphasis>, and <emphasis remap='I'>class_inited</emphasis>).
    </para>
  </listitem>
  <listitem>
    <para>
Data constants (for example, <emphasis remap='I'>resources</emphasis> and <emphasis remap='I'>num_resources</emphasis>,
<emphasis remap='I'>actions</emphasis> and <emphasis remap='I'>num_actions</emphasis>, <emphasis remap='I'>visible_interest</emphasis>,
<emphasis remap='I'>compress_motion</emphasis>,
<emphasis remap='I'>compress_exposure</emphasis>, and <emphasis remap='I'>version</emphasis>).
    </para>
  </listitem>
  <listitem>
    <para>
Widget operations (for example, <emphasis remap='I'>initialize</emphasis>, <emphasis remap='I'>realize</emphasis>, <emphasis remap='I'>destroy</emphasis>,
<emphasis remap='I'>resize</emphasis>, <emphasis remap='I'>expose</emphasis>, <emphasis remap='I'>set_values</emphasis>, <emphasis remap='I'>accept_focus</emphasis>,
and any new operations specific to
the widget).
    </para>
  </listitem>
</itemizedlist>
<para>
The <emphasis remap='I'>superclass</emphasis> field points to the superclass
global class
record, declared in the superclass private .h file.
For direct subclasses of the generic core widget,
<emphasis remap='I'>superclass</emphasis> should be initialized to the address of the
<function>widgetClassRec</function>
structure.
The superclass is used for class chaining operations and for
inheriting or enveloping a superclass's operations
(see <xref linkend='Superclass_Chaining' />,
<xref linkend='Initializing_a_Widget_Class' />, and 
<xref linkend='Inheritance_of_Superclass_Operations' />.
</para>

<para>
The <emphasis remap='I'>class_name</emphasis> field contains the text name for this class,
which is used by
the resource manager.
For example, the Label widget has the string ``Label''.
More than one widget class can share the same text class name.
This string must be permanently allocated prior to or during the
execution of the class initialization procedure and must not be
subsequently deallocated.
</para>

<para>
The <emphasis remap='I'>widget_size</emphasis> field is the size of the corresponding widget
instance structure
(not the size of the class structure).
</para>

<para>
The <emphasis remap='I'>version</emphasis> field indicates the toolkit
implementation version number and is used for
runtime consistency checking of the X Toolkit and widgets in an application.
Widget writers must set it to the
implementation-defined symbolic value
<function>XtVersion</function>
in the widget class structure initialization.
Those widget writers who believe that their widget binaries are compatible
with other implementations of the Intrinsics can put the special value
<function>XtVersionDontCheck</function>
in the <emphasis remap='I'>version</emphasis> field to disable version checking for those widgets.
If a widget needs to compile alternative code for different
revisions of the Intrinsics interface definition, it may use the symbol
<function>XtSpecificationRelease</function>,
as described in <xref linkend='Evolution_of_the_Intrinsics' />.
Use of
<function>XtVersion</function>
allows the Intrinsics implementation to recognize widget binaries
that were compiled with older implementations.
</para>

<para>
The <emphasis remap='I'>extension</emphasis> field is for future upward compatibility.
If the widget programmer adds fields to class parts,
all subclass structure layouts change,
requiring complete recompilation.
To allow clients to avoid recompilation,
an extension field at the end of each class part can point to a record
that contains any additional class information required.
</para>

<para>
All other fields are described in their respective sections.
</para>

<para>
The .c file also contains the declaration of the global class
structure pointer variable used to create instances of the class.
The following is an abbreviated version of the .c file
for a Label widget.
The resources table is described in <xref linkend='Resource_Management' />.
</para>
<literallayout>
/* Resources specific to Label */
static XtResource resources[] = {
     {XtNforeground, XtCForeground, XtRPixel, sizeof(Pixel),
        XtOffset(LabelWidget, label.foreground), XtRString,
        XtDefaultForeground},
     {XtNfont, XtCFont, XtRFontStruct, sizeof(XFontStruct *),
        XtOffset(LabelWidget, label.font),XtRString,
        XtDefaultFont},
     {XtNlabel, XtCLabel, XtRString, sizeof(String),
        XtOffset(LabelWidget, label.label), XtRString, NULL},
          .
          .
          .
}
/* Forward declarations of procedures */
static void ClassInitialize();
static void Initialize();
static void Realize();
static void SetText();
static void GetText();
     .
     .
     .
</literallayout>
<literallayout>
/* Class record constant */
LabelClassRec labelClassRec = {
  {
    /* core_class fields */
     /* superclass     */     (WidgetClass)&amp;coreClassRec,
     /* class_name     */     "Label",
     /* widget_size     */     sizeof(LabelRec),
     /* class_initialize     */     ClassInitialize,
     /* class_part_initialize     */     NULL,
     /* class_inited     */     False,
     /* initialize     */     Initialize,
     /* initialize_hook     */     NULL,
     /* realize     */     Realize,
     /* actions     */     NULL,
     /* num_actions     */     0,
     /* resources     */     resources,
     /* num_resources     */     XtNumber(resources),
     /* xrm_class     */     NULLQUARK,
     /* compress_motion     */     True,
     /* compress_exposure     */     True,
     /* compress_enterleave     */     True,
     /* visible_interest     */     False,
     /* destroy     */     NULL,
     /* resize     */     Resize,
     /* expose     */     Redisplay,
     /* set_values     */     SetValues,
     /* set_values_hook     */     NULL,
     /* set_values_almost     */     XtInheritSetValuesAlmost,
     /* get_values_hook     */     NULL,
     /* accept_focus     */     NULL,
     /* version     */     XtVersion,
     /* callback_offsets     */     NULL,
     /* tm_table     */     NULL,
     /* query_geometry     */     XtInheritQueryGeometry,
     /* display_accelerator     */     NULL,
     /* extension     */     NULL
  },
  {
    /* Label_class fields     */
     /* get_text     */     GetText,
     /* set_text     */     SetText,
     /* extension     */     NULL
  }
};
/* Class record pointer */
WidgetClass labelWidgetClass = (WidgetClass) &amp;labelClassRec;
/* New method access routines */
void LabelSetText(w, text)
     Widget w;
     String text;
{
     LabelWidgetClass lwc = (Label WidgetClass)XtClass(w);
     XtCheckSubclass(w, labelWidgetClass, NULL);
     *(lwc-&gt;label_class.set_text)(w, text)
}
/* Private procedures */
     .
     .
     .
</literallayout>
</sect2>

<sect2 id="Widget_Class_and_Superclass_Look_Up">
<title>Widget Class and Superclass Look Up</title>
<para>
To obtain the class of a widget, use
<xref linkend='XtClass' xrefstyle='select: title'/>.
</para>

<funcsynopsis id='XtClass'>
  <funcprototype>
    <funcdef>WidgetClass <function>XtClass</function></funcdef>
    <paramdef>Widget<parameter> w</parameter></paramdef>
  </funcprototype>
  </funcsynopsis>

<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>w</emphasis>
    </term>
    <listitem>
      <para>
Specifies the widget. Must be of class Object or any subclass thereof.
    </para>
  </listitem>
  </varlistentry>
</variablelist>

<para>
The
<xref linkend='XtClass' xrefstyle='select: title'/>
function returns a pointer to the widget's class structure.
</para>

<para>
To obtain the superclass of a widget, use
<function>XtSuperclass</function>.
</para>

<funcsynopsis id='XtSuperClass'>
<funcprototype>
  <funcdef>WidgetClass <function>XtSuperClass</function></funcdef>
  <paramdef>Widget<parameter> w</parameter></paramdef>
</funcprototype>
</funcsynopsis>

<para>
The
<function>XtSuperclass</function>
function returns a pointer to the widget's superclass class structure.
</para>
</sect2>

<sect2 id="Widget_Subclass_Verification">
<title>Widget Subclass Verification</title>
<para>
To check the subclass to which a widget belongs, use
<xref linkend='XtIsSubclass' xrefstyle='select: title'/>.
</para>

<funcsynopsis id='XtIsSubclass'>
<funcprototype>
  <funcdef>Boolean <function>XtIsSubclass</function></funcdef>
  <paramdef>Widget<parameter> w</parameter></paramdef>
  <paramdef>WidgetClass<parameter> widget_class</parameter></paramdef>
</funcprototype>
</funcsynopsis>

<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>w</emphasis>
    </term>
    <listitem>
      <para>
Specifies the widget or object instance whose class is to be checked.  Must be of class Object or any subclass thereof.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>widget_class</emphasis>
    </term>
    <listitem>
      <para>
Specifies the widget class for which to test. Must be <emphasis role='strong'>objectClass</emphasis> or any subclass thereof.
    </para>
  </listitem>
  </varlistentry>
</variablelist>

<para>
The
<xref linkend='XtIsSubclass' xrefstyle='select: title'/>
function returns
<function>True</function>
if the class of the specified widget is equal to
or is a subclass of the specified class.
The widget's class can be any number of subclasses down the chain
and need not be an immediate subclass of the specified class.
Composite widgets that need to restrict the class of the items they
contain can use
<xref linkend='XtIsSubclass' xrefstyle='select: title'/>
to find out if a widget belongs to the desired class of objects.
</para>

<para>
To test if a given widget belongs to a subclass of an Intrinsics-defined
class, the Intrinsics define macros or functions equivalent to
<xref linkend='XtIsSubclass' xrefstyle='select: title'/>
for each of the built-in classes.  These procedures are
<function>XtIsObject</function>,
<function>XtIsRectObj</function>,
<function>XtIsWidget</function>,
<function>XtIsComposite</function>,
<function>XtIsConstraint</function>,
<function>XtIsShell</function>,
<function>XtIsOverrideShell</function>,
<function>XtIsWMShell</function>,
<function>XtIsVendorShell</function>,
<function>XtIsTransientShell</function>,
<function>XtIsTopLevelShell</function>,
<function>XtIsApplicationShell</function>,
and
<function>XtIsSessionShell</function>.
</para>

<para>
All these macros and functions have the same argument description.
</para>

<funcsynopsis id='XtIs'>
<funcprototype>
  <funcdef>Boolean <function>XtIs</function></funcdef>
  <paramdef>Widget<parameter> w</parameter></paramdef>
</funcprototype>
</funcsynopsis>

<para>
These procedures may be faster than calling
<xref linkend='XtIsSubclass' xrefstyle='select: title'/>
directly for the built-in classes.
</para>

<para>
To check a widget's class
and to generate a debugging error message, use
<xref linkend='XtCheckSubclass' xrefstyle='select: title'/>,
defined in
<function>&lt;X11/IntrinsicP.h&gt;</function>:
</para>

<funcsynopsis id='XtCheckSubclass'>
<funcprototype>
  <funcdef>void <function>XtCheckSubclass</function></funcdef>
  <paramdef>Widget<parameter> w</parameter></paramdef>
  <paramdef>WidgetClass<parameter> widget_class</parameter></paramdef>
  <paramdef>String<parameter> message</parameter></paramdef>
</funcprototype>
</funcsynopsis>

<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>w</emphasis>
    </term>
    <listitem>
      <para>
Specifies the widget or object whose class is to be checked.  Must be of class Object or any subclass thereof.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>widget_class</emphasis>
    </term>
    <listitem>
      <para>
Specifies the widget class for which to test.  Must be <emphasis role='strong'>objectClass</emphasis> or any subclass thereof.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>message</emphasis>
    </term>
    <listitem>
      <para>
Specifies the message to be used.
    </para>
  </listitem>
  </varlistentry>
</variablelist>

<para>
The
<xref linkend='XtCheckSubclass' xrefstyle='select: title'/>
macro determines if the class of the specified widget is equal to
or is a subclass of the specified class.
The widget's class can be any number of subclasses down the chain
and need not be an immediate subclass of the specified class.
If the specified widget's class is not a subclass,
<xref linkend='XtCheckSubclass' xrefstyle='select: title'/>
constructs an error message from the supplied message,
the widget's actual class, and the expected class and calls
<xref linkend='XtErrorMsg' xrefstyle='select: title'/>.
<xref linkend='XtCheckSubclass' xrefstyle='select: title'/>
should be used at the entry point of exported routines to ensure
that the client has passed in a valid widget class for the exported operation.
</para>

<para>
<xref linkend='XtCheckSubclass' xrefstyle='select: title'/>
is only executed when the module has been compiled with the compiler symbol
DEBUG defined; otherwise, it is defined as the empty string
and generates no code.
</para>
</sect2>

<sect2 id="Superclass_Chaining">
<title>Superclass Chaining</title>
<para>
While most fields in a widget class structure are self-contained,
some fields are linked to their corresponding fields in their superclass
structures.
With a linked field,
the Intrinsics access the field's value only after accessing its corresponding
superclass value (called downward superclass chaining) or
before accessing its corresponding superclass value (called upward superclass
chaining).  The self-contained fields are</para>
<literallayout>
In all widget classes:           <emphasis remap='I'>class_name</emphasis>
                                 <emphasis remap='I'>class_initialize</emphasis>
                                 <emphasis remap='I'>widget_size</emphasis>
                                 <emphasis remap='I'>realize</emphasis>
                                 <emphasis remap='I'>visible_interest</emphasis>
                                 <emphasis remap='I'>resize</emphasis>
                                 <emphasis remap='I'>expose</emphasis>
                                 <emphasis remap='I'>accept_focus</emphasis>
                                 <emphasis remap='I'>compress_motion</emphasis>
                                 <emphasis remap='I'>compress_exposure</emphasis>
                                 <emphasis remap='I'>compress_enterleave</emphasis>
                                 <emphasis remap='I'>set_values_almost</emphasis>
                                 <emphasis remap='I'>tm_table</emphasis>
                                 <emphasis remap='I'>version</emphasis>
                                 <emphasis remap='I'>allocate</emphasis>
                                 <emphasis remap='I'>deallocate</emphasis>

In Composite widget classes:     <emphasis remap='I'>geometry_manager</emphasis>
                                 <emphasis remap='I'>change_managed</emphasis>
                                 <emphasis remap='I'>insert_child</emphasis>
                                 <emphasis remap='I'>delete_child</emphasis>
                                 <emphasis remap='I'>accepts_objects</emphasis>
                                 <emphasis remap='I'>allows_change_managed_set</emphasis>

In Constraint widget classes:    <emphasis remap='I'>constraint_size</emphasis>

In Shell widget classes:         <emphasis remap='I'>root_geometry_manager</emphasis>
</literallayout>

<para>
With downward superclass chaining,
the invocation of an operation first accesses the field from the
Object,
RectObj,
and
Core
class structures, then from the subclass structure, and so on down the class chain to
that widget's class structure.  These superclass-to-subclass fields are
</para>
<literallayout>
         <emphasis remap='I'>class_part_initialize</emphasis>
         <emphasis remap='I'>get_values_hook</emphasis>
         <emphasis remap='I'>initialize</emphasis>
         <emphasis remap='I'>initialize_hook</emphasis>
         <emphasis remap='I'>set_values</emphasis>
         <emphasis remap='I'>set_values_hook</emphasis>
         <emphasis remap='I'>resources</emphasis>
</literallayout>

<para>
In addition, for subclasses of
Constraint,
the following fields of the
<function>ConstraintClassPart</function>
and
<function>ConstraintClassExtensionRec</function>
structures are chained from the
Constraint
class down to the subclass:
</para>
<literallayout>
     <emphasis remap='I'>resources</emphasis>
     <emphasis remap='I'>initialize</emphasis>
     <emphasis remap='I'>set_values</emphasis>
     <emphasis remap='I'>get_values_hook</emphasis>
</literallayout>

<para>
With upward superclass chaining,
the invocation of an operation first accesses the field from the widget
class structure, then from the superclass structure,
and so on up the class chain to the
Core,
RectObj,
and
Object
class structures.
The subclass-to-superclass fields are
</para>
<literallayout>
     <emphasis remap='I'>destroy</emphasis>
     <emphasis remap='I'>actions</emphasis>
</literallayout>

<para>
For subclasses of
Constraint,
the following field of
<function>ConstraintClassPart</function>
is chained from the subclass up to the
Constraint class:
     <emphasis remap='I'>destroy</emphasis>
</para>
</sect2>

<sect2 id="Class_Initialization_class_initialize_and_class_part_initialize_Procedures">
<title>Class Initialization: class_initialize and class_part_initialize Procedures</title>
<para>
Many class records can be initialized completely at compile or link time.
In some cases, however,
a class may need to register type converters or perform other sorts of
once-only runtime initialization.
</para>

<para>
Because the C language does not have initialization procedures
that are invoked automatically when a program starts up,
a widget class can declare a class_initialize procedure
that will be automatically called exactly once by the Intrinsics.
A class initialization procedure pointer is of type
<function>XtProc</function>:
</para>

<para>
typedef void (*XtProc)(void);
</para>
<para>
A widget class indicates that it has no class initialization procedure by
specifying NULL in the <emphasis remap='I'>class_initialize</emphasis> field.
</para>

<para>
In addition to the class initialization that is done exactly once,
some classes perform initialization for fields in their parts
of the class record.
These are performed not just for the particular class,
but for subclasses as well, and are
done in the class's class part initialization procedure,
a pointer to which is stored in the <emphasis remap='I'>class_part_initialize</emphasis> field.
The class_part_initialize procedure pointer is of type
<function>XtWidgetClassProc</function>.
</para>

<funcsynopsis>
<funcprototype>
  <funcdef>void <function>(*XtWidgetClassProc)(WidgetClass)</function></funcdef>
  <paramdef>WidgetClass<parameter> widget_class</parameter></paramdef>
</funcprototype>
</funcsynopsis>

<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>widget_class</emphasis>
    </term>
    <listitem>
      <para>
Points to the class structure for the class being initialized.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<para>
During class initialization,
the class part initialization procedures for the class and all its superclasses
are called in superclass-to-subclass order on the class record.
These procedures have the responsibility of doing any dynamic initializations
necessary to their class's part of the record.
The most common is the resolution of any inherited methods defined in the
class.
For example,
if a widget class C has superclasses
Core,
Composite,
A, and B, the class record for C first is passed to
Core 's
class_part_initialize procedure.
This resolves any inherited Core methods and compiles the textual
representations of the resource list and action table that are defined in the
class record.
Next, Composite's
class_part_initialize procedure is called to initialize the
composite part of C's class record.
Finally, the class_part_initialize procedures for A, B, and C, in that order,
are called.
For further information,
see <xref linkend='Initializing_a_Widget_Class' />
Classes that do not define any new class fields
or that need no extra processing for them can specify NULL
in the <emphasis remap='I'>class_part_initialize</emphasis> field.
</para>

<para>
All widget classes, whether they have a class initialization procedure or not,
must start with their <emphasis remap='I'>class_inited</emphasis> field
<function>False</function>.
</para>

<para>
The first time a widget of a class is created,
<xref linkend='XtCreateWidget' xrefstyle='select: title'/>
ensures that the widget class and all superclasses are initialized, in
superclass-to-subclass order, by checking each <emphasis remap='I'>class_inited</emphasis> field and,
if it is
<function>False</function>,
by calling the class_initialize and the class_part_initialize procedures
for the class and all its superclasses.
The Intrinsics then set the <emphasis remap='I'>class_inited</emphasis> field to a nonzero value.
After the one-time initialization,
a class structure is constant.
</para>

<para>
The following example provides the class initialization procedure for a Label class.
</para>
<literallayout>
static void ClassInitialize()
{
     XtSetTypeConverter(XtRString, XtRJustify, CvtStringToJustify,
          NULL, 0, XtCacheNone, NULL);
}
</literallayout>
</sect2>

<sect2 id="Initializing_a_Widget_Class">
<title>Initializing a Widget Class</title>
<para>
A class is initialized when the first widget of that class or any
subclass is created.
To initialize a widget class without creating any widgets, use
<xref linkend='XtInitializeWidgetClass' xrefstyle='select: title'/>.
</para>

<funcsynopsis id='XtInitializeWidgetClass'>
<funcprototype>
  <funcdef>void <function>XtInitializeWidgetClass</function></funcdef>
  <paramdef>WidgetClass<parameter> object_class</parameter></paramdef>
</funcprototype>
</funcsynopsis>

<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>object_class</emphasis>
    </term>
    <listitem>
      <para>
Specifies the object class to initialize.  May be
<function>objectClass</function>
or any subclass thereof.
    </para>
  </listitem>
  </varlistentry>
</variablelist>

<para>
If the specified widget class is already initialized,
<xref linkend='XtInitializeWidgetClass' xrefstyle='select: title'/>
returns immediately.
</para>

<para>
If the class initialization procedure registers type converters,
these type converters are not available until the first object
of the class or subclass is created or
<xref linkend='XtInitializeWidgetClass' xrefstyle='select: title'/>
is called
(see <xref linkend='Resource_Conversions' />).
</para>
</sect2>

<sect2 id="Inheritance_of_Superclass_Operations">
<title>Inheritance of Superclass Operations</title>
<para>
A widget class is free to use any of its superclass's self-contained
operations rather than implementing its own code.
The most frequently inherited operations are
</para>
<itemizedlist spacing='compact'>
  <listitem>
    <para>
expose
    </para>
  </listitem>
  <listitem>
    <para>
realize
    </para>
  </listitem>
  <listitem>
    <para>
insert_child
    </para>
  </listitem>
  <listitem>
    <para>
delete_child
    </para>
  </listitem>
  <listitem>
    <para>
geometry_manager
    </para>
  </listitem>
  <listitem>
    <para>
set_values_almost
    </para>
  </listitem>
</itemizedlist>

<para>
To inherit an operation <emphasis remap='I'>xyz</emphasis>,
specify the constant
<function>XtInherit</function> <emphasis remap='I'>Xyz</emphasis>
in your class record.
</para>

<para>
Every class that declares a new procedure in its widget class part must
provide for inheriting the procedure in its class_part_initialize
procedure.
The chained operations declared in Core
and Constraint
records are never inherited.
Widget classes that do nothing beyond what their superclass does
specify NULL for chained procedures
in their class records.
</para>

<para>
Inheriting works by comparing the value of the field with a known, special
value and by copying in the superclass's value for that field if a match
occurs.
This special value, called the inheritance constant,
is usually the Intrinsics internal value
<function>_XtInherit</function>
cast to the appropriate type.
<function>_XtInherit</function>
is a procedure that issues an error message if it is actually called.
</para>

<para>
For example,
<function>CompositeP.h</function>
contains these definitions:
</para>
<literallayout>
#define XtInheritGeometryManager ((XtGeometryHandler) _XtInherit)
#define XtInheritChangeManaged ((XtWidgetProc) _XtInherit)
#define XtInheritInsertChild ((XtArgsProc) _XtInherit)
#define XtInheritDeleteChild ((XtWidgetProc) _XtInherit)
</literallayout>
<para>
Composite's class_part_initialize procedure begins as follows:
</para>
<literallayout>
static void CompositeClassPartInitialize(widgetClass)
     WidgetClass widgetClass;
{
     CompositeWidgetClass wc = (CompositeWidgetClass)widgetClass;
     CompositeWidgetClass super = (CompositeWidgetClass)wc-&gt;core_class.superclass;
     if (wc-&gt;composite_class.geometry_manager == XtInheritGeometryManager) {
         wc-&gt;composite_class.geometry_manager = super-&gt;composite_class.geometry_manager;
     }
     if (wc-&gt;composite_class.change_managed == XtInheritChangeManaged) {
         wc-&gt;composite_class.change_managed = super-&gt;composite_class.change_managed;
     }
     .
     .
     .
</literallayout>
<para>
Nonprocedure fields may be inherited in the same manner as procedure
fields.  The class may declare any reserved value it wishes for
the inheritance constant for its new fields.  The following inheritance
constants are defined:
</para>

<para>
For Object:
</para>
<itemizedlist spacing='compact'>
  <listitem>
    <para>
<function>XtInheritAllocate</function>
    </para>
  </listitem>
  <listitem>
    <para>
<function>XtInheritDeallocate</function>
    </para>
  </listitem>
</itemizedlist>
<para>
For Core:
</para>
<itemizedlist spacing='compact'>
  <listitem>
    <para>
<function>XtInheritRealize</function>
    </para>
  </listitem>
  <listitem>
    <para>
<function>XtInheritResize</function>
    </para>
  </listitem>
  <listitem>
    <para>
<function>XtInheritExpose</function>
    </para>
  </listitem>
  <listitem>
    <para>
<function>XtInheritSetValuesAlmost</function>
    </para>
  </listitem>
  <listitem>
    <para>
<function>XtInheritAcceptFocus</function>
    </para>
  </listitem>
  <listitem>
    <para>
<function>XtInheritQueryGeometry</function>
    </para>
  </listitem>
  <listitem>
    <para>
<function>XtInheritTranslations</function>
    </para>
  </listitem>
  <listitem>
    <para>
<function>XtInheritDisplayAccelerator</function>
    </para>
  </listitem>
</itemizedlist>
<para>
For Composite:
</para>
<itemizedlist spacing='compact'>
  <listitem>
    <para>
<function>XtInheritGeometryManager</function>
    </para>
  </listitem>
  <listitem>
    <para>
<function>XtInheritChangeManaged</function>
    </para>
  </listitem>
  <listitem>
    <para>
<function>XtInheritInsertChild</function>
    </para>
  </listitem>
  <listitem>
    <para>
<function>XtInheritDeleteChild</function>
    </para>
  </listitem>
</itemizedlist>
<para>
For Shell:
</para>
<itemizedlist spacing='compact'>
  <listitem>
    <para>
<function>XtInheritRootGeometryManager</function>
    </para>
  </listitem>
</itemizedlist>
</sect2>

<sect2 id="Invocation_of_Superclass_Operations">
<title>Invocation of Superclass Operations</title>
<para>
A widget sometimes needs to call a superclass operation
that is not chained.
For example,
a widget's expose procedure might call its superclass's <emphasis remap='I'>expose</emphasis>
and then perform a little more work on its own.
For example, a Composite
class with predefined managed children can implement insert_child
by first calling its superclass's <emphasis remap='I'>insert_child</emphasis>
and then calling
<xref linkend='XtManageChild' xrefstyle='select: title'/>
to add the child to the managed set.
</para>

<note>
<para>
A class method should not use
<function>XtSuperclass</function>
but should instead call the class method of its own specific superclass
directly through the superclass record.
That is, it should use its own class pointers only,
not the widget's class pointers,
as the widget's class may be a subclass of the
class whose implementation is being referenced.
</para>
</note>
<para>
This technique is referred to as <emphasis remap='I'>enveloping</emphasis> the superclass's operation.
</para>
</sect2>

<sect2 id="Class_Extension_Records">
<title>Class Extension Records</title>
<para>
It may be necessary at times to add new fields to already existing
widget class structures.  To permit this to be done without requiring
recompilation of all subclasses, the last field in a class part structure
should be an extension pointer.  If no extension fields for a class
have yet been defined, subclasses should initialize the value of the
extension pointer to NULL.
</para>

<para>
If extension fields exist, as is the case with the
Composite,
Constraint,
and
Shell
classes, subclasses can provide values for these fields by setting the
<emphasis remap='I'>extension</emphasis> pointer for the appropriate part in their class structure to
point to a statically declared extension record containing the
additional fields.
Setting the <emphasis remap='I'>extension</emphasis> field is never mandatory; code that uses fields
in the extension record must always check the <emphasis remap='I'>extension</emphasis> field and take
some appropriate default action if it is NULL.
</para>

<para>
In order to permit multiple subclasses and libraries to chain extension
records from a single <emphasis remap='I'>extension</emphasis> field, extension records should be
declared as a linked list, and each extension record definition should
contain the following four fields at the beginning of the structure
declaration:
</para>
<literallayout>
struct {
     XtPointer next_extension;
     XrmQuark record_type;
     long version;
     Cardinal record_size;
};
</literallayout>

<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>next_extension</emphasis>
    </term>
    <listitem>
      <para>
Specifies the next record in the list, or NULL.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>record_type</emphasis>
    </term>
    <listitem>
      <para>
Specifies the particular structure declaration to which
each extension record instance conforms.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>version</emphasis>
    </term>
    <listitem>
      <para>
Specifies a version id symbolic constant supplied by
the definer of the structure.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>record_size</emphasis>
    </term>
    <listitem>
      <para>
Specifies the total number of bytes allocated for the
extension record.
    </para>
  </listitem>
  </varlistentry>
</variablelist>

<para>
The <emphasis remap='I'>record_type</emphasis> field identifies the contents of the extension record
and is used by the definer of the record to locate its particular
extension record in the list.  The
<emphasis remap='I'>record_type</emphasis> field is normally assigned the
result of
<function>XrmStringToQuark</function>
for a registered string constant.  The
Intrinsics reserve all record type strings beginning with the two
characters ``XT'' for future standard uses.  The value
<emphasis role='strong'>NULLQUARK</emphasis>
may also be used
by the class part owner in extension records attached to its own class
part extension field to identify the extension record unique to that
particular class.
</para>

<para>
The <emphasis remap='I'>version</emphasis> field is an owner-defined constant that may be used to
identify binary files that have been compiled with alternate
definitions of the remainder of the extension record data structure.  The private
header file for a widget class should provide a symbolic constant for
subclasses to use to initialize this field.
The <emphasis remap='I'>record_size</emphasis> field value includes the four common header fields and
should normally be initialized with
<function>sizeof ().</function>
</para>

<para>
Any value stored in the class part extension fields of
<function>CompositeClassPart</function>,
<function>ConstraintClassPart</function>,
or
<function>ShellClassPart</function>
must point to an extension record conforming to this definition.
</para>

<para>
The Intrinsics provide a utility function for widget writers to locate a
particular class extension record in a linked list, given a widget class
and the offset of the <emphasis remap='I'>extension</emphasis> field in the class record.
</para>

<para>
To locate a class extension record, use
<xref linkend='XtGetClassExtension' xrefstyle='select: title'/>.
</para>

<funcsynopsis id='XtGetClassExtension'>
<funcprototype>
  <funcdef>XtPointer <function>XtGetClassExtension</function></funcdef>
  <paramdef>WidgetClass<parameter> object_class</parameter></paramdef>
  <paramdef>Cardinal<parameter> byte_offset</parameter></paramdef>
  <paramdef>XrmQuark<parameter> type</parameter></paramdef>
  <paramdef>long<parameter> version</parameter></paramdef>
  <paramdef>Cardinal<parameter> record_size</parameter></paramdef>
</funcprototype>
</funcsynopsis>

<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>object_class</emphasis>
    </term>
    <listitem>
      <para>
Specifies the object class containing the extension list to be searched.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>byte_offset</emphasis>
    </term>
    <listitem>
      <para>
Specifies the offset in bytes from the base of the
class record of the extension field to be searched.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>type</emphasis>
    </term>
    <listitem>
      <para>
Specifies the record_type of the class extension to be located.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>version</emphasis>
    </term>
    <listitem>
      <para>
Specifies the minimum acceptable version of the class
extension required for a match.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>record_size</emphasis>
    </term>
    <listitem>
      <para>
Specifies the minimum acceptable length of the class
extension record required for a match, or 0.
    </para>
  </listitem>
  </varlistentry>
</variablelist>

<para>
The list of extension records at the specified offset in the specified
object class will be searched for a match on the specified type,
a version greater than or equal to the specified version, and a record
size greater than or equal the specified record_size if it is nonzero.
<xref linkend='XtGetClassExtension' xrefstyle='select: title'/>
returns a pointer to a matching extension record or NULL if no match
is found.  The returned extension record must not be modified or
freed by the caller if the caller is not the extension owner.
</para>
</sect2>
</sect1>
</chapter>

MR SKB