Chapter 1

Intrinsics and Widgets

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.


1.1. Intrinsics

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.

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.

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 Chapter 12. The name of the class now at the root of the Intrinsics class hierarchy is Object. The remainder of this specification refers uniformly to widgets and Core as if they were the base class for all Intrinsics operations. The argument descriptions for each Intrinsics procedure and Chapter 12 describe which operations are defined for the nonwidget superclasses of Core. The reader may determine by context whether a specific reference to widget actually means widget or object.


1.2. Languages

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 client refers to any module, widget, or application that calls an Intrinsics procedure.

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

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


1.3. Procedures and Macros

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 which 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.

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: XtCheckSubclass, XtNew, XtNumber, XtOffsetOf, XtOffset, and XtSetArg.


1.4. Widgets

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.

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 Section 2.5.

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 Section 1.6. The predefined widget classes adhere to these conventions.

A widget instance is composed of two parts:

Much of the input/output of a widget (for example, fonts, colors, sizes, border widths, and so on) is customizable by users.

This chapter discusses the base widget classes, Core, Composite, and Constraint, and ends with a discussion of widget classing.

1.4.1. Core Widgets

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 CoreClassPart and CorePart structures. CoreClassPart Structure

All widget classes contain the fields defined in the CoreClassPart structure.

typedef struct {
WidgetClass superclass;See Section 1.6
String class_name;See Chapter 9
Cardinal widget_size;See Section 1.6
XtProc class_initialize;See Section 1.6
XtWidgetClassProc class_part_initialize;See Section 1.6
XtEnum class_inited;See Section 1.6
XtInitProc initialize;See Section 2.5
XtArgsProc initialize_hook;See Section 2.5
XtRealizeProc realize;See Section 2.6
XtActionList actions;See Chapter 10
Cardinal num_actions;See Chapter 10
XtResourceList resources;See Chapter 9
Cardinal num_resources;See Chapter 9
XrmClass xrm_class;Private to resource manager
Boolean compress_motion;See Section 7.9
XtEnum compress_exposure;See Section 7.9
Boolean compress_enterleave;See Section 7.9
Boolean visible_interest;See Section 7.10
XtWidgetProc destroy;See Section 2.8
XtWidgetProc resize;See Chapter 6
XtExposeProc expose;See Section 7.10
XtSetValuesFunc set_values;See Section 9.7
XtArgsFunc set_values_hook;See Section 9.7
XtAlmostProc set_values_almost;See Section 9.7
XtArgsProc get_values_hook;See Section 9.7
XtAcceptFocusProc accept_focus;See Section 7.3
XtVersionType version;See Section 1.6
XtPointer callback_private;Private to callbacks
String tm_table;See Chapter 10
XtGeometryHandler query_geometry;See Chapter 6
XtStringProc display_accelerator;See Chapter 10
XtPointer extension;See Section 1.6
} CoreClassPart;

All widget classes have the Core class fields as their first component. The prototypical WidgetClass and CoreWidgetClass are defined with only this set of fields.

typedef struct {
        CoreClassPart core_class;
} WidgetClassRec, *WidgetClass, CoreClassRec, *CoreWidgetClass;

Various routines can cast widget class pointers, as needed, to specific widget class types.

The single occurrences of the class record and pointer for creating instances of Core are

In IntrinsicP.h:

extern WidgetClassRec widgetClassRec;
#define coreClassRec widgetClassRec

In Intrinsic.h:

extern WidgetClass widgetClass, coreWidgetClass;

The opaque types Widget and WidgetClass and the opaque variable widgetClass 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 Intrinsic.h:

typedef struct _WidgetClassRec *WidgetClass, *CoreWidgetClass; CorePart Structure

All widget instances contain the fields defined in the CorePart structure.

typedef struct_CorePart {
Widget self;described below
WidgetClass widget_class;See Section 1.6
Widget parent;See Section 2.5
Boolean being_destroyed;See Section 2.8
XtCallbackList destroy_callbacks;See Section 2.8
XtPointer constraints;See Section 3.6
Position x;See Chapter 6
Position y;See Chapter 6
Dimension width;See Chapter 6
Dimension height;See Chapter 6
Dimension border_width;See Chapter 6
Boolean managed;See Chapter 3
Boolean sensitive;See Section 7.7
Boolean ancestor_sensitive;See Section 7.7
XtTranslations accelerators;See Chapter 10
Pixel border_pixel;See Section 2.6
Pixmap border_pixmap;See Section 2.6
WidgetList popup_list;See Chapter 5
Cardinal num_popups;See Chapter 5
String name;See Chapter 9
Screen *screen;See Section 2.6
Colormap colormap;See Section 2.6
Window window;See Section 2.6
Cardinal depth;See Section 2.6
Pixel background_pixel;See Section 2.6
Pixmap background_pixmap;See Section 2.6
Boolean visible;See Section 7.10
Boolean mapped_when_managed;See Chapter 3
} CorePart;

All widget instances have the Core fields as their first component. The prototypical type Widget is defined with only this set of fields.

typedef struct {
        CorePart core;
} WidgetRec, *Widget, CoreRec, *CoreWidget;

Various routines can cast widget pointers, as needed, to specific widget types.

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 Intrinsic.h.

typedef struct_WidgetRec *Widget, *CoreWidget; Core Resources

The resource names, classes, and representation types specified in the coreClassRec resource list are


Additional resources are defined for all widgets via the objectClassRec and rectObjClassRec resource lists; see Sections 12.2 and 12.3 for details. CorePart Default Values

The default values for the Core fields, which are filled in by the Intrinsics, from the resource lists, and by the initialize procedures, are

FieldDefault Value
selfAddress of the widget structure (may not be changed).
widget_classwidget_class argument to XtCreateWidget (may not be changed).
parentparent argument to XtCreateWidget (may not be changed).
being_destroyedParent's being_destroyed value.
ancestor_sensitivelogical AND of parent's sensitive and ancestor_sensitive values.
namename argument to XtCreateWidget (may not be changed).
screenParent's screen; top-level widget gets screen from display specifier (may not be changed).
colormapParent's colormap value.
depthParent's depth; top-level widget gets root window depth.

XtUnspecifiedPixmap is a symbolic constant guaranteed to be unequal to any valid Pixmap id, None, and ParentRelative.

1.4.2. Composite Widgets

The Composite widget class is a subclass of the Core widget class (see Chapter 3). Composite widgets are intended to be containers for other widgets. The additional data used by composite widgets are defined by the CompositeClassPart and CompositePart structures. CompositeClassPart Structure

In addition to the Core class fields, widgets of the Composite class have the following class fields.

typedef struct {
XtGeometryHandler geometry_manager;See Chapter 6
XtWidgetProc change_managed;See Chapter 3
XtWidgetProc insert_child;See Chapter 3
XtWidgetProc delete_child;See Chapter 3
XtPointer extension;See Section 1.6
} CompositeClassPart;

The extension record defined for CompositeClassPart with record_type equal to NVLLQUARK is CompositeClassExtensionRec.

typedef struct {
XtPointer next_extension;See Section 1.6.12
XrmQuarkrecord_type;See Section 1.6.12
long version;See Section 1.6.12
Cardinal record_size;See Section 1.6.12
Boolean accepts_objects;See Section 2.5.2
Boolean allows_change_managed_set;See Section 3.4.3
} CompositeClassExtensionRec, *CompositeClassExtension;

Composite classes have the Composite class fields immediately following the Core class fields.

typedef struct {
        CoreClassPart core_class;
        CompositeClassPart composite_class;
} CompositeClassRec, *CompositeWldgetClass;

The single occurrences of the class record and pointer for creating instances of Composite are

In IntrinsicP.h:

extern CompositeClassRec compositeClassRec;

In Intrinsic.h:

extern WidgetClass compositeWidgetClass;

The opaque types CompositeWidget and CompositeWidgetClass and the opaque variable compositeWidgetClass are defined for generic operations on widgets whose class is Composite or a subclass of Composite. The symbolic constant for the CompositeClassExtension version identifier is XtCompositeExtensionVersion (see Section 1.6.12). Intrinsic.h uses an incomplete structure definition to ensure that the compiler catches attempts to access private data.

typedef struct_CompositeClassRec *CompositeWidgetClass; CompositePart Structure

In addition to the Core instance fields, widgets of the Composite class have the following instance fields defined in the CompositePart structure.

typedef struct {
WidgetList children;See Chapter 3
Cardinal num_children;See Chapter 3
Cardinal num_slots;See Chapter 3
XtOrderProc insert_position;See Section 3.2
} CompositePart;

Composite widgets have the Composite instance fields immediately following the Core instance fields.

typedef struct {
        CorePart core;
        CompositePart composite;
} CompositeRec, *CompositeWidget;

Intrinsic.h uses an incomplete structure definition to ensure that the compiler catches attempts to access private data.

typedef struct _CompositeRec *CompositeWidget; Composite Resources

The resource names, classes, and representation types that are specified in the compositeClassRec resource list are

XtNnumChildrenXtCReadOnlyXtRCardinal CompositePart Default Values

The default values for the Composite fields, which are filled in from the Composite resource list and by the Composite initialize procedure, are

FieldDefault Value
insert_positionInternal function to insert at end

The children, num_children, and insert_position 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.

1.4.3. Constraint Widgets

The Constraint widget class is a subclass of the Composite widget class (see Section 3.6). 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 ConstraintClassPart and ConstraintPart structures. ConstraintClassPart Structure

In addition to the Core and Composite class fields, widgets of the Constraint class have the following class fields.

typedef struct {
XtResourceList resources;See Chapter 9
Cardinal num_resources;See Chapter 9
Cardinal constraint_size;See Section 3.6
XtInitProc initialize;See Section 3.6
XtWidgetProc destroy;See Section 3.6
XtSetValuesFunc set_values;See Section 9.7.2
XtPointer extension;See Section 1.6
} ConstraintClassPart;

The extension record defined for ConstraintClassPart with record_type equal to NULLQUARK is ConstraintClassExtensionRec.

typedef struct {
XtPointer next_extension;See Section 1.6.12
XrmQuarkrecord_type;See Section 1.6.12
long version;See Section1.6.12
Cardinal record_size;See Section 1.6.12
XtArgsProc get_values_hook;See Section 9.7.1
} ConstraintClassExtensionRec, *ConstraintClassExtension;

Constraint classes have the Constraint class fields immediately following the Composite class fields.

typedef struct_ConstraintClassRec {
        CoreClassPart core_class;
        CompositeClassPart composite_class;
        ConstraintClassPart constraint_class;
} ConstraintClassRec, *ConstraintWidgetClass;

The single occurrences of the class record and pointer for creating instances of Constraint are

In IntrinsicP.h:

extern ConstraintClassRec constraintClassRec;

In Intrinsic.h:

extem WidgetClass constraintWidgetClass;

The opaque types ConstraintWidget and ConstraintWidgetClass and the opaque variable constraintWidgetClass are defined for generic operations on widgets whose class is Constraint or a subclass of Constraint. The symbolic constant for the ConstraintClassExtension version identifier is XtConstraintExtensionVersion (see Section 1.6.12). Intrinsic.h uses an incomplete structure definition to ensure that the compiler catches attempts to access private data.

typedef struct_ConstraintClassRec *ConstraintWidgetClass; ConstraintPart Structure

In addition to the Core and Composite instance fields, widgets of the Constraint class have the following unused instance fields defined in the ConstraintPart structure

typedef struct { int empty; } ConstraintPart;

Constraint widgets have the Constraint instance fields immediately following the Composite instance fields.

typedef struct {
        CorePart core;
        CompositePart composite;
        ConstraintPart constraint;
} ConstraintRec, *ConstraintWidget;

Intrinsic.h uses an incomplete structure definition to ensure that the compiler catches attempts to access private data.

typedef struct_ConstraintRec *ConstraintWidget; Constraint Resources

The constraintClassRec core_class and constraint_class resources fields are NULL and the num_resources fields are zero; no additional resources beyond those declared by the superclasses are defined for Constraint.


1.5. Implementation-specific Types

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.

These implementation-defined types are

BooleanA 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 True.
CardinalAn unsigned integer datum with a minimum range of [0..2^16-1]
DimensionAn unsigned integer datum with a minimum range of [0..2^16-1]
PositionA signed integer datum with a minimum range of [-2^15..2^15-1]
XtPointerA 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 XtPointer and back again and the result will compare equal to the original value. In ANSI C environments it is expected that XtPointer will be defined as void*.
XtArgValA datum large enough To contain an XtPointer, Cardinal, Dimension, or Position value.
XtEnumAn integer datum large enough to encode at least 128 distinct values, two of which are the symbolic values True and False. The symbolic values TRUE and FALSE are also defined to be equal to True and False, respectively.

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


1.6. Widget Classing

The widget_class 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.

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 XtResourceList. 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).

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.

To make good use of subclassing, widget declarations and naming conventions are highly stylized. A widget consists of three files:

1.6.1. Widget Naming Conventions

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:

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 String. 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.

1.6.2. Widget Subclassing in Public .h Files

The public .h file for a widget class is imported by clients and contains

For example, the following is the public .h file for a possible implementation of a Label widget:

       #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 1.6.2.entry points */ extern void LabelSetText( ); /* Widget w */ /* String text */

extern String LabelGetText( ); /* Widget w */

#endif LABEL_H

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.

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 Constraint.h.

1.6.3. Widget Subclassing in Private .h Files

The private .h file for a widget is imported by widget classes that are subclasses of the widget and contains

For example, the following is the private .h file for a possible Label widget:

       #ifndef LABELP_H
       #define LABELP_H

#include <Xll/Label.h>

/* 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;

/* 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

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 ConstrainP.h.

1.6.4. Widget Subelassing in .c Files

The .c file for a widget contains the structure initializer for the class record variable, which contains the following parts:

The superclass field points to the superclass global class record, declared in the superclass private .h file. For direct subclasses of the generic core widget, superclass should be initialized to the address of the widgetClassRec structure. The superclass is used for class chaining operations and for inheriting or enveloping a superclass's operations (see Sections 1.6.7, 1.6.9, and 1.6.10).

The class_name 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.

The widget_size field is the size of the corresponding widget instance structure (not the size of the class structure).

The version 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 XtVersion 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 XtVersionDontCheck in the version 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 XtSpecificationRelease, as described in Chapter 13. Use of XtVersion allows the Intrinsics implementation to recognize widget binaries that were compiled with older implementations.

The extension 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.

All other fields are described in their respective sections.

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 Chapter 9.

       /* Resources specific to Label */
       static XtResource resources[ ] = {
                {XtNforeground, XtCForeground, XtRPixel, sizeof(Pixel),
                 XtOffset(LabelWidget, label.foreground), XtRString,
                {XtNfont, XtCFont, XtRFontStruct, sizeof(XFontStruct*),
                 XtOffset(LabelWidget, label.font),XtRString,
                {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( ); . .

/* Class record constant */ LabelClassRec labelClassRec = { { /* core_class fields */ /* superclass */ (WidgetClass)&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) &labelClassRec;

/* New method access routines */ void LabelSetText(w, text) Widget w; String text; { Label WidgetClass lwc = (Label WidgetClass)XtClass(w); XtCheckSubclass(w,labelWidgetClass, NULL); *(lwc->label_class.set_text)(w, text) } /* Private procedures */ . . .

1.6.5. Widget Class and Superclass Look Up

To obtain the class of a widget, use XtClass.

WidgetClass XtClass(w)
Widget w;

wSpecifies the widget. Must be of class Object or any subclass thereof.

The XtClass function returns a pointer to the widget's class structure.

To obtain the superclass of a widget, use

WidgetClass XtSuperclass(w)
Widget w;

wSpecifies the widget. Must be of class Object or any subclass thereof.

The XtSuperclass function returns a pointer to the widget's superclass class structure.

1.6.6. Widget Subclass Verification

To check the subclass to which a widget belongs, use XtIsSubclass.

Boolean XtIsSubclass(w, widget_class)
Widget w;
WidgetClass widget_class;

wSpecifies the widget or object instance whose class is to be checked. Must be of class Object or any subclass thereof.
widget_classSpecifies the widget class for which to test. Must be objectClass orany subclass thereof.

The XtIsSubclass function returns True 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 XtIsSubclass to find out if a widget belongs to the desired class of objects.

To test if a given widget belongs to a subclass of an Intrinsics-defined class, the Intrinsics define macros or functions equivalent to XtIsSubclass for each of the built-in classes. These procedures are XtIsObject, XtIsRectObj, XtIsWidget, XtIsComposite, XtIsConstraint, XtIsShell, XtIsOverrideShell, XtIsWMShell, XtIsVendorShell, XtIsTransientShell, XtIsTopLevelShell, XtIsApplicationShell and XtIsSessionShell.

All these macros and functions have the same argument description.

Boolean XtIs<class> (w)
Widget w;

wSpecifies the widget or object instance whose class is to be checked. Must be of class Object or any subclass thereof.

These procedures may be faster than calling XtIsSubclass directly for the built-in classes.

To check a widget's class and to generate a debugging error message, use XtCheckSubclass, defined in <X11/IntrinsicP.h>:

void XtCheckSubclass(w, widget_class, message)
Widget w;
WidgetClass widget_class;
String message;

wSpecifies the widget or object whose class is to be checked. Must be of class Object or any subclass thereof.
widget_classSpecifies the widget class for which to test. Must be objectClass or any subclass thereof.
messageSpecifies the message to be used.

The XtCheckSubclass 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, XtCheckSubclass constructs an error message from the supplied message, the widget's actual class, and the expected class and calls XtErrorMsg. XtCheckSubclass 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.

XtCheckSubclass 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.

1.6.7. Superclass Chaining

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

In all widget classes:class_name
In Composite widget classes:geometry_manager
In Constraint widget classes:constraint_size
In Shell widget classes:root_geometry_manager

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


In addition, for subclasses of Constraint, the following fields of the ConstraintClassPart and ConstraintClassExtensionRec structures are chained from the Constraint class down to the subclass:


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


For subclasses of Constraint, the following field of ConstraintClassPart is chained from the subclass up to the Constraint class:


1.6.8. Class Initialization: class_initialize and class_part_initialize Procedures

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.

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

typedef void (*XtProc)(void);

A widget class indicates that it has no class initialization procedure by specifying NULL in the class_initialize field.

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 class_part_initialize field. The class_part_initialize procedure pointer is of type XtWidgetClassProc.

typedef void (*XtWidgetClassProc)(WidgetClass);
WidgetClass widget class;

widget_classPoints to the class structure for the class being initialized.

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 Section 1.6.9. Classes that do not define any new class fields or that need no extra processing for them can specify NULL in the class_part_initialize field.

All widget classes, whether they have a class initialization procedure or not, must start with their class_inited field False.

The first time a widget of a class is created, XtCreateWidget ensures that the widget class and all superclasses are initialized, in superclass-to-subclass order, by checking each class_inited field and, if it is False, by calling the class_initialize and the class_part_initialize procedures for the class and all its superclasses. The Intrinsics then set the class_inited field to a nonzero value. After the one-time initialization, a class structure is constant.

The following example provides the class initialization procedure for a Label class.

       static void ClassInitialize( )
              XtSetTypeConverter(XtRString,XtRJustify, CvtStringToJustify, NULL, 0, XtCacheNone, NULL);

1.6.9. Initializing a Widget Class

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 XtInitializeWidgetClass.

void XtInitializeWidgetClass(object_class)
WidgetClass object_class;

object_classSpecifies the object class to initialize. May be objectClass or any subclass thereof.

If the specified widget class is already initialized, XtInitializeWidgetClass returns immediately.

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 XtInitializeWidgetClass is called (see Section 9.6).

1.6.10. Inheritance of Superclass Operations

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


To inherit an operation xyz, specify the constant XtInheritXyz in your class record.

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.

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 _XtInherit cast to the appropriate type. _XtInherit is a procedure that issues an error message if it is actually called.

For example, CompositeP.h contains these definitions:

#define XtInheritGeometryManager ((XtGeometryHandler) _XtInherit)
#define XtInheritChangeManaged ((XtWidgetProc) _XtInherit)
#define XtInheritInsertChild ((XtArgsProc) _XtInherit)
#de fine XtInheritDeleteChild ((XtWidgetProc) _XtInherit)

Composite's class_part_initialize procedure begins as follows:

       static void CompositeClassPartInitialize(widgetClass)
           WldgetClass widgetClass;
           CompositeWidgetClass wc = (CompositeWidgetClass)widgetClass;
           CompositeWidgetClass super = (CompositeWldgetClass)wc->core_class.superclass;

if (wc->composite_class.geometry_manager = XtInheritGeometryManager) { wc->composite_class.geometry_manager = super->composite_class.geometry_manager; }

if (wc->composite_class.change_managed = XtInheritChangeManaged) { wc->composite_class.change_managed = super->composite_class.change_managed; } . . .

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:

For Object:


For Core:


For Composite:


For Shell:


1.6.11. Invocation of Superclass Operations

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 expose 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 insert_child and then calling XtManageChild to add the child to the managed set.

A class method should not use XtSuperclass 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.

This technique is referred to as enveloping the superclass's operation.

1.6.12. Class Extension Records

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.

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 extension pointer for the appropriate part in their class structure to point to a statically declared extension record containing the additional fields. Setting the extension field is never mandatory; code that uses fields in the extension record must always check the extension field and take some appropriate default action if it is NULL.

In order to permit multiple subclasses and libraries to chain extension records from a single extension 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:

struct {
        XtPointer next_extension;
        XrmQuark record_type;
        long version;
        Cardinal record_size;

next_extensionSpecifies the next record in the list, or NULL.
record_typeSpecifies the particular structure declaration to which each extension record instance conforms.
versionSpecifies a version id symbolic constant supplied by the definer of the structure.
record_sizeSpecifies the total number of bytes allocated for the extension record.

The record_type 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 record_type field is normally assigned the result of XrmStringToQuark for a registered string constant. The Intrinsics reserve all record type strings beginning with the two characters "XT" for future standard uses. The value NULLQUARK 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.

The version 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 record_size field value includes the four common header fields and should normally be initialized with sizeof( ).

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

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 extension field in the class record.

To locate a class extension record, use XtGetClassExtension.

XtPointer XtGetClassExtension(object_class, byte_offset, type, version, record size);
WidgetClass object_class;
Cardinal byte_offset;
XrmQuark type;
long version;
Cardinal record_size;

object_classSpecifies the object class containing the extension list to be searched.
byte_offsetSpecifies the offset in bytes from the base of the class record of the extension field to be searched.
typeSpecifies the record_type of the class extension to be located.
versionSpecifies the minimum acceptable version of the class extension required for a match.
record sizeSpecifies the minimum acceptable length of the class extension record required for a match, or 0.

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. XtGetClassExtension 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 object class owner.


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