class_initialize()class_initialize()Nameclass_initialize - Object class method for one-time class initializa‐
tion.
Synopsis
typedef void (*XtProc)(void);
Description
The class_initialize() method is registered on the class_initialize
field of the Object, RectObj, or Core class structures, and is called
exactly once by the Intrinsics before any instances of the class are
created.
The class_initialize() method takes no arguments and returns no value.
It performs initialization that should be done once for this class and
all its subclasses, such as registering type converters and interning
atoms and quarks. Compare this with the class_part_initialize() method
which is called once for its class and also for every subclass, and is
responsible for initializing fields in the class part structure.
The class_initialize() method is not chained and cannot be inherited.
If a widget class does not need any one-time initialization, it should
specify NULL in its class_initialize field.
The fact that this method is not chained means that when a given widget
class, C, is initialized, only the class_initialize() method for that
class is called--the methods for the superclasses B and A are not
called during the initialization of C. If the class_initialize()
method were chained, then the method for widget class A would be
invoked whenever a subclass, such as B or C, were initialized. Since
the method is intended for one-time global initializations, this would
be inappropriate. Although this method is not chained, the Intrinsics
ensure that the superclasses of a class will be initialized before the
class itself. So, in fact, the class_initialize() methods for classes
A and B are guaranteed to have been called before the class_initial‐
ize() method for C is called.
See the "Background" section below for more information on when this
method is called.
Usage
The class_initialize() performs one-time, global initializations for
the class. It should not initialize the fields of the class struc‐
ture--that is the job of the class_part_initialize() method, which is a
chained method, and is invoked for class and all of its subclasses.
class_initialize() and class_part_initialize() perform initialization
of the widget class. It is the initialize() method that initializes
the instance structure of the widget--it is invoked whenever a widget
instance is created.
Example
The following procedure is the class_initialize() method of the Xaw
Form widget. Note that it registers two resource converters, and
interns quarks (in global variables) for use by the String-to-EdgeType
converter.
static void ClassInitialize()
{
static XtConvertArgRec parentCvtArgs[] = {
{XtBaseOffset, (XtPointer)XtOffsetOf(WidgetRec, core.parent),
sizeof(Widget)}
};
XawInitializeWidgetSet();
XtQChainLeft = XrmPermStringToQuark("chainleft");
XtQChainRight = XrmPermStringToQuark("chainright");
XtQChainTop = XrmPermStringToQuark("chaintop");
XtQChainBottom = XrmPermStringToQuark("chainbottom");
XtQRubber = XrmPermStringToQuark("rubber");
XtAddConverter( XtRString, XtREdgeType, _CvtStringToEdgeType, NULL, 0 );
XtSetTypeConverter (XtRString, XtRWidget, XmuNewCvtStringToWidget,
parentCvtArgs, XtNumber(parentCvtArgs), XtCacheNone,
NULL);
}
Background
All widget classes, whether they have a class_initialize() method 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 super‐
class-to-subclass order, by checking each class_inited field. If this
field is False, XtCreateWidget() calls the class_initialize() and the
class_part_initialize() methods 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. This
initialization process can also be performed explicitly, without creat‐
ing a widget, by calling XtInitializeWidgetClass().
See AlsoXtInitializeWidgetClass(1),
Core(3),
class_part_initialize(4), initialize(4).
Xt - Intrinsics Methods class_initialize()
