Robert P. J. Day wrote:
> in a legacy project i just inherited, i did a quick recursive grep
> and noticed a number of invocations of "offsetof". from experience,
> i've concluded that the majority of these calls are because of sloppy
> programming or laziness or just plain doing something incredibly hacky
> that could be done in a more straightforward way.
>
> are there any really compelling reasons to need to know the offset
> of a structure field? just curious.
It's most commonly used in conjunction with an application-specific
object system or generics mechanism.
Probably the best-known example is Xt widget definitions. Each widget
class definition has an array of XtResource structures identifying the
resources which the widget understands:
typedef struct _XtResource {
String resource_name; /* Resource name */
String resource_class; /* Resource class */
String resource_type; /* Representation type desired */
Cardinal resource_size; /* Size in bytes of representation */
Cardinal resource_offset;/* Offset from base to put resource value */
String default_type; /* representation type of specified default */
XtPointer default_addr; /* Address of default resource */
} XtResource, *XtResourceList;
The resource_size and resource_offset fields would be initialised by
applying sizeof() and offsetof() [1] to the field within the widget
instance structure which corresponds to the resource.
[1] Usually XtOffsetOf(); Xt predates C99 or the widespread
availablility of offsetof().
This allows the creation of generic functions (not tied to a specific
widget class) to set/get resource values for a particular widget
instance by name. IOW, roll-your-own OO with reflection.
--
Glynn Clements <[EMAIL PROTECTED]>
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