james.p.williams@usa-spaceops.com wrote:
> I'm using Young's book on mixing C++ and Motif. In it, he says to use
> XtGetSubresources() to be able to configure member data using the X
> resource database. When I try this, based on Young's example,
[SNIP]
> I get compiler warnings about the above call to the XtOffsetOf()
> macro.
>
> % g++ -Wall -Wunused -W -ansi -pedantic -c MyComponent.C
> MyComponent.C:116: warning: invalid offsetof from non-POD type
> `class MyComponent'; use pointer to member instead
>
> I understand the pointer to member syntax, but am not sure how to use
> it here to appease the compiler. Is there a correct, portable, safe
> way to do this?

You cannot use offsetof: since you have a class which is not a POD. You
cannot use member pointers either, since you have a C interface, which has
no idea about member pointers. The best I could come up with is to create a
POD type and embed that one into your class. Register that thing using the
Motif functions.

--
WW aka Attila

On Mon, 22 Sep 2003 08:44:43 -0700, james.p.william wrote:

> I'm using Young's book on mixing C++ and Motif. In it, he says to use
> XtGetSubresources() to be able to configure member data using the X
> resource database. When I try this, based on Young's example,
>
> class MyComponent: public UIComponent {
> ...
> private:
> unsigned char _orientation
> static XtResource _customResources[]; ...
> }
> }
> XtResource MyComponent::_customResources[]={
> {
> "orientation",
> "Orientation",
> XmROrientation,
> sizeof(unsigned char),
> XtOffsetOf(MyComponent,_orientation), XmRString,
> (XtPointer)"HORIZONTAL"
> },{
> ...
> }
> };
>
> I get compiler warnings about the above call to the XtOffsetOf() macro.
>
> % g++ -Wall -Wunused -W -ansi -pedantic -c MyComponent.C
> MyComponent.C:116: warning: invalid offsetof from non-POD type `class
> MyComponent'; use pointer to member instead
>
> I understand the pointer to member syntax, but am not sure how to use it
> here to appease the compiler. Is there a correct, portable, safe way to
> do this?
>
>
class MyComponent : public UIComponent {

private:
struct Resources
{
unsigned char orientation_;
};
Resources res_;
static XtResource customResources_[];
};

XtResource MyComponent::customResources_[]= {
{
"orientation",
"Orientation",
XmROrientation,
sizeof(unsigned char),
XtOffsetOf(MyComponent::Resources, orientation_), XmRString,
(XtPointer)"HORIZONTAL"
}
};

(By the way, I prefer to use trailing underscores as it avoids potential
conflicts with identifiers reserved for use by implementors.)

james.p.williams@usa-spaceops.com wrote:
> I'm using Young's book on mixing C++ and Motif. In it, he says to use
> XtGetSubresources() to be able to configure member data using the X
> resource database. When I try this, based on Young's example,
>
> class MyComponent: public UIComponent
> {
> ...
> private:
> unsigned char _orientation
> static XtResource _customResources[];
> ...
> }
>
> XtResource MyComponent::_customResources[]={
> {
> "orientation",
> "Orientation",
> XmROrientation,
> sizeof(unsigned char),
> XtOffsetOf(MyComponent,_orientation),
> XmRString,
> (XtPointer)"HORIZONTAL"
> },{
> ...
> }
> };
>
> I get compiler warnings about the above call to the XtOffsetOf()
> macro.
>
> % g++ -Wall -Wunused -W -ansi -pedantic -c MyComponent.C
> MyComponent.C:116: warning: invalid offsetof from non-POD type
> `class MyComponent'; use pointer to member instead
>
> I understand the pointer to member syntax, but am not sure how to use
> it here to appease the compiler. Is there a correct, portable, safe
> way to do this?

Yes ... and no.

Yes in the sense that C++ supports "pointer to member" types but No in
the sense that to do what Doug is doing here it will still be somewhat
non-portable.

I would find a way to silence the warning and live with it. A possible
way is to re-interpret cast a "pointer to member". If I'm not mistaken,
XtResource is defined in X Toolkit and you would have a helluva time
changing all the code that depends on that.