🚀 go-pugleaf

In article <2q5f97INNt6o@eccdb1.pms.ford.com> poorman@cc4032.pms.ford.com (Glenn M. Poorman) writes:
>In article <pauld.767898877@sarge>, pauld@verdix.com (Paul Durbin) writes:
>|> I attempt to use a member function as callback but all three arguments (to the
>|> callback) are garbage. If I make it a friend then the arguments are correct,
>|> but then I can't get to the class data, without reference thru an instance of
>|> the class and I don't want the data to be static.
>|> 
>|> So, what is causing this problem? Could I possibly use the *this pointer as
>|> the client_data to the callback, then cast to the appropriate class?
>
>The problem is that the first argument of any non-static member function in
>C++ is the "this" pointer.  So if you try to register a member function
>as a callback with (i.e. Motif), it won't work because Motif doesn't know
>that the function it is calling is a member function and it will not pass
>the "this" pointer.
>
  Your compiler should not have allowed you to pass a pointer to a member
function as the callback parameter.  Pointers to member functions are not
compatible with pointers to functions.

Try compiling this little snip of code and see what happens:
#include <Xm/Xm.h>

class bogus {
public:
  void my_callback(Widget, XtPointer, XtPointer);
};

int main(int, char **)
{
    Widget w;

    XtAddCallback(w, "my_callback", &bogus::my_callback, NULL);
}

All of my compilers give errors similar to:

CC: "testijh.C", line 12: error: bad argument  3 type for XtAddCallback(): void
(bogus::*)(Widget, XtPointer, XtPointer) ( XtCallbackProc expected) (1264)

PS. Just a nit, but it is Xt that handles callbacks not Motif.

--
===============================================================================
Ian Hogg						ianhogg@cs.umn.edu
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