Re: Typecast error

[Havoc Pennington <hp redhat com>]

Dwight Tuinstra <tuinstra northnet org> writes:
> > This program compiles and creates an executable which works
> > correctly, ie. ithe int is successfully passed to the function,
> > however I get this warning, which I would like to fix
> >
> > gui.h:873: cannot convert `int' to `void *' for argument `4' to
> > `gtk_signal_connect (GtkObject *, const gchar *, void (*) (), void
> > *)'
> >

The warning here is entirely justified; what you're doing is not
correct, because sizeof(int) != sizeof(void*) on many platforms.

So the lesson is, take warnings seriously, don't just insert
casts. ;-)

> It wants a pointer to something, so, we give it one:
> 
>   gint dummy_int = 1;
>   gtk_signal_connect( GTK_OBJECT( robot_name[1] ), "clicked",
>           GTK_SIGNAL_FUNC( display_info ), &dummy_int );
> 

No! Once you're in the callback, dummy_int will no longer exist;
you'll have a pointer to random memory.

The correct, portable, no-warnings way to write this code is:

void
display_info (GtkWidget *widget, gpointer data)
{
  gint i;

  i = GPOINTER_TO_INT (data);

  cout << i << endl;
}

...
  gtk_signal_connect (GTK_OBJECT (robot_name[1]), "clicked",
                      GTK_SIGNAL_FUNC (display_info ),
                      GINT_TO_POINTER (42));

Havoc