From: Paul Brannan <pbrannan@...>
Date: 2001-07-24T05:09:07+09:00
Subject: [ruby-talk:18375] Re: Ruby as opposed to Python?

> In C you can store function pointers inside structures, thus essentially
> pass behaviour around. You can not enherit structures, but you can point
> from one structure to another to another. Syntactically this is not always
> elegant, but conceptually C is very powerful if you want it to be. It comes
> down to programming style.

Actually, GTK+ has a very interesting way of doing single inheritance
using pure C; one struct "derives" from another simply by having an
instance of the other structure as its first element:

#define GTK_CHECK_CAST(tobj, cast_type, cast) ((cast *) (tobj))
#define GTK_WIDGET(widget) \
  (GTK_CHECK_CAST((widget), GTK_TYPE_WIDGET, GtkWidget))
struct GtkObject {
  ...
};
struct GtkWidget {
  GtkObject object;
  ...
};
struct GtkContainer {
  GtkWidget widget;
  ...
};

Thus GtkContainer derives from GtkWidget, which derives from GtkObject.
So now if we want to cast between a GtkContainer and a GtkWidget, all we
have to do is:

GtkContainer * container = foo();
GtkWidget * widget = GTK_WIDGET(container);

This obviously doesn't work for multiple inheritance, but can be pretty
powerful if used correctly (the GTK_CHECK_CAST macro actually does
run-time type checking if you compile with the right flags set).  I don't
know any easy way to do MI in C without using pointers, as you suggested.

Paul