Trying to get my head around GTK+ casting

[John E]

GTK+ contains a widget known as a FileChooserButton. When pressed, it launches a dialog box allowing the user to select a file. After the dialog box closes, the chosen file's name appears in the button and the programmer can obtain a path to the chosen file using gtk_file_chooser_get_filename(). This requires a pointer to GtkFileChooser. Note that GtkFileChooser is not the same thing as GtkFileChooserButton. GtkFileChooser is essentially a collection of helper functions for dealing with the chosen file. Here's the header file for the button:-

#if defined(GTK_DISABLE_SINGLE_INCLUDES) && !defined (__GTK_H_INSIDE__) && !defined (GTK_COMPILATION)
#error "Only <gtk/gtk.h> can be included directly."
#endif

#ifndef __GTK_FILE_CHOOSER_BUTTON_H__
#define __GTK_FILE_CHOOSER_BUTTON_H__

#include <gtk/gtkhbox.h>
#include <gtk/gtkfilechooser.h>

G_BEGIN_DECLS

#define GTK_TYPE_FILE_CHOOSER_BUTTON            (gtk_file_chooser_button_get_type ())
#define GTK_FILE_CHOOSER_BUTTON(obj)            (G_TYPE_CHECK_INSTANCE_CAST ((obj), GTK_TYPE_FILE_CHOOSER_BUTTON, GtkFileChooserButton))
#define GTK_FILE_CHOOSER_BUTTON_CLASS(klass)    (G_TYPE_CHECK_CLASS_CAST ((klass), GTK_TYPE_FILE_CHOOSER_BUTTON, GtkFileChooserButtonClass))
#define GTK_IS_FILE_CHOOSER_BUTTON(obj)         (G_TYPE_CHECK_INSTANCE_TYPE ((obj), GTK_TYPE_FILE_CHOOSER_BUTTON))
#define GTK_IS_FILE_CHOOSER_BUTTON_CLASS(klass) (G_TYPE_CHECK_CLASS_TYPE ((klass), GTK_TYPE_FILE_CHOOSER_BUTTON))
#define GTK_FILE_CHOOSER_BUTTON_GET_CLASS(obj)  (G_TYPE_INSTANCE_GET_CLASS ((obj), GTK_TYPE_FILE_CHOOSER_BUTTON, GtkFileChooserButtonClass))

typedef struct _GtkFileChooserButton        GtkFileChooserButton;
typedef struct _GtkFileChooserButtonPrivate GtkFileChooserButtonPrivate;
typedef struct _GtkFileChooserButtonClass   GtkFileChooserButtonClass;

struct _GtkFileChooserButton
{
  /*< private >*/
  GtkHBox parent;

  GtkFileChooserButtonPrivate *GSEAL (priv);
};

struct _GtkFileChooserButtonClass
{
  /*< private >*/
  GtkHBoxClass parent_class;

  void (* file_set) (GtkFileChooserButton *fc);

  void (*__gtk_reserved1);
  void (*__gtk_reserved2);
  void (*__gtk_reserved3);
  void (*__gtk_reserved4);
  void (*__gtk_reserved5);
  void (*__gtk_reserved6);
  void (*__gtk_reserved7);
};


GType                 gtk_file_chooser_button_get_type         (void) G_GNUC_CONST;
GtkWidget *           gtk_file_chooser_button_new              (const gchar          *title,
								GtkFileChooserAction  action);

#ifndef GTK_DISABLE_DEPRECATED
GtkWidget *           gtk_file_chooser_button_new_with_backend (const gchar          *title,
								GtkFileChooserAction  action,
								const gchar          *backend);
#endif /* GTK_DISABLE_DEPRECATED */

GtkWidget *           gtk_file_chooser_button_new_with_dialog  (GtkWidget            *dialog);
G_CONST_RETURN gchar *gtk_file_chooser_button_get_title        (GtkFileChooserButton *button);
void                  gtk_file_chooser_button_set_title        (GtkFileChooserButton *button,
								const gchar          *title);
gint                  gtk_file_chooser_button_get_width_chars  (GtkFileChooserButton *button);
void                  gtk_file_chooser_button_set_width_chars  (GtkFileChooserButton *button,
								gint                  n_chars);
gboolean              gtk_file_chooser_button_get_focus_on_click (GtkFileChooserButton *button);
void                  gtk_file_chooser_button_set_focus_on_click (GtkFileChooserButton *button,
                                                                  gboolean              focus_on_click);

G_END_DECLS

#endif /* !__GTK_FILE_CHOOSER_BUTTON_H__ */

and here's a cut-down version of the header file for GtkFileChooser (showing function gtk_file_chooser_get_filename() at the end):-

#if defined(GTK_DISABLE_SINGLE_INCLUDES) && !defined (__GTK_H_INSIDE__) && !defined (GTK_COMPILATION)
#error "Only <gtk/gtk.h> can be included directly."
#endif

#ifndef __GTK_FILE_CHOOSER_H__
#define __GTK_FILE_CHOOSER_H__

#include <gtk/gtkfilefilter.h>
#include <gtk/gtkwidget.h>

G_BEGIN_DECLS

#define GTK_TYPE_FILE_CHOOSER             (gtk_file_chooser_get_type ())
#define GTK_FILE_CHOOSER(obj)             (G_TYPE_CHECK_INSTANCE_CAST ((obj), GTK_TYPE_FILE_CHOOSER, GtkFileChooser))
#define GTK_IS_FILE_CHOOSER(obj)          (G_TYPE_CHECK_INSTANCE_TYPE ((obj), GTK_TYPE_FILE_CHOOSER))

typedef struct _GtkFileChooser      GtkFileChooser;

typedef enum
{
  GTK_FILE_CHOOSER_ACTION_OPEN,
  GTK_FILE_CHOOSER_ACTION_SAVE,
  GTK_FILE_CHOOSER_ACTION_SELECT_FOLDER,
  GTK_FILE_CHOOSER_ACTION_CREATE_FOLDER
} GtkFileChooserAction;

typedef enum
{
  GTK_FILE_CHOOSER_CONFIRMATION_CONFIRM,
  GTK_FILE_CHOOSER_CONFIRMATION_ACCEPT_FILENAME,
  GTK_FILE_CHOOSER_CONFIRMATION_SELECT_AGAIN
} GtkFileChooserConfirmation;

GType gtk_file_chooser_get_type (void) G_GNUC_CONST;

/* GError enumeration for GtkFileChooser */
/**
 * GTK_FILE_CHOOSER_ERROR:
 *
 * Used to get the #GError quark for #GtkFileChooser errors.
 */
#define GTK_FILE_CHOOSER_ERROR (gtk_file_chooser_error_quark ())

typedef enum {
  GTK_FILE_CHOOSER_ERROR_NONEXISTENT,
  GTK_FILE_CHOOSER_ERROR_BAD_FILENAME,
  GTK_FILE_CHOOSER_ERROR_ALREADY_EXISTS,
  GTK_FILE_CHOOSER_ERROR_INCOMPLETE_HOSTNAME
} GtkFileChooserError;

GQuark gtk_file_chooser_error_quark (void);

/* Filename manipulation */
gchar *  gtk_file_chooser_get_filename       (GtkFileChooser *chooser);

/* ... and all the other function declarationss */

G_END_DECLS

#endif /* __GTK_FILE_CHOOSER_H__ */

The programmer creates a button and uses it like this:-

GtkWidget* file_chooser_button = gtk_file_chooser_button_new ("Choose a file", GTK_FILE_CHOOSER_ACTION_OPEN);

// Then - after the user has chosen his file:-

gchar* chosen_file = gtk_file_chooser_get_filename (GTK_FILE_CHOOSER(file_chooser_button));

Thanks for persevering so far! Here's the bit I don't understand.... GTK_FILE_CHOOSER() effectively casts a GtkFileChooserButton* to a GtkFileChooser*. But looking at the two header files I can't see anything that would allow one object to be cast to the other. And yet it does work! What am I failing to see