How do I decide whether a method-function pointer belongs in the instance structure or class structure in GObject? but in gstreamer more function pointers put into instance structure，why？

[zhang007z]

in glib GObject for example:
//--------------------------------------------------------//
typedef struct _MyInstance MyInstance;
struct _MyInstance {
        GObject parent;
        ......//instance variable
        ......//this place is method function pointer in instance structure.      
};

typedef struct _MyInstanceClass MyInstanceClass;
struct _MyInstanceClass {
        GObjectClass parent_class;
       ......//class variable
       ......//this place is method function pointer in class structure.
};
//--------------------------------------------------------//
I don't understand that the method function pointer puts in class or
instance structure that it seen to same . what difference are they? I think
that class function method pointers each of instance object is used to call
same, so puting this functions to class structure. However, How to use
instance object's function method pointers about method pointers of its own?
because I think the function method pointers of instance object for itself
is same, How to understand instance object function method? below I give
that gstreamer Gstpad.h code snippet as example :
//-------------------------------------------------------//
struct _GstPad {
  GstObject         object;

  /*< public >*/
  gpointer          element_private;

  GstPadTemplate        *padtemplate;

  GstPadDirection        direction;

  /*< public >*/ /* with STREAM_LOCK */
  /* streaming rec_lock */
  GStaticRecMutex       *stream_rec_lock;
  GstTask           *task;
  /*< public >*/ /* with PREROLL_LOCK */
  GMutex            *preroll_lock;
  GCond             *preroll_cond;

  /*< public >*/ /* with LOCK */
  /* block cond, mutex is from the object */
  GCond             *block_cond;
  GstPadBlockCallback        block_callback;
  gpointer           block_data;

  /* the pad capabilities */
  GstCaps           *caps;
  GstPadGetCapsFunction     getcapsfunc;
  GstPadSetCapsFunction     setcapsfunc;
  GstPadAcceptCapsFunction   acceptcapsfunc;
  GstPadFixateCapsFunction   fixatecapsfunc;

  GstPadActivateFunction     activatefunc;
  GstPadActivateModeFunction     activatepushfunc;
  GstPadActivateModeFunction     activatepullfunc;

  /* pad link */
  GstPadLinkFunction         linkfunc;
  GstPadUnlinkFunction       unlinkfunc;
  GstPad            *peer;

  gpointer           sched_private;

  /* data transport functions */
  GstPadChainFunction        chainfunc;
  GstPadCheckGetRangeFunction    checkgetrangefunc;
  GstPadGetRangeFunction     getrangefunc;
  GstPadEventFunction        eventfunc;

  GstActivateMode        mode;

  /* generic query method */
  GstPadQueryTypeFunction    querytypefunc;
  GstPadQueryFunction        queryfunc;

  /* internal links */
#ifndef GST_DISABLE_DEPRECATED
  GstPadIntLinkFunction      intlinkfunc;
#else
#ifndef __GTK_DOC_IGNORE__
  gpointer intlinkfunc;
#endif
#endif

  GstPadBufferAllocFunction      bufferallocfunc;

  /* whether to emit signals for have-data. counts number
   * of handlers attached. */
  gint               do_buffer_signals;
  gint               do_event_signals;

  /* ABI added */
  /* iterate internal links */
  GstPadIterIntLinkFunction     iterintlinkfunc;

  /* free block_data */
  GDestroyNotify block_destroy_data;

  /*< private >*/
  union {
    struct {
      gboolean                      block_callback_called;
      GstPadPrivate                *priv;
    } ABI;
    gpointer _gst_reserved[GST_PADDING - 2];
  } abidata;
};

struct _GstPadClass {
  GstObjectClass    parent_class;

  /* signal callbacks */
  void      (*linked)       (GstPad *pad, GstPad *peer);
  void      (*unlinked)     (GstPad *pad, GstPad *peer);
  void      (*request_link)     (GstPad *pad);
  gboolean  (*have_data)        (GstPad *pad, GstMiniObject *data);

  /*< private >*/
  gpointer _gst_reserved[GST_PADDING];
};
//-------------------------------------------------------//
you can see function method pointer in instance structure and class
structure from giving above code snippet. intance function method pointers
are initialized by gst_pad_init function, as below:
//--------------------------------------------------------------------------------//
static void
gst_pad_init (GstPad * pad)
{
  ........//other no important code
  GST_PAD_CHAINFUNC (pad) = NULL;    
  GST_PAD_LINKFUNC (pad) = NULL;    
  GST_PAD_CAPS (pad) = NULL;
  GST_PAD_GETCAPSFUNC (pad) = NULL;  
  GST_PAD_ACTIVATEFUNC (pad) = gst_pad_activate_default;
  GST_PAD_EVENTFUNC (pad) = gst_pad_event_default;
  GST_PAD_QUERYTYPEFUNC (pad) = gst_pad_get_query_types_default;
  GST_PAD_QUERYFUNC (pad) = gst_pad_query_default;
#ifndef GST_REMOVE_DEPRECATED
  GST_PAD_INTLINKFUNC (pad) = gst_pad_get_internal_links_default;
#endif
  GST_PAD_ITERINTLINKFUNC (pad) = gst_pad_iterate_internal_links_default;

  GST_PAD_ACCEPTCAPSFUNC (pad) = gst_pad_acceptcaps_default;

 ............//other no important code
}
//--------------------------------------------------------------------------------//
so I confuse this issue,why there are function method pointers in intance
structure rather than puting into class class structure?




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