[gtkmm] Drang and Drop data changes between source's signal and target drop signal

[erik echohome org]

Everyone,
    I've got a really intersting one here.  It appears like the data is
being changed from the source objects drag_data_get method, and the
targets drop_drag_data_rec method, specificall, I'm packaging a
pointer and then want to get it back out of the DragContext in the
target's method.  I'm probably packaging the data incorrectly in some
manner, would you help me out and take a look at it and let me know? 
I'd appreciate it.

Here is the debug output statements that make me think this:

MTBList::drag_data_get(312):Packaged pointer: 0x83508f0
ImageNotebook::drop_drag_data_rec(136):passed pointer: 0x84b9968

Here are the code snippets:
gtk_selection_data_set( selection_data, selection_data->target,
    8,    // data format
(const guchar*)pMTB,	// data pointer
sizeof( pMTB ) );      // data size

pMTB = reinterpret_cast<MTB*>( selection_data->data );
context->drag_finish( TRUE, FALSE, time ); // ok


I for the life of me am not seeing where I've gone wrong here.

    Thanks for your help,
         Erik.