mono/docs/object-layout

Object and VTable layout
========================

The first pointer inside an Object points to a MonoClass structure. Objects
also contains a MonoThreadsSync structure which is used by the mono Thread
implementation. 
 
typedef struct {
	MonoClass *class;
	MonoThreadsSync synchronisation;
	
	/* object specific data goes here */
} MonoObject;

The MonoClass contains all Class infos, the VTable and a pointer to static
class data.

typedef struct {
	/* various class infos */
	MonoClass  *parent;
	const char *name;
	const char *name_space;

	...

	/* interface offset table */
	gint  *interface_offsets;

	gpointer data; /* a pointer to static data */

	/* the variable sized vtable is included at the end */
	gpointer vtable [vtable_size];
} MonoClass;


Calling virtual functions:
==========================

Each MonoMethod (if virtual) has an associated slot, which is an index into the
VTable. So we can use the following code to compute the address of a virtual
function: 
 
method_addr = object->class->vtable [method->slot];


Calling interface methods:
==========================

Each interface class is associated with an unique ID. The following code
computes the address of an interface function:

offset_into_vtable = object->class->interface_offsets [interface_id];
method_addr = object->class->vtable [offset_into_vtable + method->slot];