Interface pointer

Sorry, I don't understand what you mean. Are you asking how COM interfaces work?

Basically, an "interface" is just a struct with a pointer to a vtbl that has those methods. If you don't have the Windows SDK you can look at MinGW headers, or Wine, to see the order of the methods. They go in order.

For example this is the .idl file for the IFileSystemBindData interface:

Code:

/*****************************************************************************
 * IFileSystemBindData interface
 */
[
    object,
    uuid(01e18d10-4d8b-11d2-855d-006008059367),
    pointer_default(unique),
    local
]
interface IFileSystemBindData : IUnknown
{
    HRESULT SetFindData(
                [in] const WIN32_FIND_DATAW *pfd);
    HRESULT GetFindData(
                [out] WIN32_FIND_DATAW *pfd);
}

So it has IUnknown methods first, then IFileSystemBindData, so it's like this:

Code:

struct IFileSystemBindData
{
  IFileSystemBindDataVtbl* lpVtbl;
};

struct IFileSystemBindDataVtbl
{
  IUnknown_QueryInterface function pointer;
  IUnknown_AddRef function pointer;
  IUnknown_Release function pointer;
  SetFindData function pointer;
  GetFindData function pointer;
};

So you have to implement that vtbl, put it somewhere in your object, and pass that to the Windows shell API. Let's say your object layout is like this:

Code:

struct obj
{
  member1;
  member2;
  member3;
  member4;
};

You can place the interface vtbl anywhere:

Code:

struct obj
{
  member1;
  member2;
  member3;
  IFileSystemBindData here;
  member4;
};

Then you store your own vtbl's address into "here.lpVtbl" (which is usually in read-only memory since it doesn't change). Your vtbl will contain function pointers to your own methods.

To obtain the pointer to your "struct obj" from within an interface's method, just do some byte subtraction, i.e.:

Code:

; assume the "This" pointer is in rcx
lea rax, [rcx - offset_to_here_in_struct]
; rax is now pointer to the object

; e.g. if member1, member2, and member3 are all pointers, then offset_to_here_in_struct is 3 * 8 = 24

Remember when the windows API wants the interface, you pass it the address of "here" above. It doesn't know the object's struct, nor does it need to. All it knows is that there's a vtbl at the pointer passed that implements the interface. That's the point of interfaces.

You could pass a different object, with completely different layout and different vtbl, and the Windows API would work with it with no changes. It's "abstract". It's same as virtual functions in C++. (in fact that's how it is implemented in C++)

Implementing multiple interfaces on the same object is the same, just put more vtbls, and then QueryInterface return address of each interface when asked. But since IFileSystemBindData inherits from IUnknown, you don't have to implement IUnknown as a separate vtbl: just return the same address as IFileSystemBindData in its QueryInterface. It will "just work".