[PATCH] tools/widl/header.c: Add a way to declare interface data members.

[Max TenEyck Woodbury]

On 11/11/2012 01:01 AM, Nikolay Sivov wrote:
> On 11/11/2012 05:00, Max TenEyck Woodbury wrote:
>> I mentioned this a few days ago.  It would have helped if you had
>> raised this point then.
>>
>> As it stands, it is simply a way to adding data members to an aggregate
>> with an interface.
>
> Data members to an aggregate? What are you talking about and what it has
> to do with interface definition?

An aggregate is a collection of information, like a class, struct or
union.

Some aggregates include 'interface's, a COM, OLE or RPC thingy.  The
interface can have only methods defined, but those methods might want
access to some additional data.  To get to that data, the method now
has to build a pointer to the containing aggregate and reference the
data through that pointer.  This introduces complications to the code
since the data may not be in same place in the aggregate in each
instance where the interface is used.  You need a slightly different
code sequence for each different place the method is needed.  However,
the source code will be virtually identical for each instance.

This patch allows those aggregate data members associated with the
interface, which are not technically part of the interface, to be
placed in a fixed relation to the interfaces Vtbl pointer.
(Practically the Vtbl pointer is the interface.)  By establishing such
a relationship, the need to convert from the pointer to the interface
(specifically to its Vtbl pointer) to a pointer to its containing
aggregate in order to get to the relevant data is removed.

Now, technically, the associated data is not part of the interface.  It
is part of the aggregate containing and implementing the interface.
Moving the declaration from the aggregate to the end of the struc for
the interface is a hack that lets simpler and more general code to be
generated.

So, it's a hack, that you only use if you want to, to speed up
execution and simplify maintenance.