[PATCH 2/2] mscoree: There can be only one QI implementation i a COM object!

[Michael Stefaniuc]

On 09/09/2016 04:25 PM, Vincent Povirk wrote:
>> Uhm... those are two different interfaces but they were already
>> implemented in the same COM object aka struct ConfigFileHandler.
>> See my first patch were the AddRef and Release methods were already
>> forwarded from ISAXErrorHandler to ISAXContentHandler. So I just cleaned
>> up COM for the single COM object.
> 
> A COM object is not the same as a struct. A single struct can
> implement multiple COM objects (we do this for many interfaces in
> windowscodecs where we have a "container" object that always contains
First time I hit that. All the others were COM violations.

> at least one child), or a single COM object can be implemented via
> multiple structs.
I have merged quite a few of those during the COM cleanup as there is no
advantage in splitting up the IFoo_iface field over multiple structs but
I have seen different downsides (complex/more code, refcounting done
wrong, memory leaks).

> The identity of a COM object is determined by the result of calling
> QueryInterface with IID_IUnknown. In this case, QueryInterface returns
> different IUnknown pointers for each interface in the struct, so they
Then you need to comment that as this is the exception of how COM is
implemented in the rest of Wine. And more importantly it is
indistinguishable from a COM violation bug,

> are different objects. This imposes some requirements on their
> behavior, but there's no requirement that the refcounts of two
> different objects be unrelated, or that the reference counting be
> implemented in any specific way.
In this particular case you are right. But in general the refcount is
visible to applications. while(IFoo_Release(iface)); is a common pattern.

> There's no reason they have to be different objects here, but there
That's my point. Principle of the least surprise, if you don't need to
use a non standard construct then don't do it.

> are many other structs in windowscodecs that implement multiple COM
> objects which must be separate.
> 
>> The alternative is to split the struct ConfigFileHandler. But as this is
>> not part of the API/ABI and the applications cannot get to those COM
>> interfaces it doesn't matters. Also the ISAXXMLReader just needs the two
>> COM interfaces, how those are implemented it doesn't matters either.
> 
> There's no requirement that a single struct can only implement one COM
> object. The implementation of most encoders and decoders in
> windowscodecs is greatly simplified by putting multiple objects in one
> struct.
Then just use a union for all those interface fields. That makes it
obvious on what is going on. But afair there are other patterns used
throughout Wine that deal with a similar problem.

bye
	michael