Practical ATL: Understanding the class object

In my previous article in this series, I explained how to create a simple COM server and implement a method that returns an enumerator object.

It was all done very easily, but the ATL wizards hide some of the things that you really ought to know for my next article. For starters: where is the class object?

The class object is the COM object responsible for creating new instances of the CStuff COM object that I demonstrated in a previous article. But we didn’t have to write any code for it, nor is it even visible… so what’s up with that?

Some more on class objects

The class object of a COM server is the object that is actually responsible handing out instances of the COM object. The class object is a COM object itself, but only 1 instance of it normally exists in a module (DLL or EXE) per defined COM object type.

For example, the ‘Stuff’ COM object that I implemented in my previous article uses the standard class factory with IClassFactory as the interface for handing out new CStuff instances.

The most important method in IClassFactory is ‘CreateInstance’ which simply creates a new instance of the COM object and then passes an interface pointer back to the caller.

Normally, this process is invisible to the client application, because ‘CoCreateInstance’ uses IClassFactory behind the scenes so that the client doesn’t have to bother with it.

First it calls ‘CoGetClassObject’ to get the class object for the requested COM object, and then it uses the IClassFactory interface of that class object to create a new instance. That instance is then passed to the caller, as an interface pointer of the type that was requested in the call to ‘CoCreateInstance’.

I think it is safe to say that ‘IClassFactory’ (or one of its relatives) is the interface on the class object of 99.99% of all COM objects that have a class object.

The default class factory

By default, CStuff uses CComClassFactory as its default class object. But looking at the declaration of CStuff, it is not immediately apparent how it does this.

CStuff derives from CCoClass<T>, which has the macro DECLARE_CLASSFACTORY in its implementation.

For EXE servers, this macro expands to DECLARE_CLASSFACTORY_EX(ATL::CComClassFactory), which in turn expands to

typedef ATL::CComCreator< ATL::CComObjectNoLock< ATL::CComClassFactory > > _ClassFactoryCreatorClass;

DLL servers use CComObjectCached instead of CComObjectNoLock, but I don’t cover DLL servers in this article.

As you can remember from the first article that featured CStuff, the C++ COM classes are missing the IUnknown core by default, because the implementation of that core depends on how we want to the object to behave.

This means we have to shove our class into a COM object to make it whole. For class objects, the CComObjectNoLock is most appropriate, because the lifetime of the object should not contribute to the lifetime of the server executable. Otherwise there would be a deadlock.

The server would start, and create the class object. And then the server would continue to live until the class object was destroyed. But since the COM runtime will always have a reference to the class object, the reference count would never reach zero, the class object would never be destroyed, and the server would never be able to shut down.

And finally, this class definition is shoved into a creator object which will help us by creating a properly initialized instance of the class object itself via its static CreateInstance function.

Associating the COM object C++ class with the class factory

So the COM object C++ class has a typedef that identifies the class object type. But how does the executable use this fact?.

This is where things get a little hairy, and possibly start to make ominous squishy sounds J

After the class declaration of CStuff, there is the innocuous line OBJECT_ENTRY_AUTO(__uuidof(Stuff), CStuff)

The OBJECT_ENTRY_AUTO is a macro that expands to this:

#define OBJECT_ENTRY_AUTO(clsid, class) \
      __declspec(selectany) ATL::_ATL_OBJMAP_ENTRY __objMap_##class = {&clsid, class::UpdateRegistry, class::_ClassFactoryCreatorClass::CreateInstance, class::_CreatorClass::CreateInstance, NULL, 0, class::GetObjectDescription, class::GetCategoryMap, class::ObjectMain }; \
      extern “C” __declspec(allocate(“ATL$__m”)) __declspec(selectany) ATL::_ATL_OBJMAP_ENTRY* const __pobjMap_##class = &__objMap_##class; \

Don’t worry if the sight of this code makes you nauseous. That’s normal. I highlighted the important parts

What it does is simple. It’s just how it does it that is ugly. It creates a map entry for the ATL innards of our server executable. In this map entry, it associates the static method for creating the class object with the static method for creating an object instance.

Creating the class object

Now that the class object and the COM object are associated with each other, the next bit of code in the ATL headers is where the magic happens:

  if (m_pObjMap != NULL)
    pEntry = m_pObjMap;
    while (pEntry->pclsid != NULL && hr == S_OK)
      hr = pEntry->RegisterClassObject(dwClsContext, dwFlags);

When the ATL module initializes, it will iterate over all elements in the object map and register each one. And this registration process entails the following:

    IUnknown* p = NULL;
    if (pfnGetClassObject == NULL)
      return S_OK;
    HRESULT hRes = pfnGetClassObject(
                         (LPVOID*) &p);
    if (SUCCEEDED(hRes))
      hRes = CoRegisterClassObject(
               *pclsid, p, dwClsContext, dwFlags, &dwRegister);
    if (p != NULL)
    return hRes;

If you do a bit of spelunking in the headers, you’ll see that pfnGetClassObject is a function pointer that maps to _ClassFactoryCreatorClass::CreateInstance. So this code simply creates a new class object and registers it.

Again, this is specific to EXE servers. In DLL servers, the class object is created on demand in the DllGetClassObject function, which is called by CoGetClassObject if the COM server is a DLL.

Creating CStuff instances

If you look at the _ClassFactoryCreatorClass::CreateInstance, you’ll see that its first parameter is a void *. And the value that is passed in this parameter by the ATL module is the address of the static method for creating new object instances.

_ClassFactoryCreatorClass::CreateInstance simply creates a new class object ‘p’, and then calls p->SetVoid(pv); to pass the aforementioned function pointer to the class object.

And the SetVoid method is no more complicated then this:

  void SetVoid(void* pv)
    m_pfnCreateInstance = (_ATL_CREATORFUNC*)pv;
  _ATL_CREATORFUNC* m_pfnCreateInstance;

I guess the original developer hadn’t had his morning coffee yet when he came up with this name. It manages to be 100% factually correct, while being 100% opaque at the same time. But it does the job.

When the class object has to create a new instance, it simply does the following:

  STDMETHOD(CreateInstance)(LPUNKNOWN pUnkOuter, REFIID riid, void** ppvObj)
    // snip irrelevant code
        hRes = m_pfnCreateInstance(pUnkOuter, riid, ppvObj);
    // snip irrelevant code
    return hRes;

Providing a new class factory

Now that you understand how the class object plumbing is hooked up, it is easy to see how you can implement your own class object.

Remember how DECLARE_CLASSFACTORY_EX maps a class object to the _ClassFactoryCreatorClass typedef?

In C++, a typedef in a derived class can override a typedef in a base class. So even though the CCoClass<T> base class says that the _ClassFactoryCreatorClass maps to ATL::CComCreator< ATL::CComObjectNoLock< ATL::CComClassFactory > >,

We can override this by simply putting DECLARE_CLASSFACTORY_EX(CStuffCreator) In the declaration for CStuff. That’s really all there is to it. The macro magic takes care of the rest.


If changing the class object for a COM server is as trivial as adding a single line to your COM server C++ class, then why did I just write 4 pages of stuff?

I wrote all this because if I tell someone to use a macro to do something non-trivial, I want him or her to understand what is going on. There are already far too many programmers on this planet who do things without understanding what it is they do, or how it works. And I am not going to add some more by glossing over the details.

Of course, judging by the number of visitors on this corner of the internet that I call ‘home’, I stand little risk of doing so no matter how crappy I would write J

This article was originally part of my article on implementing a custom class object. But after I spent more than 3 pages on the previous explanation, I thought it would be best to put it in a separate article.

A big thank-you goes to Jim Springfield of the Architects team within the developers division (I think) of Microsoft for taking the time to review this article for correctness, and to Amit Mohindra for helping my questions and articles reach the right people.

And you can find the demo project attached to this article under the MIT license.



One thought on “Practical ATL: Understanding the class object”

  1. So good article!I have kown lot of detals about COM,Thanks very much! I think I will read more of your blog articles in the feature !

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