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Posted 21 May 2006


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COM in plain C, Part 5

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21 May 2006CPOL18 min read
Add a connectable object (source/sink).



Often, it's convenient for a DLL function we call to "callback" into one of our own functions so that we can do some additional tasks at a particular point, or be notified that something has happened. For example, consider the standard C library function qsort. The fourth argument passed to qsort is a pointer to some function we provide to compare two items. In this way, when we call qsort, qsort allows us to determine the order that items are sorted, while qsort itself does the work of actually reordering the items.

We can't just provide any arbitrary function to qsort. The guy who wrote qsort specified exactly what arguments would be passed to our "compare" callback function, what our function returns, and of course, specified exactly what the purpose of our callback function is. Furthermore, qsort determines when our callback function is actually called (because it is qsort itself that calls our function). The guy who designed qsort mandated that our compare callback function must be defined as so:

int (__cdecl *compare)(const void *elem1, const void *elem2);

Let's create our own version of qsort, which we'll call Sort. (Actually, we'll cheat to make this simpler, and just do a bubble sort.) And let's assume that we'll put this in a DLL named ISort.dll.

Instead of passing Sort a pointer to the compare function each time, let's design this so that the app will first call a separate SetCompare function to allow our DLL to store the pointer in some global. Let's also put a function in the DLL called UnsetCompare that an app can call to clear that function pointer. So here's our DLL source code (that we'll put in a file called ISort.c):

// A global to store the pointer to the app's Compare function
int (STDMETHODCALLTYPE *CompareFunc)(const void *, const void *);

     int (STDMETHODCALLTYPE *compare)(const void *, const void *))
   // Save the compare function ptr in a global
   CompareFunc = compare;

void STDMETHODCALLTYPE UnsetCompare(void)
   CompareFunc = 0;

        DWORD numElems, DWORD sizeElem)
   void   *hi;
   void      *p;
   void   *lo;
   void   *tmp;

   // Has the app set its Compare function pointer yet?
   if (!CompareFunc) return(E_FAIL);

   // Do the (bubble) sort
   if ((tmp = GlobalAlloc(GMEM_FIXED, sizeElem)))
      hi = ((char *)base + ((numElems - 1) * sizeElem));
      lo = base;

      while (hi > base)
         lo = base;
         p = ((char*)base + sizeElem);
         while (p <= hi)
            if ((*CompareFunc)(p, lo) > 0) lo = p;
            (char *)p += sizeElem;

         CopyMemory(tmp, lo, sizeElem);
         CopyMemory(lo, hi, sizeElem);
         CopyMemory(hi, tmp, sizeElem);
         (char *)hi -= sizeElem;



Now, let's write an app that uses our DLL to sort an array of five DWORDs. First, the app calls SetCompare to specify its compare callback function (which we'll name Compare). Then, the app calls Sort. Finally, when our app is finished using Sort, and we wish to make sure that any additional calls to Sort do not accidentally cause it to call our Compare function, we call UnsetCompare.

// An array of 5 DWORDs to be sorted
DWORD Array[5] = {2, 3, 1, 5, 4};

// Our Compare function
int STDMETHODCALLTYPE Compare(const void *elem1, const void *elem2)
   // Do a compare of the two elements. We know that
   // we passed an array of DWORD values to Sort()
   if (*((DWORD *)elem1) == *((DWORD *)elem2)) return 0;
   if (*((DWORD *)elem1) *lt; *((DWORD *)elem2)) return -1;
   return 1;

int main(int argc, char **argv)
   // Give ISort.dll our Compare function ptr

   // Sort of array of 5 DWORDs
   Sort(&Array[0], 5, sizeof(DWORD));


   return 0;

Wrapping a callback function in a COM object

COM also has a facility to allow callbacks, but as you may have feared, it's not quite as straight-forward as our example above. In the directory named ISort, you'll find the files to a COM object that implements our above Sort DLL.

First of all, we have to wrap our Sort function inside some COM object. After all, we're going to turn ISort.dll into a COM component. Let's define an ISort object, using the DECLARE_INTERFACE_ macro Microsoft provides. Like all COM objects, its VTable begins with the QueryInterface, AddRef, and Release functions. We won't bother adding any IDispatch function to it. (Our ISort is not useable by script languages such as VBScript). And then, we'll add the Sort function as the first extra function. Here's the definition that we'll put in ISort.h:

#define INTERFACE  ISort
   STDMETHOD  (QueryInterface) (THIS_ REFIID, void **) PURE;
   STDMETHOD  (Sort)           (THIS_ void *, DWORD, DWORD) PURE;

So our app is going to get an instance of our ISort object and call its Sort function. You've now had plenty of experience writing C/C++ apps that get a COM object and call its functions. That's nothing new.

And of course, we need to run GUIDGEN.EXE to generate a GUID for our ISort object and its VTable. We'll give them names of CLSID_ISort and IID_ISort respectively, and put them in ISort.h.

// ISort object's GUID
// {619321BA-4907-4596-874A-AEFF082F0014}
DEFINE_GUID(CLSID_ISort, 0x619321ba, 0x4907, 0x4596,
 0x87, 0x4a, 0xae, 0xff, 0x8, 0x2f, 0x0, 0x14);

// ISort VTable's GUID
// {4C9A7D40-D0ED-45ea-9520-1CB9095973F8}
DEFINE_GUID(IID_ISort, 0x4c9a7d40, 0xd0ed, 0x45ea,
 0x95, 0x20, 0x1c, 0xb9, 0x9, 0x59, 0x73, 0xf8);

And like we typically do, we'll need to add at least one, extra, private data member to our ISort object -- a reference count member. So, we'll define a MyRealISort inside ISort.c that adds this extra private member to our ISort:

typedef struct {
   ISortVtbl   *lpVtbl;   // ISort's VTable
   DWORD       count;     // ISort's reference count
} MyRealISort;

By now, all of this should be quite familiar to you.

All that's left is to provide some way for an app to give us a pointer to its Compare function so our Sort function can call it. So, do we simply add SetCompare and UnsetCompare functions to our ISort VTable? Nope.

Here's where it starts to get complicated. Microsoft needed some standard way to allow an app to provide its callback functions to a COM object. Microsoft decided that an app should wrap its callback functions in a COM object. What particular COM object? That depends.

Remember how it was the author of qsort who determined what arguments were passed to the callback, and what it returned? Well, since we're the guys writing our ISort object, we get to make up our own COM object that the app must use to wrap any callback functions we require. So, let's just define our own COM object that we'll call an ICompare. Of course, we must start its VTable with the three functions QueryInterface, AddRef, and Release. So, what do we want next? Let's see... Do we want to support script languages supplying us a compare function (such as a VBScript function)? If so, we'd put some IDispatch functions next. No, let's skip that, and for now, only allow a C/C++ app to provide us with a compare callback.

Let's add an extra function to ICompare. We'll call it Compare. That will be the app's compare callback. (So now, you see how the app has to wrap its callback in our own defined COM object.) Here's the definition of our ICompare (which we'll add to ISort.h):

#define INTERFACE  ICompare
   STDMETHOD  (QueryInterface) (THIS_ REFIID, void **) PURE;
   STDMETHOD_ (long, Compare)  (THIS_ const void *, 
                                const void *) PURE;

We do need to run GUIDGEN.EXE to create a GUID for our ICompare's VTable. (We won't need a GUID for the object itself). We'll give it the name of DIID_Compare. (Traditionally, Microsoft prepends a D to the names of GUIDs intended for VTables that wrap callbacks. We'll just follow the tradition.)

// ICompare VTable's GUID
// {4115B8E2-1823-4bbc-B10D-3D33AAA12ACF}
DEFINE_GUID(DIID_ICompare, 0x4115b8e2, 0x1823, 0x4bbc,
 0xb1, 0xd, 0x3d, 0x33, 0xaa, 0xa1, 0x2a, 0xcf);

Let's go back to our app. Now, we have to put a COM object inside our app code. Specifically, we have to create an ICompare object, and have our Compare callback be its extra function. Because we'll need only one ICompare object, we'll simply declare it static. We'll also have to #include ISort.h because that contains the definition of the ICompare object, its VTable, and its GUID. So, here's the code we insert to replace the Compare function in our above app code:

#include "ISort.h"

// Here's our app's ICompare object. We need only one of these,
// so we'll just declare it static. That way, we don't have to
// allocate it, free it, nor maintain any reference count.
static ICompare   MyCompare;

// This is ICompare's QueryInterface. It returns a pointer
// to our ICompare object.
HRESULT STDMETHODCALLTYPE QueryInterface(ICompare *this,
      REFIID vTableGuid, void **ppv)
   // Since this is an ICompare object, we must recognize
   // ICompare VTable's GUID, which is defined for us in
   // ISort.h. Our ICompare can also masquerade as an IUnknown.
   if (!IsEqualIID(vTableGuid, &IID_IUnknown) &&
      !IsEqualIID(vTableGuid, &DIID_ICompare))
      *ppv = 0;

   *ppv = this;

   // Normally, we'd call our AddRef function here. But since
   // our ICompare isn't allocated, we don't need to bother
   // with reference counting.

   // Our one and only ICompare isn't allocated. Instead, it's
   // statically declared above (MyCompare). So we'll just
   // return 1.

   // Our one and only ICompare isn't allocated, so we
   // never need worry about freeing it.

// This is the extra function for ICompare. It is called
// Compare. The ISort object will call this function when
// we call ISort's Sort function.
long STDMETHODCALLTYPE Compare(ICompare *this,
      const void *elem1, const void *elem2)
   // Do a compare of the two elements. We know that
   // we passed an array of DWORD values to Sort().
   if (*((DWORD *)elem1) == *((DWORD *)elem2)) return 0;
   if (*((DWORD *)elem1) < *((DWORD *)elem2)) return -1;
   return 1;

// Our ICompare VTable. We need only one of these, so we can
// declare it static.
static const ICompareVtbl ICompare_Vtbl = {QueryInterface,

Notice that our Compare function is there. It's simply wrapped inside an ICompare object now. (And the first argument passed to it is now a pointer to our ICompare object, which will be our statically declared MyCompare, in this case.)

Now, we're left with the really convoluted job. How does our app give its ICompare object to our ISort object (inside ISort.dll)? I really wish I could say that this is going to be simple and easy-to-understand, but unfortunately, due to some dubious design decisions by the MS guys who devised this whole scheme, it's going to be a painfully twisted trip through the house of horrors.

IConnectionPointContainer and IConnectionPoint objects

To let an app give its ICompare object to our ISort object, we have to add some things to our ISort object. Specifically, we have to add two sub-objects to it. (Hopefully, in the last chapter, you have read and understand how to create an object that has sub-objects.)

One good thing is that Microsoft has already defined these two sub-objects (and their VTable GUIDs) in some include files that ship with your compiler. The two sub-objects are called IConnectionPointContainer and IConnectionPoint. All we need to do is add them to our ISort object. Since we've already defined a MyRealISort, we'll just embed these two sub-objects inside it. And since we know that an app is going to give us a pointer to an ICompare object, let's also add a place to store that inside MyRealISort.

typedef struct {
   ISortVtbl                 *lpVtbl;
   DWORD                     count;
   IConnectionPointContainer container;
   IConnectionPoint          point;
   ICompare                  *compare;
} MyRealISort;

Note that our base object is our ISort itself. We could notate it like so:

typedef struct {
   ISort                     iSort;
   DWORD                     count;
   IConnectionPointContainer container;
   IConnectionPoint          point;
   ICompare                  *compare;
} MyRealISort;

But since an ISort has only its one lpVtbl member, both definitions amount to the same thing.

As you'll remember from the last chapter, the base object's QueryInterface must recognize the GUIDs of all of its sub-object's VTables, as well as its own VTable. So our ISort QueryInterface must look like so:

        REFIID vTableGuid, void **ppv)
   // Because an IConnectionPointContainer
   // is a sub-object of our ISort, we
   // must return a pointer to this sub-object
   // if the app asks for it. Because
   // we've embedded our IConnectionPointContainer
   // object inside of our
   // MyRealISort, we can get that sub-object
   // very easily using pointer arithmetic.
   if (IsEqualIID(vTableGuid, &IID_IConnectionPointContainer))
      *ppv = ((unsigned char *)this + 
               offsetof(MyRealISort, container));

   else if (IsEqualIID(vTableGuid, &IID_IUnknown) || 
            IsEqualIID(vTableGuid, &IID_ISort))
      *ppv = this;

      *ppv = 0;



But wait. Did we forget to also check for our IConnectionPoint sub-object's VTable GUID, and return a pointer to that? No, we didn't forget. Unfortunately, the MS guys who thought up this design kind of broke the rules. The IConnectionPoint sub-object is not gotten by calling the base object's QueryInterface (nor the QueryInterface of our IConnectionPointContainer sub-object). In a moment, we'll see how it's gotten.

The IConnectionPointContainer sub-object can serve two purposes.

First of all, by passing an IConnectionPointContainer VTable GUID to our ISort's QueryInterface function, an app can determine whether our ISort accepts callback functions. Incidentally, in COM documentation, such an object (i.e., one that provides an IConnectionPointContainer) is described as sourcing events. If QueryInterface returns a pointer to an IConnectionPointContainer, then the object can source events. (If not, then a 0 is returned.)

Secondly, the IConnectionPointContainer has a function to get an IConnectionPoint sub-object. That function is called FindConnectionPoint.

So, let's write the functions of our IConnectionPointContainer sub-object. Because it's a sub-object of ISort, and ISort is the base object, our IConnectionPointContainer's QueryInterface, AddRef, and Release functions simply delegate to ISort's QueryInterface, AddRef, and Release.

STDMETHODIMP QueryInterface_Connect(IConnectionPointContainer *this,
                                    REFIID vTableGuid, void **ppv)
   // Because this is a sub-object of our ISort (ie, MyRealISort) object,
   // we delegate to ISort's QueryInterface. And because we embedded the
   // IConnectionPointContainer directly inside of MyRealISort, all we need
   // is a little pointer arithmetic to get our ISort.
   return(QueryInterface((ISort *)((char *)this - 
          offsetof(MyRealISort, container)), vTableGuid, ppv));

STDMETHODIMP_(ULONG) AddRef_Connect(IConnectionPointContainer *this)
   // Because we're a sub-object of ISort, delegate to its AddRef()
   // in order to increment ISort's reference count.
   return(AddRef((ISort *)((char *)this - offsetof(MyRealISort, container))));

STDMETHODIMP_(ULONG) Release_Connect(IConnectionPointContainer *this)
   // Because we're a sub-object of ISort, delegate to its Release()
   // in order to decrement ISort's reference count.
   return(Release((ISort *)((char *)this - offsetof(MyRealISort, container))));

For now, we're going to use a stub for the EnumConnectionPoints function.

STDMETHODIMP EnumConnectionPoints(IConnectionPointContainer *this,
             IEnumConnectionPoints **enumPoints)
   *enumPoints = 0;

The real work is in FindConnectionPoint. The app passes us a handle to where it wants us to return ISort's IConnectionPoint sub-object. Since we've embedded it directly inside MyRealISort, it's easy to locate.

The app also passes us the VTable GUID we defined for our ICompare (callback) object. That's how we know that the app really does mean to provide us with the correct object that we require. We don't want to return a IConnectionPoint if the app isn't prepared to give us an ICompare object.

STDMETHODIMP FindConnectionPoint(IConnectionPointContainer *this,
             REFIID vTableGuid, IConnectionPoint **ppv) 
   // Is the app asking us to return an IConnectionPoint object it can use
   // to give us its ICompare object? The app asks this by passing us
   // ICompare VTable's GUID (which we defined in ISort.h).
   if (IsEqualIID(vTableGuid, &DIID_ICompare))
      MyRealISort  *iSort;

      // The app obviously wants to give its ICompare object to our
      // ISort. In order to do that, we need to give the app a standard
      // IConnectionPoint. This is easy to do since we embedded both
      // our IConnectionPointContainer and IConnectionPoint inside of our 
      // ISort. All we need is a little pointer arithmetic.
      iSort = (MyRealISort *)((char *)this - offsetof(MyRealISort, container));
      *ppv = &iSort->point;

      // Because we're giving the app a pointer to our IConnectionPoint,
      // and our IConnectionPoint is a sub-object of ISort, we need to
      // increment ISort's reference count. The easiest way to do this is
      // to call our IConnectionPointContainer's AddRef, because all we do
      // there is delegate to our ISort's AddRef.


   // We don't support any other app callback objects for our ISort.
   // All we've defined, and support, is an ICompare object. Tell
   // the app we don't know anything about the GUID he passed to us, and
   // do not give him any IConnectionPoint object.
   *ppv = 0;

That takes care of our IConnectionPointContainer's functions. Now we need to write our IConnectionPoint's functions.

Since IConnectionPoint is also a sub-object of ISort (just like IConnectionPointContainer), its QueryInterface, AddRef, and Release will delegate to ISort's QueryInterface, AddRef, and Release. I won't bother reproducing those functions since they are almost identical to IConnectionPointContainer's.

For now, we're going to use a stub for the EnumConnections function.

The GetConnectionInterface function simply copies ICompare's VTable GUID to a buffer that the app passes. Later, we'll see the use for this.

The GetConnectionPointContainer function returns a pointer to the IConnectionPointContainer sub-object that created this IConnectionPoint object. What this implies is that, as long as the app is holding onto some IConnectionPoint sub-object that our IConnectionPointContainer gives to the app, our IConnectionPointContainer must stick around. This is no problem here since we've embedded both our IConnectionPointContainer and IConnectionPoint inside our MyRealISort (and our MyRealISort is going to stick around until all its sub-objects and base object are Released).

The above three functions are fairly trivial, so I'll simply refer you to the source code in Sort.c.

The real work is in the Advise and Unadvise functions. This is how an app actually gives us its ICompare. Advise essentially does the same task as our SetCompare function did. And Unadvise does what our UnsetCompare did.

Let's look at Advise:

STDMETHODIMP Advise(IConnectionPoint *this, IUnknown *obj, DWORD *cookie) 
   HRESULT     hr;
   MyRealISort *iSort;

   // Get our MyRealISort that this IConnectionPoint sub-object belongs
   // to. Because this IConnectPoint sub-object is embedded directly
   // inside its MyRealISort, all we need is a little pointer arithmetic.
   iSort = (MyRealISort *)((char *)this - offsetof(MyRealISort, point));

   // We allow only one ICompare for our ISort, so see if the app already
   // called our Advise(), and we got one. If so, let the app know that it
   // is trying to give us more ICompares than we allow.
   if (iSort->compare) return(CONNECT_E_ADVISELIMIT);
   // Ok, we haven't yet gotten the one ICompare we allow from the app. Get
   // the app's ICompare object. We do this by calling the QueryInterface
   // function of the app object passed to us. We pass ICompare VTable's
   // GUID (which we defined in ISort.h).
   // Save the app's ICompare pointer in our ISort compare member, so we
   // can get it when we need it.
   hr = obj->lpVtbl->QueryInterface(obj, &DIID_ICompare, &iSort->compare);

   // We need to return (to the app) some value that will clue our Unadvise()
   // function below how to locate this app ICompare. The simpliest thing is
   // to just use the app's ICompare pointer as that return value.
   *cookie = (DWORD)iSort->compare;


Actually, the app doesn't pass a pointer to its ICompare object. That would be too easy, straight-forward, and understandable, and obviously that wasn't the goal for the MS programmers who designed this thing. Rather, the app passes us some app object from which we can request the app to give us its ICompare. Our Advise function is expected to call that object's QueryInterface function, passing the ICompare VTable's GUID to request the app's ICompare. I guess that MS thought this would be a good way to double-check that the app really is giving us an ICompare (except, don't you know that if an app is so badly written, it can't figure out what it should be doing with Advise, then its QueryInterface will probably return the wrong object too?). Our course, the app's QueryInterface will automatically do an AddRef on the ICompare it gives to us, so we are expected to Release the app's ICompare when done with it.

Our Advise function stores the app's ICompare pointer in our MyRealISort's compare member, so we can access it when we need it, and finally Release it.

Advise must return some DWORD value, of our own choosing, to the app. The app is expected to store this DWORD value, and later pass it to our Unadvise function. We can choose any DWORD value we want, but this value should somehow help our Unadvise function when the app later wants us to give back (i.e., Release) its ICompare. Indeed, the app later calls our Unadvise when the app no longer wants us to call its callback functions, and to instead Release its ICompare. So, let's look at Unadvise:

STDMETHODIMP Unadvise(IConnectionPoint *this, DWORD cookie) 
   MyRealISort *iSort;

   // Get the MyRealISort that this IConnectionPoint sub-object belongs
   // to. Because this IConnectPoint sub-object is embedded directly
   // inside its MyRealISort, all we need is a little pointer arithmetic.
   iSort = (MyRealISort *)((char *)this - offsetof(MyRealISort, point));

   // Use the passed value to find wherever we stored his ICompare pointer.
   // Well, since we allow only one ICompare for our ISort, we already
   // know we stored it in our ISort->compare member. And Advise()
   // returned that pointer as the "cookie" value. So we already got the
   // ICompare right now.
   // Let's just make sure the cookie he passed is really the pointer we
   // expect.
   if (cookie && (ICompare *)cookie == iSort->compare)
      // Release the app's ICompare
      ((ICompare *)cookie)->lpVtbl->Release((ICompare *)cookie);

      // We no longer have the app's ICompare, so clear the ISort
      // compare member.
      iSort->compare = 0;

The rest of the code in Sort.c is almost exactly the same as in our very first chapter's IExample.c. The only difference is that, after our IClassFactory's CreateInstance allocates our MyRealISort, we must initialize the sub-objects inside it, and initialize any other private data members.

You can compile our COM object into ISORT.DLL. To register it, simply take the register utility for IExample (in the directory RegIExample), and replace all instances of IExample with ISort.

An example C app

In the directory ISortApp is an example C app that uses our ISort DLL. Previously, we discussed the ICompare functions, so all that's really new here is how the app calls our Advise function to give us its ICompare object, and then later calls Unadvice to ask us to release it. This is a fairly trivial example, so you can peruse the comments in ISortApp.c.

Because COM event connections can be fairly complicated, I've supplied a second example of an object that uses callbacks. In the directory IExampleEvts is the source to another COM DLL. This DLL implements an object (IExampleEvts) that has an extra function called DoSomething. This function has the ability to call any of up to five different app callbacks. So we have defined an IFeedback object to wrap all five of those callback functions. Its definition is in IExampleEvts.h. (A callback object can contain numerous extra, callback functions. And each function can have a different purpose, take different arguments, and return different values.)

Take the time to peruse these examples and fully understand what is going on. Things are going to get more complicated as we go on from here.

Adding support for script languages

In order to support script languages, we need to add some IDispatch functions. Let's take our IExampleEvts COM object and adapt it for use with script languages.

First, let's create a new directory named IExampleEvts2, and copy the original sources there. Then, we'll rename the files, replacing all instances of IExampleEvts with IExampleEvts2, and all instances of IFeedback with IFeedback2. I've done this for you already. I also ran GUIDGEN.EXE and made new GUIDs for the objects and their VTables. Since we're going to also need a type library, I created a new GUID for that too.

We need to add IDispatch functions to both our IExampleEvts2 VTable and our IFeedback2 VTable. You can compare the original IExampleEvts.h with the new IExampleEvts2.h to see the changes. Note that both VTables now have the five standard IDispatch functions added, and also the macros now specify that both VTables are based on an IDispatch (instead of IUnknown).

In IExampleEvts2.c, I've added the five IDispatch functions to our IExampleEvts2 object and its statically declared VTable. I also added a static variable MyTypeInfo to hold an ITypeInfo that Microsoft's GetTypeInfoOfGuid creates for us (and which we use with DispInvoke). This is same thing we did when we added support to our IExample2 object back in chapter 2. There's nothing new to any of these changes so far.

We do have to change our DoSomething function. No longer can we call any callback function directly (referencing it via the lpVtbl of the object). This is because a script engine will not actually have function pointers in its VTable for those extra callback functions. Instead, we must call it indirectly using the callback object's (IFeedback's) Invoke function. We must pass the appropriate DISPID of the function we wish to call.

The only new thing appears in our IExampleEvts2.IDL file (i.e., the file that MIDL.EXE compiles into our .TLB type library file). Notice how we've defined IFeedback2's VTable (which we arbitrarily label as IFeedback2VTbl). This is similar to how we define our IExampleEvts2 VTable, except that we don't use the dual keyword on IFeedback2's VTable. Script engines can't utilize a dual VTable for a callback object. I've arbitrarily chosen to use DISPID 1 for the first callback function (Callback1), 2 for the second callback function, etc.

The other new thing is how we define our IExampleEvts2 object itself as so:

coclass IExampleEvts2
   [default] interface IExampleEvts2VTbl;
   [source, default] interface IFeedback2VTbl;

The first interface line should look familiar. It's the same as how we've always marked an object's VTable in any IDL file. But the second interface line tells anyone using our type library that IExampleEvts2 supports a callback object whose VTable is described by IFeedback2Vtbl. The "source" keyword indicates this is a callback VTable. The "default" keyword means that this VTable is the one a script engine should use for any callback object associated with our IExampleEvts2 object.

You can compile our COM object into IExampleEvts2.DLL. To register it, simply take the register utility for IExample2 (in the directory RegIExample2) and replace all instances of IExample2 with IExampleEvts2.

Another example C app

In the directory IExampleEvts2App is an example C app that uses our IExampleEvts2 DLL. Compare this with the original source in IExampleEvtsApp. Notice that we've added the five standard IDispatch functions to our IFeedback2 object.

Although IExampleEvts2.IDL did not define the IFeedback2 VTable as "dual", we've nevertheless included the extra functions (Callback1 through Callback5) in our actual statically declared VTable (IFeedback2_Vtbl). In this way, we can fool DispInvoke into using this VTable as if it really was declared as dual. Also, notice that the type library we load is IExampleEvts2's type library, because that is where the IFeedback2 VTable is described. And the ITypeInfo we ask GetTypeInfoOfGuid to create is for an IFeedback2 VTable. We use this ITypeInfo with our IFeedback2's Invoke function.

A VBScript example

In the IExampleEvts2 directory is a VBScript example named IExampleEvts2.vbs as so:

Set myobj = WScript.CreateObject("IExampleEvts2.object", "label_")

sub label_Callback1()
   Wscript.echo "Callback1 is called"
end sub

The first line uses the Windows Scripting Shell's CreateObject to get an instance of our IExampleEvts2 object (which should have been registered with the ProdID of IExampleEvts2.object). The reason we use the Script Shell's CreateObject instead of the VB engine's CreateObject, is because the former allows us to pass a second argument -- a string that is prepended to the name of any callback function. Here, we've specified the string label. So for example, when our IExampleEvts2 DoSomething calls the script engine's IFeedback2's Invoke and passes a DISPID of 1 (i.e., for Callback1), then the VBScript function that gets called is label_Callback1.

In fact, we've provided a VB function by that name, which simply displays a message box.

Multiple types of callback objects

An object can support numerous types of callback objects. For example, if we wanted, we could have our ISort object support being given both an ICompare and IFeedback2 callback objects. In this case, we would need a separate IConnectionPoint object for each. In that case, we'd need to put both the ICompare and IFeedback2 VTables in our IDL file, and reference them both in our object like so:

coclass IExampleEvts2
   [default] interface IExampleEvts2VTbl;
   [source, default] interface IFeedback2VTbl;
   [source] interface ICompareVtbl;

But note that only one source interface can be marked as default, and a script engine can use only the default one (i.e., a script can't provide us any ICompare callback functions).

Furthermore, we have to provide real code for our IConnectionPointContainer's EnumConnectionPoints function. This function should return another object that can enumerate each of the different types of IConnectPoints that we support. (The object it returns is a standard enumerator object such as the enumerator we used in our Collections chapter.)

Because callback objects are usually used in conjunction with scripting (and script engines can't support multiple types of callback objects per a single coclass object), I'm going to delve further into multiple types of callback objects.

Multiple callback objects

In our example objects, we allowed an app (or script engine) to provide us only one IFeedback2 object (which we stored in our MyRealIExampleEvts2's feedback member). But theoretically, an app may provide us with as many IFeedback2 callback objects as it desires. In our IConnectionPoint's Advise, we simply refused to accept more than one IFeedback2 from an app/engine. If you wish to allow an app/engine to provide more IFeedback2s, then instead of a feedback member, you may wish to define another struct that can be linked into a list, and also has a member to store an IFeedback2 pointer. You would GlobalAlloc one of these structs in Advise, and link them into a list whose head would be stored in some MyRealIExampleEvts2 member.

You would also need to modify DoSomething to loop through the entire list, and call each IFeedback2's Invoke.

But again, since the primary use of callback objects is for scripting, and most uses will involve only one callback object, we'll skip developing an example to illustrate.

What's next?

In the next article, we'll examine how to turn a C app into an ActiveX script host (i.e., how to run scripts from your C app, and how a script calls C functions in your app, and your app calls functions in the script, to control your app and exchange data with a script).


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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Comments and Discussions

QuestionI need to find a way to call the vbscript inside an html file Pin
gupse gunes11-Apr-14 0:55
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QuestionEvent Sink in DLL not Firing Pin
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rveach17-Nov-11 3:23
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GeneralGot Code: Could not connect object Error: 80020009 Pin
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QuestionWhere is the "IFeedback2's Invoke" in VBScript example? Pin
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Questionare these used to create events Pin
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QuestionCan a COM server dll be implemented as a regular dll? Pin
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QuestionDoes COM need to be Rebased? Pin
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GeneralRe: Does COM need to be Rebased? Pin
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GeneralRe: Does COM need to be Rebased? Pin
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GeneralExe and DCOM error Pin
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GeneralRe: Exe and DCOM error Pin
Jeff Glatt28-Jun-06 4:58
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GeneralDifferences between an OCX and a DLL Pin
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GeneralRe: Differences between an OCX and a DLL Pin
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GeneralCOM development. Pin
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GeneralRe: COM development. Pin
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GeneralRe: COM development. [modified] Pin
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GeneralRe: COM development. Pin
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GeneralRe: COM development. [modified] Pin
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