COM - Implementing DllGetClassObject

Question

I am attempting to understand creating/using COM components without the help of MFC/ATL to know its inner workings. I am using this codeguru article for reference.Following are the steps followed by me.

Created a Wind32 Dll,
Added a MIDL file and declared the interface IAdd and library name DemoMath; compiled the code using MIDL compiler.
Created CAddObj class deriving IAdd interface,provided implementation for IAdd and IUnknown interfaces.
Created class CAddFactory deriving from IClassFactory interface;provided implementation for IClassFactory methods.

Now creating DllGetClassObject to give client an option to invoke this function to get an instance of the class factory.

Following is the code:

#include "stdafx.h"
#include <objbase.h>
#include "AddObjFactory.h"
#include "IAdd_i.c"
STDAPI DllGetClassObject(const CLSID& clsid,
                         const IID& iid,
                         void** ppv)
{
    //
    //Check if the requested COM object is implemented in this DLL
    //There can be more than 1 COM object implemented in a DLL
    //

    if (clsid == CLSID_AddObject)
    {
        //
        //iid specifies the requested interface for the factory object
        //The client can request for IUnknown, IClassFactory,
        //IClassFactory2
        //
        CAddFactory *pAddFact = new CAddFactory;
        if (pAddFact == NULL)
            return E_OUTOFMEMORY;
        else
        {
            return pAddFact->QueryInterface(iid , ppv);
        }
    }

    //
    //if control reaches here then that implies that the object
    //specified by the user is not implemented in this DLL
    //

    return CLASS_E_CLASSNOTAVAILABLE;
}

Now where is CLSID_AddObject constant suppose to be defined or Is it generated while compiling MIDL file(I didn't find it though)?

In many pre-C++98-standard compilers, new returned zero on failure. However, that hasn't been the case for over a decade, even on Microsoft's compilers. You need to catch the std::bad_alloc exception instead. — Euro Micelli
– Euro Micelli, Commented Jun 11, 2013 at 11:20
You have a memory leak in your DllGetClassObject() implementation. QueryInterface() increments the reference count if successful. If failed, your object does not get freed. You should start the object with a reference count of 1, call QueryInterface() on it, and then call Release() on it regardless of whether QueryInterface() succeeds or fails. That way, if successful, the reference count is only 1 (in the caller) when DllGetClassObject() exits, and if failed then the object gets freed correctly. — Remy Lebeau
– Remy Lebeau, Commented Jun 11, 2013 at 19:54

Roman Ryltsov · Accepted Answer · 2013-06-11 11:48:08Z

1

coclass IDL item will typically get you CLSID:

library Foo
{
//...
    [
        //...
    ]
    coclass AddObject
    {
        //...
    };

then on your "IAdd_i.c" you are already including:

MIDL_DEFINE_GUID(CLSID, CLSID_AddObject, ...);

this is what defines CLSID_AddObject.

edited Jun 11, 2013 at 11:48

answered Jun 11, 2013 at 11:04

Roman Ryltsov

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3 Comments

ZoomIn Over a year ago

thanks for the answer,did u mean interface IAdd instead of IAddObject(since this does not exist) in coclass AddObject? and I assume there needs to be another uuid I should associate the coclass with,right?

Roman Ryltsov Over a year ago

IAddObject is irrelevant, just typical declaration. I updated to only leave the relevant part. coclass itself is the important thing.

ZoomIn Over a year ago

@Hans Passant I didn't get why you removed your answer!

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