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POCOM - Implementing DllGetClassObject
 

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  1. POCOM - Implementing DllGetClassObject
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    <p>I am attempting to understand creating/using COM components without the help of MFC/ATL to know its inner workings. I am using <a href="http://www.codeguru.com/cpp/com-tech/activex/tutorials/article.php/c5567/Step-by-Step-COM-Tutorial.htm%20this%20article" rel="nofollow">this codeguru article</a> for reference.Following are the steps followed by me.</p> <ul> <li>Created a Wind32 Dll,</li> <li>Added a MIDL file and declared the interface <code>IAdd</code> and library name <code>DemoMath</code>; compiled the code using MIDL compiler.</li> <li>Created <code>CAddObj</code> class deriving <code>IAdd</code> interface,provided implementation for <code>IAdd</code> and <code>IUnknown</code> interfaces.</li> <li>Created class <code>CAddFactory</code> deriving from <code>IClassFactory</code> interface;provided implementation for <code>IClassFactory</code> methods.</li> </ul> <p>Now creating <code>DllGetClassObject</code> to give client an option to invoke this function to get an instance of the class factory.</p> <p>Following is the code:</p> <pre><code>#include "stdafx.h" #include &lt;objbase.h&gt; #include "AddObjFactory.h" #include "IAdd_i.c" STDAPI DllGetClassObject(const CLSID&amp; clsid, const IID&amp; 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-&gt;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; } </code></pre> <p>Now where is <code>CLSID_AddObject</code> constant suppose to be defined or Is it generated while compiling MIDL file(I didn't find it though)?</p>
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    1. COIn 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.
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      1. POCOM - Implementing DllGetClassObject
      1. USEuro Micelli
    2. COYou 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.
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      1. POCOM - Implementing DllGetClassObject
      1. USRemy Lebeau
 

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