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    <p>Think of COM as a binary compatible way to share interfaces across DLL boundaries. C++ classes can't easily be exported from DLLs because of the non-standard name mangling done between various compiler versions. COM allows code in one DLL or executable, to create an implementation of an interface from another DLL or EXE as long as that implementation follows a defined interface and calling convention. This is what allows a COM class to be written in C# and invoked from C++, Python, and many other COM-aware languages.</p> <p>COM interfaces are just standard C++ classes containing all pure virtual functions and derived from IUnknown. IUnknown is a pre-defined interface that all compliant COM interfaces must derive from that provides common facilities like reference counting and the ability to query whether an object implements a particular interface.</p> <p>DLLs that wish to advertise the fact they can create implementations of COM interfaces do so by exporting 4 functions:</p> <ul> <li>DllGetClassObject => Return a class factory of a requested interface</li> <li>DllCanUnloadNow => Whether all instances handed out have since been released</li> <li>DllRegisterServer => Register the types this DLL supplies in the registry</li> <li>DllUnregisterServer => Unregister this DLL and its types from the registry</li> </ul> <p>So to answer your questions:</p> <blockquote> <p>Q1. About Windows DLL Based on my understanding, a Windows DLL can export functions, types(classes) and global variables. Is this understanding all right?</p> </blockquote> <p>DLLs can export functions and classes (not sure about global variables, but you don't want to be exporting them DLLs even if you can! :-) ) However, exported classes will be name mangled and hence only usable by other DLLs or EXEs that happen to have the same name mangling (not a good way to do business). Functions with C-style calling convention are not name mangled and hence can be exported and called from elsewhere without problems.</p> <blockquote> <p>Q2. A COM DLL exports both functions such as DllRegisterServer() and DllGetClassObject(), and also the Classes which implements the IUnknown interface. Is this understanding all right?</p> </blockquote> <p>Halfway, there are 4 functions to export to be a full COM compliant DLL (shown above). You can search MSDN for any of those names to see the full signatures fro them. Your implementation for DllGetClassObject is going to be the main one which outside parties will use. They can use that to get an IClassFactory to an interface your DLL provides, and then use that to create an instance.</p> <p>COM is a large and complicated beast, but its foundations are fairly straightforward. You can also check out this link <a href="http://www.codeproject.com/KB/COM/comintro.aspx" rel="noreferrer">COM Intro</a> for some more info. Good luck! </p>
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    1. CO+1 COM isn't that crazy. It enables you to bridge high-level languages across a low-level gap.
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