Most ATL classes are template based. ATL is shipped as a set of source files. To use ATL classes, we include ATL's *.h and *.cpp files in our project. For basic ATL projects, the core files we use are atlbase.h and atlcom.h. ATL files can be found in the Visual C++ ATL\include and ATL\src directories.
ATL provides built-in support for COM servers, class factories, and IUnknown. The general idea behind using ATL is to plug your application classes into the ATL class hierarchy. You add one C++ class for each COM object in a server. Following are brief summaries of ATL core classes.
ATL class CComModule provides support for COM server functionality. It provides methods to register and unregister COM objects, check server lock and references counts, and get class objects. All ATL-based programs have a global instance of CComModule (or a class derived from it) called _Module.
Class CComObject provides QueryInterface, AddRef, and Release. These methods call into internal methods in CComObjectRoot. Class CComObjectRoot (and its base class, CComObjectRootBase) provides an internal data-driven implementation of IUnknown, called from QueryInterface, AddRef, and Release in CComObject.
Class CComCoClass provides a data-driven implementation of IClassFactory.
An interesting feature of ATL is that your application classes are not the most derived classes! An instance of CComObject derives from your class.

Your class derives CComObjectRoot and CComCoClass.
Assume we are implementing COM object PhBookObj in C++ class CPhBookObj. The following diagram depicts how CPhBookObj fits into the ATL class hierarchy.

Normally, we do not manually code required in-process server functions (for example, DllGetClassObject) or our COM object implementation classes. Instead, we use the ATL COM AppWizard to generate a skeleton server and add our COM implementation classes using Visual C++ tools. This is the topic of the next lesson.