This topic pertains only to applications written in C++. For an introduction to programming for DirectX in Visual Basic, see Visual Basic Programming Topics.

To C++ programmers, a COM interface is like an abstract base class. That is, it defines a set of signatures and semantics but not the implementation, and no state data is associated with the interface. In a C++ abstract base class, all methods are defined as pure virtual, which means that they have no code associated with them.

Pure virtual C++ functions and COM interfaces both use a device called a vtable. A vtable contains the addresses of all functions that implement the interface. If you want an application or object to use these functions, use the QueryInterface method to verify that the interface exists on an object and to obtain a pointer to that interface. After sending QueryInterface, the object sends your application or object a pointer to the vtable, through which this method can call the interface methods implemented by the object. This mechanism isolates any private data that the object uses and the calling client process from one another .

Another similarity between COM objects and C++ objects is that a method's first argument is the name of the interface or class, called the this argument in C++. Because COM objects and C++ objects are completely binary-compatible, the compiler treats COM interfaces like C++ abstract classes and assumes the same syntax. This results in less complex code. For example, the this argument in C++ is treated as an understood parameter and not coded, and the indirection through the vtable is handled implicitly in C++.