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 they have no code associated with them.
Pure virtual C++ functions and COM interfaces both employ a device called a vtable. A vtable contains the addresses of all functions that implement the given interface. If you want a program or object to use these functions, you can use the QueryInterface method to verify the interface exists on an object, and to obtain a pointer to that interface. What your program or object actually receives from the object after sending QueryInterface is a pointer to the vtable, through which this method can call the interface methods implemented by the object. This mechanism totally isolates private data used by the object and the calling client process.
Another similarity of COM objects to C++ objects is that the first argument of a method is the name of the interface or class, called the this argument in C++. Because COM objects and C++ objects are totally 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++.