FAQ 34.09 What is an interface?

Pure specification without any implementation.

A COM interface is a set of related methods that have a well-defined contract but have no implementation. A COM class implements one or more interfaces, and all the services a COM class exports are defined by the interfaces it implements. Callers access the services of a COM object by calling the methods defined by its interfaces. COM interfaces serve to decouple the caller's view of the services provided by a COM class from how the COM class implements those interfaces.

In C++ terms, an interface is an abstract class that defines a set of public, pure virtual functions and contains no data members. By convention, the names of COM interfaces begin with a capital I (for interface). The following interface is called IUnknown and it defines three methods QueryInterface, AddRef, and Release (for more information on IUnknown, see FAQ 34.10).

 class IUnknown { public:     virtual HRESULT QueryInterface(REFIID riid, void** ppv) = 0;     virtual unsigned long AddRef() = 0;     virtual unsigned long Release()= 0; }; 

COM interfaces and COM classes are distinct concepts, and COM interfaces are specified independently of any particular COM class and independently of any particular implementation of the interface. In this way, COM interfaces resemble Java interfaces. Unfortunately, C++ provides only one construct, the class, for representing classes and interfaces, and therefore a certain amount of programmer discipline is required when using C++ to achieve the same effect.

Once defined, a COM interface and its contract can never change. Thus, every implementation of the interface must satisfy the same contract no matter what programming language is used, no matter what operating system is used, no matter what data structures are used, and no matter what performance optimizations are performed. In this way, objects and their callers can work together even when they are implemented by different people using different tools and working in different organizations and who don't know or communicate with each other. In other words, COM interfaces are substitutable.

Notice that an interface cannot define concrete data members (whereas C++ permits data members to be declared in the class body). However, an interface can define get/set member functions, thus indicating that there is some abstract state that can be manipulated through the interface. The COM class that implements the interface is responsible for defining the concrete data members.

A key element of the COM standard is the binary specification of COM interfaces. A COM interface is an array of function pointers similar to the virtual function table generated by most C++ compilers. With COM interfaces, unlike C++ virtual function tables, the location of the interface pointer is not linked to the location of the object's data. COM makes the location of the COM object's data invisible to the caller in the interests of decoupling and guaranteeing language and location transparency. This separation of the interface from the concrete data promotes location transparency because it means that a COM interface can be accessed by a caller (through a proxy) even when the actual COM object (and the object's data) is located in another process.

Microsoft has defined a set of COM interfaces (usually referred to as the standard interfaces) that cover a wide range of services including object creation, memory allocation, structured storage, and compound documents, and more are being added all the time. You can define your own interfaces, too; these are referred to as custom interfaces.