FAQ 34.14 How does COM provide language transparency?

The COM binary object model.

COM is language independent due to its binary specification of how COM classes and interfaces are laid out in memory. For example, the binary definition of an interface is a pointer to a table of function pointers.

Thus, even though the examples use C++ to define the IUnknown and IStack interfaces, the key step is for the C++ compiler to turn the C++ class definitions into a pointer to a table of function pointers (or a virtual function table).

This binary definition of classes and interfaces makes it possible for any language that can create the correct memory structures to define COM classes and interfaces. For example, the C language can define a COM interface using structs and pointers to functions; many other languages use similar features. Languages that cannot build these constructs cannot be used for defining COM classes.

This binary specification also means that a COM class can be used by any programming language that can call functions using a function pointer. Clearly, C++ and C are capable of this trick.

All this makes C++ a natural programming language for defining and using COM classes since C++ compilers are capable of automatically constructing the virtual function tables (so that the programmer does not have to lay them out by hand) and calling functions through virtual function tables.

Of course, other programming languages and development environments can automate the definition of COM classes in the same way. For instance, Visual Basic can be used for creating COM classes since the Visual Basic development environment can interpret the Visual Basic code and emit the correct interface declarations, class declarations, MIDL code, and so on.