Chapter 4: Type Information and Language Independence

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Objectives:

• Understand the reasons for using the Interface Definition Language (IDL).

• Examine the output files generated by the MIDL compiler.

• Learn to define interfaces, coclasses, and type libraries using IDL.

• Understand how to define custom enumerations in IDL.

• Understand the Visual Basic and Java COM language mappings.

• Use C++ smart pointers to simplify client-side COM code.

There are two solid reasons why every COM-based project should begin with the Interface Definition Language (IDL): language independence and location transparency.

IDL is the "official" language of COM that provides a way to specify the functionality of a COM server such that all COM language mappings can access them. In this chapter we will investigate how to create type information for a COM server and how various target languages (specifically VB and Java) can make use of it. We also come to learn how to ease C++ client-side COM development using smart pointer technology.

As for the other benefit of IDL (location transparency), the COM architecture keeps a client encapsulated from a server's actual physical location using stubs and proxies, which are often created using MIDL-generated code, as we will see in Chapter 5.

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