Question

I’m new to COM, and I don’t know what it is or why it exists.

Is this a programming methodology like OOP? Do programming languages have to support it? (with some special keywords or something)

When I asked my professor about it, he said:

COM is a binary-stable way to do OOP. We need to know binary-layout (something..something..)

I’ve no idea what it means. Some people say it is used for code reuse. OOP does a good job at that already, so then why did this COM evolve in the first place?

What is it with C++ and COM? Wherever I see COM, it’s is always described with abstract C++ examples. Is it only for C++?

Can any one show me a case or exmaple so that I can understand the need for COM? What are the requirements for learning this, so I can write my own components?

Answer 1

COM at its core is a way of providing a data-passing contract which is independent of any specific language. It is provably not language dependent, as there are many languages which support COM (there are C++, C, .NET, and Java implementations)

In practice it is useful for a couple of different examples:

1. Communication between different languages: Because COM is language independent, it is possible to use COM to pass data between components in different languages. For instance you can use COM to talk between C++, Java, and .NET code.
2. Threading Semantics: COM allows you to define threading semantics for a particular component to ensure it is created in the appropriate thread context no matter where it is used.
3. General componentization.