What is the Component Object Model (COM)? Is it language-dependent?

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.

Answer 2

COM was first created as a mechanism to allow Microsoft Office applications to communicate with one another, then, in its second iteration it was modified and extended to become a specification for how binary code components could--if they were constructed according to the specification-- communicate with each other, and share data, no matter what language or OS they were built upon (as long as the binary file (the compiled .dll or .exe) conformed to the COM specification).

The purpose was to allow "binary reuse" which means that a code component could be reused by multiple client code components, that the original code component knew nothing about, and which were not even in existence when the component was originally compiled. To quote from one of the original architects of COM, Don Box:

[...] The design paradigm of COM was that component contracts are expressed as type definitions. This was a step forward from the world COM replaced, in which contracts were expressed only as simple functional entry points.In this respect, COM was a major advance because it brought the dynamic loading of code and the type system together in a fairly consistent manner.