Why COM classes need constructor? How to use COM class without registration?

Question

I am looking into DirectShow samples from Windows SDK. Many of the classes feature non-default constructor. How those constructors are called? Who supplies arguments?

Can I use those classes in C++ programs without registration? If so I could use class constructor directly.

If I use a COM class without registration what happens in the following code fragment:

Foo * foo = new Foo(.....); // note, not using CoCreateInstance
Bar * bar = foo->QueryInterface(...);
bar->Release();
delete foo; // CRASH?

Thank you!

You don't know how Release is implemented in the general case (it could even be a no-op, or use another memory allocator). Here, it is likely doing delete this. You should use CoCreateInstance + Release, this is the only safe thing to do. — Alexandre C.
– Alexandre C., Commented Apr 6, 2012 at 15:49

Roman Ryltsov · Accepted Answer · 2012-04-06 15:42:18Z

2

It has nothing to do with COM, it is just the framework is built this way and constructor arguments in DirectShow BaseClasses are necessary to get everything together - the class, the ancestor, the instantiating factory.

Using COM class without registration might be or might not be possible. I suspect you might be interested in using DirectShow filter without registration instead, and there is a good article on the topic: Using Filters Without Registration.

edited Apr 6, 2012 at 15:42

answered Apr 6, 2012 at 15:25

Roman Ryltsov

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Hans Passant · Accepted Answer · 2012-04-06 16:12:12Z

2

A C++ class that's exposed as a real COM coclass cannot in general have a constructor that takes arguments. There's no mechanism for the COM client code to pass arguments to the CoCreateInstance() function. A COM interface typically has an Initialize() method to supply required initialization. Technically that can be doctored as well, CoCreateInstance is just a convenience function that hides the class factory (IClassFactory). A custom class factory with a custom CreateInstance() method is possible, but rarely done.

Microsoft uses the interface-based programming model that's common in COM code in non-COM code as well. DirectX is a good example. Think of Direct3DCreate9Ex() as the class factory.

edited Apr 6, 2012 at 16:12

answered Apr 6, 2012 at 15:33

Hans Passant

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