Comment by jlarocco
5 days ago
I don't like Windows, but I've always thought COM was pretty cool. It's a nightmare using it directly from low level languages like C++ and Rust, though. It's a perfect place to use code generation or metaprogramming.
In Python, Ruby and the Microsoft languages COM objects integrate seamlessly into the language as instances of the built-in class types.
Also, there's a fairly straightfoward conversion from C# to C++ signatures, which becomes apparent after you see a few of them. It might be explicitly spelled out in the docs somewhere.
COM is basically just reference counting and interfaces. Also, the HRESULT type tries to give some structure to 32 bit error codes.
I remember a few years back hearing hate about COM and I didn't feel like they understood what it was.
I think the legit criticisms include:
* It relies heavily on function pointers (virtual calls) so this has performance costs. Also constantly checking those HRESULTs for errors, I guess, gives you a lot more branching than exceptions.
* The idea of registration, polluting the Windows registry. These days this part is pretty optional.
As somebody who's been, for whatever reason, toying around with writing a COM-style ABI layer in Rust, there's a lot of good ideas in there and I think a lot of the hatred comes from the DLL hell that was spawned by registration; along with the, unfortunately necessary, boilerplate.
Virtual dispatch absolutely has an overhead, but absolutely nobody in their right mind should be using COM interfaces in a critical section of code. When we're talking things like UI elements, HTTP clients, whatever, the overhead of an indirect call is negligible compared to the time spent inside a function.
The one thing I'm personally trying to see if there's any room for improvement on in a clean slate design, is error handling / HRESULT values. Exceptions get abused for flow control and stack unwinding is expensive, so even if there was a sane way to implement cross-language exception handling it's a non starter. But HRESULT leads to IErrorInfo, ISupportErrorInfo, thread local state SetErrorInfo/GetErrorInfo, which is a whole extra bunch of fun to deal with.
There's the option of going the GObject and AppKit route, using an out parameter for an Error type - but you have to worry about freeing/releasing this in your language bindings or risk leaking memory.
> Virtual dispatch absolutely has an overhead, but absolutely nobody in their right mind should be using COM interfaces in a critical section of code.
I could definitely be wrong, but I think C++ style "virtual dispatch" (ie, following two pointers instead of one to get to your function) doesn't really cost anything anymore, except for the extra pointers taking up cache space.
Don't all of the Windows DirectX gaming interfaces use COM? And isn't AAA gaming performance critical?
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Registration free COM has existed since Windows XP, if I get my timeline right without bothering to look it up.
All modern Windows APIs introduced since Vista have been COM, classical Win32 C APIs are seldom introduced nowadays.
Certainly current Windows 11 performance problems have nothing to do with using COM all over the place, rather Webwidgets instead of native code, hiring people that apparently never did Windows programming, that apparently do AI driven coding.
Ah, macOS and iDevices driver model is equally based in COM like design, one would expect drivers to be something where performance matters.
Then there is XPC, Android IPC, and one could consider D-BUS as well, if it was more widely adopted across the GNU/Linux world.
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WinRT is certainly not a "clean slate design", but still a useful comparison to see where Microsoft themselves iterated on the COM design with decades of hindsight.
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> COM is basically just reference counting and interfaces. > I remember a few years back hearing hate about COM and I didn't feel like they understood what it was.
Even in "core" COM there's also marshaling, the whole client/server IPC model, and apartments.
And, I think most people encounter COM with one of its friends attached (like in this case, OLE/Automation in the form of IDispatch), which adds an additional layer of complexity on top.
Honestly I think that COM is really nice, though. If they'd come up with some kind of user-friendly naming scheme instead of UUIDs, I don't even think it would get that much hate. It feels to me that 90% of the dislike for COM is the mental overhead of seeing and dealing with UUIDs when getting started.
Once you get past that part, it's really fast to do pretty complex stuff in; compared to the other things people have come up with like dbus or local gRPC and so on, it works really well for coordinating extensibility and lots of independent processes that need to work together.
Even the UUIDs aren't bad, they're a reasonable solution to Zooko's triangle. You can't globally assign names.
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You might have been hearing some of that hate from me. I definitely don't understand COM, but I've had to use it once or twice. It's pretty far outside what I normally work on, which is all high-level garbage collected languages. I don't know if that's even the right dimension to distinguish it. I couldn't figure out how to use COM or what it's purpose was.
The task was some automated jobs doing MS word automation. This all happened about 20 years ago. I never did figure out how to get it to stop leaking memory after a couple days of searching. I think I just had the process restart periodically.
Compared to what I was accustomed to COM seemed weird and just unnecessarily difficult to work with. I was a lot less experienced then, but I haven't touched COM since. I still don't know what the intent of COM is or where it's documented, and nor have I tried to figure it out. But it's colored my impression of COM ever since.
I think there may be a lot of people like me. They had to do some COM thing because it was the only way to accomplish a task, and just didn't understand. They randomly poked it until it kind of worked, and swore never to touch it again.
> I still don't know what the intent of COM is
COM is an ABI (application binary interface). You have two programs, compiled in different languages with different memory management strategies, potentially years apart. You want them to communicate. You either
-1 use a Foreign Function Interface (FFI) provided to those languages -2 serialize/deserialize data and send it over some channel like a socket
(2) is how the internet works so we've taken to doing it that way for many different systems, even if they don't need it. (1) is how operating systems work and how the kernel and other subsystems are exposed to user space.
The problem with FFI is that it's pretty barebones. You can move bytes and call functions, but there's no standard way of composing those bytes and function calls into higher level constructs like you use in OOP languages.
COM is a standard for defining that FFI layer using OOP patterns. Programs export objects which have well defined interfaces. There's a root interface all objects implement called "Unknown", and you can find out if an object supports another interface by calling `queryInterface()` with the id of a desired interface (all interfaces have a globally unique ID). You can make sure the object doesn't lose its data out of nowhere by calling `addRef()` to bump its reference count, and `release()` to decrement it (thus removing any ambiguity over memory management, for the most part - see TFA for an example where that fails).
> where it's documented
https://learn.microsoft.com/en-us/windows/win32/com/the-comp...
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> I couldn't figure out how to use COM or what it's purpose was.
A shorter version than the other reply:
COM allows you to have a reference counted object with callable virtual methods. You can also ask for different virtual methods at runtime (QueryInterface).
Some of the use cases include: maybe those methods are implemented in a completely different programming language that you are using, for example I think one of the historical ones is JavaScript or vbscript interacting with C++. It standardizes the virtual calls in such a way that you can throw in such an abstraction. And since reference counting happens via a virtual call, memory allocation is also up to the callee. Another historical use case is to have the calls be handled in a different process.
I'd say COM is also run-time type safe casting, and importantly the reference counting is uniform which might help writing wrappers for dynamic and garbage collected languages.
I'm still not sure that it brings a lot to the table for ordinary application development.
It's been a while since I've written it professionally, but I felt the fact that it has consistent idioms and conventions helped me be somewhat more productive writing C++. In the vast landscape of C++ features it winds up making some decisions for you. You can use whatever you want within your component but the COM interfaces dictate how you talk to outside.
Not if using Delphi or C++ Builder.
For whatever reason all attempts to make COM easier to use in Visual C++, keep being sabotaged by internal teams.
It is like Windows team feels like it is a manhood test to use such low level tooling.
Using COM in Perl was pretty seamless back in its heyday.