random technical notes

The Windows Runtime C++ Template Library (WRL) is a COM-based template library that provides a low-level way to use Windows Runtime components.

The Windows Runtime is implemented by using Component Object Model (COM) technology. COM depends on reference-counting to manage the lifetime of objects, and other housekeeping techniques, and on testing HRESULT values to determine whether an operation succeeded or failed. To successfully write a COM app or library, you must carefully follow COM rules and techniques.

The Visual C++ component extensions (C++/CX) is a high-level, language-based way to use Windows Runtime components. Both the WRL and C++/CX simplify the writing of code for the Windows Runtime by automatically performing COM housekeeping tasks on your behalf.

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Additional info: Visual C++ WinRT FAQ – WRL vs C++/CX

• WRL is a non-extension-based ISO compliant alternative to using C++/CX when targeting WinRT. It’s fairly obvious that C++/CX is far simpler to use than WRL, specially when creating components (consuming components is relatively easier).
• The big thing with WRL is that you can use ISO C++. You don’t have to learn a new syntactic extension that you cannot use outside the Microsoft-world. Most C++ devs would feel comfortable using WRL, specially if they’ve used ATL before. That said portability is a myth, since WRL code is as tied into Windows as would be C++/CX code.
• Do you want to totally avoid exceptions (perhaps to remain in sync with existing code that doesn’t use exceptions)? If so, you have to use WRL since C++/CX uses exceptions.
• Performance wise, will you see any difference? As stated above, C++/CX uses exceptions while WRL uses HRESULTs. So the performance implications of using exceptions will obviously come into play. There is also the non-trivial conversion between HRESULTs and RT exceptions. Outide of that, I don’t think there’s going to be any noticable difference in performance.
• Not sure to what extent you can do this, but since WRL exposes the underlying COM architecture, you can fine-tune your code to some degree (since WinRT is built on top of COM). I haven’t read or heard about any scenarios where this has actually made a difference.
• The psycological aspect. While this is the least technical of the reasons, it might be the biggest factor here. Many C++ devs would simply hate anything that they see as foreign syntax. And C++/CX is certainly not ISO C++. Its close similarity with C++/CLI (which many C++ devs found disgusting) doesn’t help either. If your C++ dev team comprises mainly of a bunch of these guys, I reckon it’d be wise to just use WRL.