How to host an IContextMenu, part 11 - Composite extensions - composition

re: How to host an IContextMenu, part 11 - Composite extensions - composition

Jack Mathews — Thu, 07 Oct 2004 16:42:00 GMT

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Since a C++ constructor cannot fail, there are various conventions for how one handles failure during construction. One convention, which I use here, is to put the bulk of the work in an Initialize method, which can return an appropriate error code if the initialization fails.
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Actually, the way you fail a C++ ctor is to throw an exception. It will unwind the stack properly and will destroy appropriate obects if you throw in the middle of a ctor initialization.

By putting your work in Initialize methods, you run the risk of not having fully formed objects, so that if an exception happens in between your ctor and initialize, your dtor could crash, causing all sorts of badness.

re: How to host an IContextMenu, part 11 - Composite extensions - composition

Mike Dunn — Thu, 07 Oct 2004 17:07:00 GMT

I'll read the full post during my lunch break, but after a quick skim I'm confused by your CCompositeContextMenu::QueryInterface. If you do these steps:
IUnknown* punk;
punk = static_cast<IContextMenu*>(this);
*ppv = punk;
Won't the first assignment counteract the cast, since by C++ rules the rvalue gets converted to an IUnknown*? ((IContextMenu*)this) is going to be a different value than ((IUnknown*)this), so wouldn't *ppv end up pointing at the wrong place in the vtbl?

re: How to host an IContextMenu, part 11 - Composite extensions - composition

Raymond Chen — Thu, 07 Oct 2004 17:50:00 GMT

It's great to use a throwing model if your entire program follows it, but COM does not allow exceptions to be thrown across the COM boundary.

Mike: I discussed this yesterday in a comment.

http://weblogs.asp.net/oldnewthing/archive/2004/10/06/238630.aspx#239019

The double-cast works due to the rules for COM object layout. COM requires that

(void*)(IBaseInterface*)(IDerivedInterface*)p == (void*)(IDerivedInterface*)p

In other words, the implicit cast to IUnknown is required by COM rules not to change the numeric value of p.

Most C++ compilers for Windows will conform to COM object layout rules if you set the right compiler flags or use the right macros.

http://weblogs.asp.net/oldnewthing/archive/2004/10/05/238050.aspx

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