Best practices for using com_ptr to create factory objects? about wil HOT 6 CLOSED

I've created #93 which adds put_void() and put_unknown() to com_ptr and allows you to pass a CLSID to wil::CoCreateInstance.

from wil.

Wow, that was quick, awesome!
I like the new overloads for CoCreateInstance -- now my code will look much nicer.

The new put_void member fn matches other smart COM pointers, so it makes transition a bit easier, so that's also nice. I wonder about put_unknown -- IUnknown is the base interface, but in some cases com calls do return other interfaces, e.g. ID2D1Factory::CreateWicBitmapRenderTarget returns ID2D1RenderTarget, where you might actually want to hold ID2D1BitmapRenderTarget. So, I'm thinking it might be better to have a member function template, say put_as<T> (with a better name though!), that does the reinterpret_cast, possibly with an extra check to ensure that T is the base class of whatever interface is held by the smart pointer?

from wil.

Wow talk about consistency :/
Since this is still a PR, would you be fine with just put_as<void> instead of put_void?

from wil.

Absolutely! Whatever you think is best.
Just pointing out, that a separate put_void is consistent with com_ptr::put_void in WinRT, so that's one less search & replace when switching between the two. Personally, I don't care about that (at all), so feel free to get rid of it...

from wil.

but in some cases com calls do return other interfaces, e.g. ID2D1Factory::CreateWicBitmapRenderTarget returns ID2D1RenderTarget, where you might actually want to hold ID2D1BitmapRenderTarget.

After rereading don't think this is a great idea. You should use an wil::com_ptr<ID2D1RenderTarget> and use QueryInterface after the call to make sure it is actually an ID2D1BitmapRenderTarget, because you never know when that implementation detail may be changed.

from wil.

That's a very good point! In that case, there's nothing else to be added to your fix, I think.
Thanks!

from wil.