On May 26, 2009, at 10:21 AM, Darin Adler wrote:
On May 26, 2009, at 10:16 AM, Drew Wilson wrote:
OK, I've got two strong votes for the interface + static factory
approach. Any objections from the rest of the WebKit team? If I
don't hear any counter proposals, I'll do that.
I think it's unpleasant to pay run-time cost for a compile-time
choice. Sure, sometimes the extra cost is no big deal, but sometimes
it can be a big deal and I see no reason to choose idioms that use
virtual functions if there are equally good or better ones that don't.
Are there really no better techniques than abstract base classes and
virtual functions for this sort of compile-time switch? How about
the technique used for ResourceRequest and ResourceResponse? Maybe
Darin Fisher can explain that one.
I agree with Darin's comments here. We've tried hard to avoid using
runtime polymorphism for compile-time choices. Here it's probably not
performance-critical, but it can be avoided.
The ResourceRequestBase / ResourceRequest model (due to Darin Fisher)
seems pretty clean to me. I would like to see more of our classes with
port-specific implementation details move to this style. I think it
could work for SharedWorkerRepository.
The basic idea is this. Let's say you have a class FooBar.
- You define a FooBarBase class that has the cross-platform interface
and data members. But not all the methods are actually implemented in
the cross-platform code. All of its constructors are protected so the
class cannot be instantiated directly.
- Each port subclasses FooBarBase to define FooBar, adding
constructors, platform-specific data members, and any needed platform-
specific private helpers or type conversions.
- Each port implements the methods of FooBarBase that are platform-
specific, freely downcasting to FooBar when needed since we have
guaranteed that every instance of FooBarBase is actually a FooBar.
- Cross-platform code using the class just uses FooBar. The Base class
is an implementation detail.
(Darin F., please correct me if I have not done justice to this
technique.)
Note that this method has no runtime cost - there's no need to use
virtual methods or other forms of runtime indirection. And there's no
need to #ifdef any headers, everything is controlled purely by
including the right platform specific FooBar.h so it can be handled by
include paths. It's a little subtle at first but I think it results in
nice, understandable code.
I think we should document this technique as the preferred way to make
classes with port-specific implementation details and convert more of
WebCore/platform/ to this technique, as well as using it for new
classes.
Regards,
Maciej
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