Issue 5258 - Implement Filter::As<>() method to make working with filters easier from C++ code

compiled/filter/ActiveFilter.h

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-present eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 50 matching lines @@
   FILTER_PROPERTY(unsigned int, mHitCount, FILTER_HITCOUNT,
       GetHitCount, SetHitCount);
   FILTER_PROPERTY(unsigned int, mLastHit, FILTER_LASTHIT,
       GetLastHit, SetLastHit);
   bool BINDINGS_EXPORTED IsActiveOnDomain(DependentString& docDomain,
       const String& sitekey) const;
   bool BINDINGS_EXPORTED IsActiveOnlyOnDomain(DependentString& docDomain) const;
   bool BINDINGS_EXPORTED IsGeneric() const;
   OwnedString BINDINGS_EXPORTED Serialize() const;
 };
+
+template<>
+inline ActiveFilter* Filter::As<ActiveFilter>()
+{
+  if ((mType & Type::ACTIVE) == Type::ACTIVE)

[sergei, 2017/10/12 12:24:34]

I wonder whether concrete type value can be a class member (enum, static const,
etc) and this method can be a template for all classes.

[hub, 2017/10/12 13:51:33]

You can have static const, but then for linkage purpose it needs to be put in a
.cpp.

You can also have a class method like this

  static Type GetClassType()
  {
    return Type::FILTER;
  }

And then have Filter::As<> call T::GetClassType()

It will be statically looked up.

And since Filter doesn't have it calling As<Filter>() will fail.

[sergei, 2017/10/12 13:57:31]

A method is also fine for me, can we use constexpr? I would expect no overhead
from any of the approaches, even from static const which actually looks very
questionable but I have googled a bit regarding it while reviewing some another
review and it seems fine too.

[hub, 2017/10/12 14:03:01]

Yes you can. I recommend doing so.

[Wladimir Palant, 2017/10/13 11:43:28]

Done.

+    return nullptr;
+
+  return static_cast<ActiveFilter*>(this);
+}