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

compiled/filter/ElemHideBase.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 @@
   explicit ElemHideBase(Type type, const String& text, const ElemHideData& data);
   static Type Parse(DependentString& text, ElemHideData& data);
 
   const DependentString BINDINGS_EXPORTED GetSelector() const
   {
     return mData.GetSelector(mText);
   }
 
   OwnedString BINDINGS_EXPORTED GetSelectorDomain() const;
 };
+
+
+template<>
+inline ElemHideBase* Filter::As<ElemHideBase>()
+{
+  if ((mType & Type::ELEMHIDEBASE) == Type::ELEMHIDEBASE)
+    return nullptr;
+
+  return static_cast<ElemHideBase*>(this);
+}