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

compiled/filter/Filter.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
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  * GNU General Public License for more details.
  *
  * You should have received a copy of the GNU General Public License
  * along with Adblock Plus.  If not, see <http://www.gnu.org/licenses/>.
  */
 
 #pragma once
 
+#include <type_traits>
 #include <vector>
 
 #include "../String.h"
 #include "../intrusive_ptr.h"
 #include "../debug.h"
 #include "../bindings/runtime.h"
 
 class Filter : public ref_counted
 {
 protected:
   OwnedString mText;
 
 public:
   enum Type
   {
     UNKNOWN = 0,
     INVALID = 1,
     COMMENT = 2,
-    BLOCKING = 3,
-    WHITELIST = 4,
-    ELEMHIDE = 5,
-    ELEMHIDEEXCEPTION = 6,
-    ELEMHIDEEMULATION = 7,
-    VALUE_COUNT = 8
+    ACTIVE = 4,
+    REGEXP = ACTIVE | 8,
+    BLOCKING = REGEXP | 16,
+    WHITELIST = REGEXP | 32,
+    ELEMHIDEBASE = ACTIVE | 64,
+    ELEMHIDE = ELEMHIDEBASE | 128,
+    ELEMHIDEEXCEPTION = ELEMHIDEBASE | 256,
+    ELEMHIDEEMULATION = ELEMHIDEBASE | 512
   };
 
   explicit Filter(Type type, const String& text);
   ~Filter();
 
   Type mType;
 
   const String& BINDINGS_EXPORTED GetText() const
   {
     return mText;
   }
 
   OwnedString BINDINGS_EXPORTED Serialize() const;
 
   static Filter* BINDINGS_EXPORTED FromText(DependentString& text);
+
+  template<typename T,
+    typename std::enable_if<std::is_base_of<Filter, T>::value>::type* = nullptr>
+  T* As()
+  {
+    if ((mType & T::classType) != T::classType)
+      return nullptr;
+
+    return static_cast<T*>(this);
+  }
+
+  template<typename T,
+    typename std::enable_if<std::is_base_of<Filter, T>::value>::type* = nullptr>
+  const T* As() const
+  {
+    if ((mType & T::classType) != T::classType)
+      return nullptr;
+
+    return static_cast<const T*>(this);
+  }
 };
 
 typedef intrusive_ptr<Filter> FilterPtr;