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

compiled/subscription/Subscription.h

Index: compiled/subscription/Subscription.h
===================================================================
--- a/compiled/subscription/Subscription.h
+++ b/compiled/subscription/Subscription.h
@@ -12,17 +12,16 @@
  * 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 "../filter/Filter.h"
 #include "../String.h"
 #include "../FilterNotifier.h"
 #include "../intrusive_ptr.h"
 #include "../debug.h"
 #include "../bindings/runtime.h"
@@ -95,12 +94,27 @@ public:
   Filter* BINDINGS_EXPORTED FilterAt(Filters::size_type index);
   int BINDINGS_EXPORTED IndexOfFilter(const Filter& filter);
   OwnedString BINDINGS_EXPORTED Serialize() const;
   OwnedString BINDINGS_EXPORTED SerializeFilters() const;
 
   static Subscription* BINDINGS_EXPORTED FromID(const String& id);
 
   template<typename T>
-  T* As();
+  T* As()
+  {
+    if (mType != T::classType)
+      return nullptr;
+
+    return static_cast<T*>(this);
+  }
+
+  template<typename T>
+  const T* As() const
+  {
+    if (mType != T::classType)
+      return nullptr;
+
+    return static_cast<const T*>(this);
+  }
 };
 
 typedef intrusive_ptr<Subscription> SubscriptionPtr;