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

compiled/subscription/Subscription.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 81 matching lines @@
     return mFilters.size();
   }
 
   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();
+  template<typename T,
+    typename std::enable_if<std::is_base_of<Subscription, T>::value>::type* = nullptr>

[sergei, 2017/10/13 12:34:36]

static_cast does not allow down casting at a compile time in the case of absent
corresponding inheritance, so I would expect that one can remove this SFINAE.
Even if it's required, in addition it might be a bit shorter (C++14)
typename = std::enable_if_t<std::is_base_of<Subscription, T>::value>.

[Wladimir Palant, 2017/10/13 16:58:18]

You are correct, removed.

+  T* As()
+  {
+    if (mType != T::classType)
+      return nullptr;
+
+    return static_cast<T*>(this);

[sergei, 2017/10/13 12:34:36]

In addition, since T::classType seems already a convention, the body of this
method could be in some another function abp_down_cast where one can put any
kind of SFINAE checks regarding the possibility of inheritance or correctness of
T::classType, etc.

[Wladimir Palant, 2017/10/13 16:58:18]

Note that we have different checks for Filter and Subscription subclasses, the
latter getting the simpler version. With Subscription not having an extended
hierarchy I didn't consider implementing mType as a bitmask justified.

+  }
+
+  template<typename T,
+    typename std::enable_if<std::is_base_of<Subscription, T>::value>::type* = nullptr>
+  const T* As() const
+  {
+    if (mType != T::classType)
+      return nullptr;
+
+    return static_cast<const T*>(this);
+  }
 };
 
 typedef intrusive_ptr<Subscription> SubscriptionPtr;