Issue 4125 - [emscripten] Convert filter classes to C++

compiled/intrusive_ptr.h

 // Parts of this code have been copied from boost/smart_ptr/intrusive_ptr.hpp.
 //
 //  Copyright (c) 2001, 2002 Peter Dimov
 //
 // Distributed under the Boost Software License, Version 1.0. (See
 // accompanying file LICENSE_1_0.txt or copy at
 // http://www.boost.org/LICENSE_1_0.txt)
 
 #pragma once
 
@@ skipping 16 matching lines @@
     if (--mRefCount == 0)
       delete this;
   }
 
 protected:
   ref_counted()
       : mRefCount(1)
   {
   }
 
-  // We need this virtual destructor, otherwise the numerical value of
-  // a ref_counted pointer and the numerical value of a pointer to a derived
-  // class will no longer be identical - despite still pointing to the same
-  // object. So for example `(int)(Filter*)ptr - (int)(ref_counted*)ptr` will be
-  // four, something that presents an issue for our JavaScript bindings which
-  // don't know how to convert pointers between different types.
   virtual ~ref_counted()
   {
     assert(mRefCount == 0, u"Destroying a ref-counted object with a non-zero reference count"_str);
   }
 
 private:
   int mRefCount;
 };
 
 template<typename T,
     class = typename std::enable_if<std::is_base_of<ref_counted,T>::value>::type>
 class intrusive_ptr
 {
 public:
-  intrusive_ptr()
+  explicit intrusive_ptr()
       : mPointer(nullptr)
   {
   }
 
   explicit intrusive_ptr(T* pointer)
       : mPointer(pointer)
   {
     // Raw pointers always had their reference count increased by whatever gave
     // us the pointer so we don't need to do it here.
   }
@@ skipping 129 matching lines @@
 inline bool operator==(const T* a, const intrusive_ptr<U>& b)
 {
   return a == b.get();
 }
 
 template<typename T, typename U>
 inline bool operator!=(const T* a, intrusive_ptr<U> const& b)
 {
   return a != b.get();
 }