Add static_assert to upcasting constructor; add rcp_dynamic_cast()

include/rcp.h

@@ -23,6 +23,7 @@
 #pragma once
 
 #include <atomic>
+#include <type_traits>
 
 /**
  * Reference-counting pointer.
@@ -97,10 +98,11 @@ public:
         }
     }
 
-    /* Casting constructor: allows rcp<Base> -> rcp<Derived> */
+    /* Upcasting constructor: allows rcp<Derived> -> rcp<Base> */
     template <typename U>
     rcp(const rcp<U> & other) : ptr(static_cast<T *>(other.get_raw()))
     {
+        static_assert(std::is_base_of<T, U>::value, "rcp: implicit cast must be an upcast");
         if (ptr)
         {
             ptr->rcp_inc();
@@ -154,6 +156,12 @@ public:
     }
 };
 
+template <typename T, typename U>
+rcp<T> rcp_dynamic_cast(const rcp<U> & other)
+{
+    return rcp<T>(dynamic_cast<T *>(other.get_raw()));
+}
+
 #define rcp_managed_root(classname) \
     public: \
         void rcp_inc() const \