From 7880adef89bde253a982b01275b92d73ba18f5e6 Mon Sep 17 00:00:00 2001
From: Josh Holtrop <jholtrop@gmail.com>
Date: Tue, 24 Feb 2026 15:48:41 -0500
Subject: [PATCH] Add static_assert to upcasting constructor; add
 rcp_dynamic_cast()

---
 include/rcp.h  | 10 +++++++++-
 test/tests.cpp | 27 +++++++++++++++++++++++++++
 2 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/include/rcp.h b/include/rcp.h
index 51d28ec..50a6d7d 100644
--- a/include/rcp.h
+++ b/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 \
diff --git a/test/tests.cpp b/test/tests.cpp
index ac8a3de..dcca39a 100644
--- a/test/tests.cpp
+++ b/test/tests.cpp
@@ -181,6 +181,30 @@ void test_move_assignment_releases_existing()
     assert(mybase_destruct == destructed_before + 2);
 }
 
+void test_upcast()
+{
+    rcp<MyDerived> derived = MyDerived::create(1.5);
+    rcp<MyBase> base = derived;
+    assert(base);
+    assert(base->x == 1);
+}
+
+void test_dynamic_cast_success()
+{
+    rcp<MyDerived> derived = MyDerived::create(1.5);
+    rcp<MyBase> base = derived;
+    rcp<MyDerived> back = rcp_dynamic_cast<MyDerived>(base);
+    assert(back);
+    assert(back->v == 1.5);
+}
+
+void test_dynamic_cast_failure()
+{
+    rcp<MyBase> base = MyBase::create(1, 2);
+    rcp<MyDerived> derived = rcp_dynamic_cast<MyDerived>(base);
+    assert(!derived);
+}
+
 int main(int argc, char * argv[])
 {
     test_class_hierarchy();
@@ -192,5 +216,8 @@ int main(int argc, char * argv[])
     test_move_constructor();
     test_move_assignment();
     test_move_assignment_releases_existing();
+    test_upcast();
+    test_dynamic_cast_success();
+    test_dynamic_cast_failure();
     return 0;
 }