Allow comparison to nullptr on left side of operator

include/rcp.h

@@ -224,6 +224,18 @@ public:
     friend rcp<V> rcp_dynamic_cast(rcp<W> && other);
 };
 
+template <typename T>
+bool operator==(std::nullptr_t, const rcp<T> & p)
+{
+    return p == nullptr;
+}
+
+template <typename T>
+bool operator!=(std::nullptr_t, const rcp<T> & p)
+{
+    return p != nullptr;
+}
+
 namespace std
 {
     template <typename T>

tests.cpp

@@ -84,6 +84,8 @@ void test_booleans()
     rcp<MyDerived> myderived;
     assert(!myderived);
     assert(myderived == nullptr);
+    assert(nullptr != mybase);
+    assert(nullptr == myderived);
 }
 
 void test_create()