singleton: add swap modifier

singleton.hpp

@@ -49,6 +49,22 @@ namespace cruft {
         }
 
 
+        /// Swap the stored object pointer for the provided one and return
+        /// the old value.
+        ///
+        /// Useful for bypassing `instantiate` by initialising the pointer
+        /// to a pre-existing object.
+        ///
+        /// The caller to swap must free any resources associated with the
+        /// prior value and arrange for the new object to be safely destructed
+        /// in the future.
+        static SelfT*
+        swap (SelfT *next)
+        {
+            return std::exchange (instance, next);
+        }
+
+
         /// returns a reference to sole instantiated value
         ///
         /// `instantiate` must have already been called before `get` is called.