Let's =default to avoid an out of line definition question

CppCoreGuidelines.md

@@ -2122,7 +2122,7 @@ This will force every derived class to compute a center -- even if that's non-tr
         // ...
         // ... no data members ...
         // ...
-        virtual ~Shape() = 0;
+        virtual ~Shape() = default;
     };
 
 ##### Enforcement