exceptionsand const

Added to E.15

CppCoreGuidelines.md

@@ -14139,10 +14139,16 @@ To prevent slicing.
         // ...
     }
 
-Instead, use:
+Instead, use a reference:
 
     catch (exception& e) { /* ... */ }
 
+of - typically better stil - a `const` reference:
+
+    catch (const exception& e) { /* ... */ }
+
+Most handlers do not modify their exception and in general we [recommend use of `const`](#Res-const).
+
 ##### Enforcement
 
 Flag by-value exceptions if their types are part of a hierarchy (could require whole-program analysis to be perfect).