json/tree: explain lack of scalar type operators

json/tree.hpp

@@ -67,6 +67,9 @@ namespace json { namespace tree {
             virtual boolean& as_boolean (void);
             virtual null&    as_null    (void);
 
+            // we don't provide operators for conversion due to ambiguities
+            // introduced when using indexing operators with pointer
+            // arguments.
             virtual bool   as_bool   (void) const;
             virtual float  as_float  (void) const;
             virtual double as_double (void) const;