I’ve derived a class from error_category to categorize errors associated with my class.  Elsewhere, I want to test to see if an error code I’ve gotten (from an exception) is one of those, or not.  A line like

          if (X.code().category != mycategory) ...

should do the trick.  ‘mycategory’ is the same object used to create error codes in my class: a single instance of the derived error_category class.

 

However, this gives ambiguities in matching the operator.

 

This makes it quite unfriendly to use.  I have to cast the right-hand-side to the base class reference, for example.

 

If I reversed the arguments, the list of possible matches the compiler mentioned did not even include the one I wanted.

 

As this is a TR2 candidate, I think such rough edges should be examined and avoided.

 

--John