delphi - Why do I get "type has no typeinfo" error with an enum type

Question

I have declared the following enum type in which I want the first member to have the ordinal value of 1 (one) rather than the usual 0 (zero):

  type
    TMyEnum = (
               meFirstValue = 1,
               meSecondValue,
               meThirdValue
              );

If I call TypeInfo(), e.g. as part of a call to GetEnumName(), I get a compiler error:

  GetEnumName(TypeInfo(TMyEnum), Ord(aValue));

ERROR: "E2134: Type 'TMyEnum' has no typeinfo"

Why is this?

I know that classes only have typeinfo if they are compiled with the $M compiler option enabled or (derive from some class which was, such as TPersistent) but I didn't think there were any special conditions for having typeinfo for enum types.

Answer 1

Discontiguous enumerations, and enumerations which don't start at zero, don't have typeinfo. For typeinfo to be implemented, it would need to be in a different format from the existing tkEnumeration, owing to backward compatibility issues.

I considered implementing a tkDiscontiguousEnumeration (or possibly better named member) for Delphi 2010, but the benefit seemed small considering their relative scarcity and the difficulties in enumeration - how do you encode the ranges efficiently? Some encodings are better for some scenarios, worse for others.