On Sun, 25 Oct 2009 02:22:51 +0300, Michel Fortin
<[email protected]> wrote:
On 2009-10-24 18:45:10 -0400, Justin Johansson <[email protected]> said:
Let me try again. You have a set of N things which are represented by
an enum definition. There is some subset of this set containing M
things (so M < N) which represents the things meaningful in a public
sense. The remaining Q=N-M members of the whole set are only meaningful
to some internal processing so this subset containing Q members are
designated private. I'm trying to think about this problem in an
abstract
way; sometimes giving concrete examples like Color colors (pun intended)
the abstract problem.
Sounds like you want enum inheritance: one enum being a superset of
another. :-)
It was previously proposed, but it was rejected because it works
"backwards": downcasting is allowed while upcasting is prohibited.
Consider the following example (taken from DMD):
enum Token
{
colon,
semicolon,
eof,
// ...
max,
}
enum AsmToken : Token // "extend" an existing enum
{
dword, // = Token.max + 1 implicitly
even,
far,
naked,
near,
ptr,
qword,
seg,
word,
}
Now this is very useful, and less error prone than manual enum values
management. Less jump to the usage:
AsmToken tok1 = Token.colon; // fine
Token tok2 = AsmToken.dword; // error
The two lines above are exact opposite of class/interface inheritance
behavior:
class Token {}
class AsmToken : Token {}
AsmToken tok1 = new Token(); // error
Token tok2 = new AsmToken(); // fine
which makes sense but confusing for novices.
IIRC, Walter rejected the feature for exactly this reason.
