• d-bugmail puremagic.com Nov 06 2009
• d-bugmail puremagic.com Nov 08 2009
• d-bugmail puremagic.com Nov 20 2009
• d-bugmail puremagic.com Nov 20 2009

d-bugmail puremagic.com writes:

http://d.puremagic.com/issues/show_bug.cgi?id=3483

           Summary: Eliminate read-modify-write operations for enums
           Product: D
           Version: unspecified
          Platform: Other
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: andrei metalanguage.com


--- Comment #0 from Andrei Alexandrescu <andrei metalanguage.com> 2009-11-06
22:16:03 PST ---
Consider:

enum E { a = 5, b = 10, c = 15; }

Arithmetic involving an E are accepted, e.g. E.a + 42, and that's fine because
they yield type int which is not convertible back to E. The problem is that
read-modify-write expressions are allowed too:

E.a x;
x += 42;

Such operations are nonsensical because they take an enum value easily in
places that have nothing to do with the actual defined values.

All operations <op>= and also ++ and -- should be disabled for enums.

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d-bugmail puremagic.com writes:

http://d.puremagic.com/issues/show_bug.cgi?id=3483


Stewart Gordon <smjg iname.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |smjg iname.com


--- Comment #1 from Stewart Gordon <smjg iname.com> 2009-11-08 06:51:35 PST ---
This is going to break uses of enums as bit flags, which is a valid use
according to the docs.

Maybe we need a separate "bitflags" type, which would differ in this and other
respects....

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d-bugmail puremagic.com writes:

http://d.puremagic.com/issues/show_bug.cgi?id=3483



--- Comment #2 from Stewart Gordon <smjg iname.com> 2009-11-20 14:55:18 PST ---
Moreover, "next value" is a perfectly valid concept for enums, so I don't get
why you want ++/-- removed at all.

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d-bugmail puremagic.com writes:

http://d.puremagic.com/issues/show_bug.cgi?id=3483



--- Comment #3 from Andrei Alexandrescu <andrei metalanguage.com> 2009-11-20
16:54:22 PST ---
(In reply to comment #2)
 Moreover, "next value" is a perfectly valid concept for enums, so I don't get
 why you want ++/-- removed at all.


It would be just too odd and requiring a lot of complication to define
properly. 

enum E { a = 5, b = 8, c = 15; }

Increment of an enum really becomes a series of if/else statements (if it's 5
make it 8 etc.) or would just increment the actual value leading to values that
are not part of the enum. What to do? Then what do you do when you are at the
end of the scale and want to increment?

Answers could be found, the problem is that reasonable people may think
different answers are best, and expect different behaviors.

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