Re: Enum bitfield (Visual Studio Bug or not?)
Jason Hise ha scritto:
I opened the following bug report.
https://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=366557
To summarize, when you use an enum as a bitfield, MSVC will sign-
extend its value when performing comparisons. This leads to the
unintuitive result that if the bitfield is just large enough to
represent all possible enum values, the values in the second half of
the range will not compare equal to themselves.
Consider:
enum foo { a, b, c, d };
struct bar { foo f : 2; };
// In some function...
bar b;
b.f = d;
if (b.f != d) cout << "This gets printed.";
VC++ considers b.f to hold the value -1, because it sees the bitfield
as signed. This results in a comparison of -1 against 3, which of
course is false.
G++ does not exhibit this behavior, and appears to compare enum
bitfields the way you would expect.
Which of these compilers, if either, is in violation of the standard?
What is the correct behavior?
G++ is right and VC++ is wrong. 9.6/4: "If the value of an enumerator is
stored into a bit-field of the same enumeration type and the number of
bits in the bit-field is large enough to hold all the values of that
enumeration type, the original enumerator value and the value of the
bit-field shall compare equal."
HTH,
Ganesh
--
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]