Re: (no subject)
Re: (no subject)
- Subject: Re: (no subject)
- From: Steve Checkoway <email@hidden>
- Date: Fri, 24 Feb 2006 15:41:27 -0800
On Feb 24, 2006, at 2:17 PM, Brant Sears wrote:
Hi. I have a question about some code that is OK in gcc 3.3, but
gcc 4 doesn't like.
for (NSCellAttribute ca = NSCellDisabled; ca
<=NSCellAllowsMixedState; ++(int)ca) {
[myCell setCellAttribute:ca to:[modelCell cellAttribute:ca ]];
}
The warning that I'm getting states that "ca" is not really an
lvalue. I don't understand this. Why isn't "ca" an lvalue in the
above code fragment?
It's not an lvalue because you're casting it to an int. If you're
using c++, you can cast it to an int& but there's a comment in some
code I have that says:
/*
* Safely add an integer to an enum.
*
* This is illegal:
*
* ((int&)val) += iAmt;
*
* It breaks aliasing rules; the compiler is allowed to assume that
"val" doesn't
* change (unless it's declared volatile), and in some cases, you'll
end up getting
* old values for "val" following the add. (What's probably really
happening is
* that the memory location is being added to, but the value is stored
in a register,
* and breaking aliasing rules means the compiler doesn't know that
the register
* value is invalid.)
*
* Always do these conversions through a union.
*/
and the code is (more or less)
template<typename t>
static inline void enum_add( T &val, int iAmt )
{
union conv { T value; int i; } c;
c.value = val;
c.i += iAmt;
val = c.value;
}
Try just using an int instead of NSCellAttribute and casting it to
NSCellAttribute if the compiler complains.
Is it just philosophically wrong to try to iterate through an enum
like this?
Yes. If you really want you can use the above code with enum_add( ca,
1 ).
- Steve
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| | >(no subject) (From: "Brant Sears" <email@hidden>) |