Re: [boost] Re: warnings in 1.28.0

From: Pete Becker (petebecker_at_[hidden])
Date: 2002-05-23 08:31:07

At 12:53 PM 5/23/2002 +0100, John Maddock wrote:
> >   enum _Ios_Seekdir { _M_ios_seekdir_end = 1L << 16 };
> >
> >     // 27.4.2.1.5  Type seekdir
> >     typedef _Ios_Seekdir seekdir;
> >     static const seekdir beg =          seekdir(0);
> >     static const seekdir cur =          seekdir(SEEK_CUR);
> >     static const seekdir end =          seekdir(SEEK_END);
>
>What? You're kidding me: surely this can't be legal code, none of
>ios_base::beg, cur or end are valid values of ios_base::seekdir are they?

The representation of an enumeration must have enough bits to hold the 
largest declared value. So with _Ios_Seekdir having an enumerator with the 
value 1 << 16, the enum must have 16 bits. As long as SEEK_CUR and SEEK_END 
fit in 16 bits they are legal values.

The reason for that rule is to allow things like:

enum flags { none = 0, first = 0x01, second = 0x02, third = 0x04 };
flags fst = flags(first | second | third);

        -- Pete

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