I'm having trouble understanding how 22.2.2.1.2 "num_get virtual
function" allows conversion of sequences that start with the
characters
"0x".  I.e., I think the following should be ok,

   istringstream iss("0x1");
   iss.setf(ios_base::hex, ios_base::basefield);
   int one = 2;
   iss >> one;
   assert(one == 1);
   assert(iss);

but I can't see how this is permitted by 22.2.2.1.2.  In particular,
when we hit the 'x', it looks to me like the Stage 2 "as if" code
fragment sets c to '\0':

<snip>
Stage 2: If in==end then stage 2 terminates. Otherwise a charT
is taken from in and local variables are initialized as if by

   char_type ct = *in ;
   char c = src[find(atoms, atoms + sizeof(src) - 1, ct) - atoms];
   if ( ct == use_facet<numpunct<charT> >(loc).decimal_point() )
       c = '.';
   bool discard =
       ( ct == use_facet<numpunct<charT> >(loc).thousands_sep()
           &&
       use_facet<numpunct<charT> >(loc).grouping().length() != 0 );

where the values src and atoms are defined as if by:

   static const char src[] = "0123456789abcdefABCDEF+-";
   char_type atoms[sizeof(src)];
   use_facet<ctype<charT> >(loc).widen(src, src + sizeof(src),
atoms);

for this value of loc.
</snip>

Why?  Because there's no 'x' in src, so the find "fails", returning
atoms+sizeof(src)-1, and hence c='\0'.  Stage 2 should stop at this
point, just as it would if ct were 'y' or 'z', and the conversion
should be applied to the accumulated characters so far, setting i to 0
and leaving "x1" unprocessed.

But that's not what happens, and I don't think it's what anyone
intended.  So what have I missed?

John

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On Dec 5, 11:29 pm, jksalmon <john.sal...@gmail.com> wrote:
> I'm having trouble understanding how 22.2.2.1.2 "num_get virtual
> function" allows conversion of sequences that start with the
> characters "0x".
...
> <snip>
>    ...
>    static const char src[] = "0123456789abcdefABCDEF+-";
>    char_type atoms[sizeof(src)];
>    use_facet<ctype<charT> >(loc).widen(src, src + sizeof(src),
> atoms);
>
> for this value of loc.
> </snip>
>
> Why?  Because there's no 'x' in src, so the find "fails",
> ...

                   ]

Aha!  I see that this has changed in the Draft Standard, N2914-09-104
dated 2009-06-22.  The letters 'X' and 'x' appear in the src string.

John



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