Incorrect implementation of codecvt<char, char, mbstate_t> in codecvt.cc

[Bo Persson]

Kristian Spangsege wrote:

> The implementation of codecvt<char, char, mbstate_t> is not in
> agreement with DR19 (TC). It may be due to a misinterpretation of
> the text there, and if so, the problem could also be present in the
> implementation of codecvt<wchar_t, char, mbstate_t> - I haven't
> checked that.
>
> From the latest SVN version of "codecvt.cc":
>
>> codecvt_base::result
>>  codecvt<char, char, mbstate_t>::
>>  do_out(state_type&, const intern_type* __from,
>>         const intern_type*, const intern_type*& __from_next,
>>         extern_type* __to, extern_type*,
>>         extern_type*& __to_next) const
>>  {
>>    // _GLIBCXX_RESOLVE_LIB_DEFECTS
>>    // According to the resolution of DR19, "If returns noconv [...]
>>    // there are no changes to the values in [to, to_limit)."
>>    __from_next = __from;
>>    __to_next = __to;
>>    return noconv;
>>  }
>
>
> '__from_next' is set equal to '__from'. According to DR19 it must
> instead be set equal to '__from_end' (the next argument after
> '__from'.)
>
>
> From DR19:
>
>> If returns noconv, internT and externT are the same type and the
>> converted sequence is identical to the input sequence
>> [from,from_next). to_next is set equal to to, the value of state is
>> unchanged, and there are no changes to the values in [to,
>> to_limit).
>
>
> The way I interpret this is as follows: If the return value is
> 'noconv' then it indicates that the initial section [from,from_next)
> of the input can be used directly as output since the converted
> sequence would by identical had it been computed.

Yes, but as there has been no conversion at all, what should the "end 
of conumed input sequence" be? Setting __from_next = __from indicates 
that no characters were used.


> The problem with the implementation above is that if I implement my
> application to conform with DR19, then it will enter an inifinite
> loop because the input position '__from_next' is not advanced.
>

I think your code will have to special case 'noconv' anyway, since it 
will have to output the from-sequence (all of it). Other cases will 
have to output the to-sequence (possibly in a loop, handling partial 
conversions).


Bo Persson