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ctype::do_widen() return on failureSection: 28.3.4.2.2.3 [locale.ctype.virtuals] Status: NAD Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The Effects and Returns clauses of the do_widen() member function of
the ctype facet fail to specify the behavior of the function on failure.
That the function may not be able to simply cast the narrow character
argument to the type of the result since doing so may yield the wrong value
for some wchar_t encodings. Popular implementations of ctype<wchar_t> that
use mbtowc() and UTF-8 as the native encoding (e.g., GNU glibc) will fail
when the argument's MSB is set. There is no way for the the rest of locale
and iostream to reliably detect this failure.
[Kona: This is a real problem. Widening can fail. It's unclear
what the solution should be. Returning WEOF works for the wchar_t
specialization, but not in general. One option might be to add a
default, like narrow. But that's an incompatible change.
Using traits::eof might seem like a good idea, but facets
don't have access to traits (a recurring problem). We could
have widen throw an exception, but that's a scary option;
existing library components aren't written with the assumption
that widen can throw.]
[ 2009-07 Frankfurt ]
NAD. The behavior is specified for all of the facets that an implementation is required to provide, for the basic character set.
Proposed resolution: