Hi Jonathan,
> This adds the missing std::from_chars overloads for floating-point
> types, as required for C++17 conformance.
>
> The implementation is a hack and not intended to be used in the long
> term. Rather than parsing the string directly, this determines the
> initial portion of the string that matches the pattern determined by the
> chars_format parameter, then creates a NTBS to be parsed by strtod (or
> strtold or strtof).
>
> Because creating a NTBS requires allocating memory, but std::from_chars
> is noexcept, we need to be careful to minimise allocation. Even after
> being careful, allocation failure is still possible, and so a
> non-conforming std::no_more_memory error code might be returned.
>
> Because strtod et al depend on the current locale, but std::from_chars
> does not, we change the current thread's locale to "C" using newlocale
> and uselocale before calling strtod, and restore it afterwards.
>
> Because strtod doesn't have the equivalent of a std::chars_format
> parameter, it has to examine the input to determine the format in use,
> even though the std::from_chars code has already parsed it once (or
> twice for large input strings!)
>
> By replacing the use of strtod we could avoid allocation, avoid changing
> locale, and use optimised code paths specific to each std::chars_format
> case. We would also get more portable behaviour, rather than depending
> on the presence of uselocale, and on any bugs or quirks of the target
> libc's strtod. Replacing strtod is a project for a later date.
two of the new tests FAIL on Solaris 11.3 only:
+FAIL: 20_util/from_chars/4.cc (test for excess errors)
+UNRESOLVED: 20_util/from_chars/4.cc compilation failed to produce executable
+FAIL: 20_util/from_chars/5.cc (test for excess errors)
+UNRESOLVED: 20_util/from_chars/5.cc compilation failed to produce executable
Excess errors:
/vol/gcc/src/hg/master/local/libstdc++-v3/testsuite/20_util/from_chars/4.cc:41:
error: no matching function for call to 'from_chars(char*, char*, double&,
std::chars_format&)'
[...]
AFAICT that's due to the fact that Solaris 11.3 (unlike 11.4) lacks
uselocale.
Rainer
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Rainer Orth, Center for Biotechnology, Bielefeld University
