[committed] libstdc++: Add std::from_chars for floating-point types

[Jonathan Wakely]

On 27/07/20 10:46 +0100, Jonathan Wakely wrote:
>On 27/07/20 11:41 +0200, Rainer Orth wrote:
>>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.
>
>Ah yes, the tests should not run unconditionally. I'll fix that today,
>thanks.

This should be fixed at 2251b4a5423efa8ee0d7e67537b63e404a1f6afa.