Re: [Boost-bugs] [Boost C++ Libraries] #10639: lexical_cast<double>(string) wrong in C++11

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #10639: lexical_cast<double>(string) wrong in C++11
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2014-10-11 09:40:47

#10639: lexical_cast<double>(string) wrong in C++11
--------------------------------+--------------------------
  Reporter:  richard.kreckel@…  |      Owner:  apolukhin
      Type:  Bugs               |     Status:  assigned
 Milestone:  To Be Determined   |  Component:  lexical_cast
   Version:  Boost 1.56.0       |   Severity:  Problem
Resolution:                     |   Keywords:
--------------------------------+--------------------------
Changes (by apolukhin):

 * status:  new => assigned

Comment:

 The problem seems to be much worse than it looks like.

 CLANG-3.4 and GCC-4.8.2 produce same results in C++03 and C++11 mode.
 Moreover, code in LexicalCast that does the conversion '''must work
 exactly the same way''' in C++11 and C++03. It is pretty simple and does
 not use real number types until the very end:

 {{{
     const wide_result_t result =
 std::pow(static_cast<wide_result_t>(10.0), pow_of_10) * mantissa;
     value = static_cast<T>( has_minus ? (boost::math::changesign)(result)
 : result);
 }}}

 This makes me think that there's probably some precision error in
 `std::pow`.

 Unfortunately I have no access to GCC-4.9 and Clang-3.5 at this moment, so
 I can not investigate this issue further. Please, could someone do it?


 Thanks for the [http://www.exploringbinary.com/real-c-rounding-is-perfect-

 gcc-now-converts-correctly/ link]! I'll put the test cases from it to the
 lexical cast auto tests and in case of errors will fallback to something
 like std::num_get. Maybe even with tests passing fallback to num_get will
 be done: current algo heavily relies on hardware precision and does not
 work in some cases (issue #6975).

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/10639#comment:2>
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