| Subject: Lexical_cast improvement not included in 1_31 release | | I proposed some time ago an improvement to lexical_cast to | deal with the corner cases where there were insufficient | decimal digits to deal with some float values. For example, | a 'loopback' test should work for any type T and any value l: | | template <typename T> bool test(T l) | { | cout << l << ' '; | string sl = lexical_cast<string>(l); | cout << sl << ' '; | T ll = lexical_cast<T >(sl); | cout << ll << ' ' | << (l == lexical_cast<T>(lexical_cast<string>(l))) | // Convert type T to string and back to T. | << endl; // eg for long 2147483647 2147483647 2147483647 true | return (l == lexical_cast<T>(lexical_cast<string>(l))); | } // test | | The current version fails this test. | | There seemed no dissent at the time to calculating the | significant digits using std::numeric_limits<Target>::digits10 +1. | | But alas my suggestion (below) has not been included in the | latest release. | | lexical_stream() | { | stream.unsetf(std::ios::skipws); | // was: | /*if(std::numeric_limits<Target>::is_specialized) | stream.precision(std::numeric_limits<Target>::digits10 +1 ); | else if(std::numeric_limits<Source>::is_specialized) | stream.precision(std::numeric_limits<Source>::digits10 +1 );*/ | | Suggested: | | // New calculation to give more accurate number of | significant decimal digits: | // 32 significand bits | digits10 = 6 significant_digits10 = 9 | // 53 significand bits | digits10 = 15 significant_digits10 = 17 | // 64 significand bits | digits10 = 18 significant_digits10 = 21 | // 106 significand bits | digits10 = 31 significant_digits10 = 33 | // 113 significand bits | digits10 = 33 significant_digits10 = 36 | // 128 significand bits | digits10 = 38 significant_digits10 = 40 | | if(std::numeric_limits<Target>::is_specialized && | std::numeric_limits<Target>::radix == 2) { | stream.precision(2 + std::numeric_limits<Target>::digits * | 301/1000); } else | if(std::numeric_limits<Source>::is_specialized && | std::numeric_limits<Source>::radix == 2) { | stream.precision(2 + std::numeric_limits<Source>::digits * | 301/1000); } // Else if not specialized or unusual radix then | use default stream precision, probably 6. // All builtin | types, bool, char, integer and floating point, should be | specialized. } // lexical_stream() | | It is also irritating that current version produces a long | list of spurious 'error' messages which can avoided by | inheriting from boost noncopyable (suggested by Reece Dunn). | | What should I do to progress this? | | Prepare a Boost test for lexical_cast using the Boost Test framework? | | Paul | | Paul A Bristow, | Prizet Farmhouse, Kendal, Cumbria UK LA8 8AB | +44 1539 561830 +44 7714 330204 | mailto: pbristow@hetp.u-net.com | |