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  1. // Copyright Alexander Nasonov & Paul A. Bristow 2006.
  2.  
  3. // Use, modification and distribution are subject to the
  4. // Boost Software License, Version 1.0.
  5. // (See accompanying file LICENSE_1_0.txt
  6. // or copy at http://www.boost.org/LICENSE_1_0.txt)
  7.  
  8. #ifndef BOOST_DETAIL_LCAST_PRECISION_HPP_INCLUDED
  9. #define BOOST_DETAIL_LCAST_PRECISION_HPP_INCLUDED
  10.  
  11. #include <climits>
  12. #include <ios>
  13. #include <limits>
  14.  
  15. #include <boost/config.hpp>
  16. #include <boost/integer_traits.hpp>
  17.  
  18. #ifndef BOOST_NO_IS_ABSTRACT
  19. // Fix for SF:1358600 - lexical_cast & pure virtual functions & VC 8 STL
  20. #include <boost/mpl/if.hpp>
  21. #include <boost/type_traits/is_abstract.hpp>
  22. #endif
  23.  
  24. #if defined(BOOST_NO_LIMITS_COMPILE_TIME_CONSTANTS) || \
  25.   (defined(BOOST_MSVC) && (BOOST_MSVC<1310))
  26.  
  27. #define BOOST_LCAST_NO_COMPILE_TIME_PRECISION
  28. #endif
  29.  
  30. #ifdef BOOST_LCAST_NO_COMPILE_TIME_PRECISION
  31. #include <boost/assert.hpp>
  32. #else
  33. #include <boost/static_assert.hpp>
  34. #endif
  35.  
  36. namespace mars_boost {} namespace boost = mars_boost; namespace mars_boost { namespace detail {
  37.  
  38. class lcast_abstract_stub {};
  39.  
  40. #ifndef BOOST_LCAST_NO_COMPILE_TIME_PRECISION
  41. // Calculate an argument to pass to std::ios_base::precision from
  42. // lexical_cast. See alternative implementation for broken standard
  43. // libraries in lcast_get_precision below. Keep them in sync, please.
  44. template<class T>
  45. struct lcast_precision
  46. {
  47. #ifdef BOOST_NO_IS_ABSTRACT
  48.     typedef std::numeric_limits<T> limits; // No fix for SF:1358600.
  49. #else
  50.     typedef BOOST_DEDUCED_TYPENAME mars_boost::mpl::if_<
  51.         mars_boost::is_abstract<T>
  52.       , std::numeric_limits<lcast_abstract_stub>
  53.       , std::numeric_limits<T>
  54.       >::type limits;
  55. #endif
  56.  
  57.     BOOST_STATIC_CONSTANT(bool, use_default_precision =
  58.             !limits::is_specialized || limits::is_exact
  59.         );
  60.  
  61.     BOOST_STATIC_CONSTANT(bool, is_specialized_bin =
  62.             !use_default_precision &&
  63.             limits::radix == 2 && limits::digits > 0
  64.         );
  65.  
  66.     BOOST_STATIC_CONSTANT(bool, is_specialized_dec =
  67.             !use_default_precision &&
  68.             limits::radix == 10 && limits::digits10 > 0
  69.         );
  70.  
  71.     BOOST_STATIC_CONSTANT(std::streamsize, streamsize_max =
  72.             mars_boost::integer_traits<std::streamsize>::const_max
  73.         );
  74.  
  75.     BOOST_STATIC_CONSTANT(unsigned int, precision_dec = limits::digits10 + 1U);
  76.  
  77.     BOOST_STATIC_ASSERT(!is_specialized_dec ||
  78.             precision_dec <= streamsize_max + 0UL
  79.         );
  80.  
  81.     BOOST_STATIC_CONSTANT(unsigned long, precision_bin =
  82.             2UL + limits::digits * 30103UL / 100000UL
  83.         );
  84.  
  85.     BOOST_STATIC_ASSERT(!is_specialized_bin ||
  86.             (limits::digits + 0UL < ULONG_MAX / 30103UL &&
  87.             precision_bin > limits::digits10 + 0UL &&
  88.             precision_bin <= streamsize_max + 0UL)
  89.         );
  90.  
  91.     BOOST_STATIC_CONSTANT(std::streamsize, value =
  92.             is_specialized_bin ? precision_bin
  93.                                : is_specialized_dec ? precision_dec : 6
  94.         );
  95. };
  96. #endif
  97.  
  98. template<class T>
  99. inline std::streamsize lcast_get_precision(T* = 0)
  100. {
  101. #ifndef BOOST_LCAST_NO_COMPILE_TIME_PRECISION
  102.     return lcast_precision<T>::value;
  103. #else // Follow lcast_precision algorithm at run-time:
  104.  
  105. #ifdef BOOST_NO_IS_ABSTRACT
  106.     typedef std::numeric_limits<T> limits; // No fix for SF:1358600.
  107. #else
  108.     typedef BOOST_DEDUCED_TYPENAME mars_boost::mpl::if_<
  109.         mars_boost::is_abstract<T>
  110.       , std::numeric_limits<lcast_abstract_stub>
  111.       , std::numeric_limits<T>
  112.       >::type limits;
  113. #endif
  114.  
  115.     bool const use_default_precision =
  116.         !limits::is_specialized || limits::is_exact;
  117.  
  118.     if(!use_default_precision)
  119.     { // Includes all built-in floating-point types, float, double ...
  120.       // and UDT types for which digits (significand bits) is defined (not zero)
  121.  
  122.         bool const is_specialized_bin =
  123.             limits::radix == 2 && limits::digits > 0;
  124.         bool const is_specialized_dec =
  125.             limits::radix == 10 && limits::digits10 > 0;
  126.         std::streamsize const streamsize_max =
  127.             (mars_boost::integer_traits<std::streamsize>::max)();
  128.  
  129.         if(is_specialized_bin)
  130.         { // Floating-point types with
  131.           // limits::digits defined by the specialization.
  132.  
  133.             unsigned long const digits = limits::digits;
  134.             unsigned long const precision = 2UL + digits * 30103UL / 100000UL;
  135.             // unsigned long is selected because it is at least 32-bits
  136.             // and thus ULONG_MAX / 30103UL is big enough for all types.
  137.             BOOST_ASSERT(
  138.                     digits < ULONG_MAX / 30103UL &&
  139.                     precision > limits::digits10 + 0UL &&
  140.                     precision <= streamsize_max + 0UL
  141.                 );
  142.             return precision;
  143.         }
  144.         else if(is_specialized_dec)
  145.         {   // Decimal Floating-point type, most likely a User Defined Type
  146.             // rather than a real floating-point hardware type.
  147.             unsigned int const precision = limits::digits10 + 1U;
  148.             BOOST_ASSERT(precision <= streamsize_max + 0UL);
  149.             return precision;
  150.         }
  151.     }
  152.  
  153.     // Integral type (for which precision has no effect)
  154.     // or type T for which limits is NOT specialized,
  155.     // so assume stream precision remains the default 6 decimal digits.
  156.     // Warning: if your User-defined Floating-point type T is NOT specialized,
  157.     // then you may lose accuracy by only using 6 decimal digits.
  158.     // To avoid this, you need to specialize T with either
  159.     // radix == 2 and digits == the number of significand bits,
  160.     // OR
  161.     // radix = 10 and digits10 == the number of decimal digits.
  162.  
  163.     return 6;
  164. #endif
  165. }
  166.  
  167. template<class T>
  168. inline void lcast_set_precision(std::ios_base& stream, T*)
  169. {
  170.     stream.precision(lcast_get_precision<T>());
  171. }
  172.  
  173. template<class Source, class Target>
  174. inline void lcast_set_precision(std::ios_base& stream, Source*, Target*)
  175. {
  176.     std::streamsize const s = lcast_get_precision(static_cast<Source*>(0));
  177.     std::streamsize const t = lcast_get_precision(static_cast<Target*>(0));
  178.     stream.precision(s > t ? s : t);
  179. }
  180.  
  181. }}
  182.  
  183. #endif //  BOOST_DETAIL_LCAST_PRECISION_HPP_INCLUDED
  184.  
  185.  
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