boost/detail/lcast_precision.hpp

0001 // Copyright Alexander Nasonov & Paul A. Bristow 2006.
0002
0003 // Use, modification and distribution are subject to the
0004 // Boost Software License, Version 1.0.
0005 // (See accompanying file LICENSE_1_0.txt
0006 // or copy at http://www.boost.org/LICENSE_1_0.txt)
0007
0008 #ifndef BOOST_DETAIL_LCAST_PRECISION_HPP_INCLUDED
0009 #define BOOST_DETAIL_LCAST_PRECISION_HPP_INCLUDED
0010
0011 #include <climits>
0012 #include <ios>
0013 #include <limits>
0014
0015 #include <boost/config.hpp>
0016
0017 namespace boost { namespace detail {
0018
0019 // Calculate an argument to pass to std::ios_base::precision from
0020 // lexical_cast.
0021 template<class T>
0022 struct lcast_precision
0023 {
0024     using limits = std::numeric_limits<T>;
0025
0026     static constexpr bool use_default_precision =
0027             !limits::is_specialized || limits::is_exact
0028         ;
0029
0030     static constexpr bool is_specialized_bin =
0031             !use_default_precision &&
0032             limits::radix == 2 && limits::digits > 0
0033         ;
0034
0035     static constexpr bool is_specialized_dec =
0036             !use_default_precision &&
0037             limits::radix == 10 && limits::digits10 > 0
0038         ;
0039
0040     static constexpr std::streamsize streamsize_max =
0041             (std::numeric_limits<std::streamsize>::max)()
0042         ;
0043
0044     static constexpr unsigned int precision_dec = limits::digits10 + 1U;
0045
0046     static_assert(!is_specialized_dec ||
0047             precision_dec <= streamsize_max + 0UL
0048         , "");
0049
0050     static constexpr unsigned long precision_bin =
0051             2UL + limits::digits * 30103UL / 100000UL
0052         ;
0053
0054     static_assert(!is_specialized_bin ||
0055             (limits::digits + 0UL < ULONG_MAX / 30103UL &&
0056             precision_bin > limits::digits10 + 0UL &&
0057             precision_bin <= streamsize_max + 0UL)
0058         , "");
0059
0060     static constexpr std::streamsize value =
0061             is_specialized_bin ? precision_bin
0062                                : is_specialized_dec ? precision_dec : 6
0063         ;
0064 };
0065
0066 template<class T>
0067 inline void lcast_set_precision(std::ios_base& stream, T*)
0068 {
0069     stream.precision(lcast_precision<T>::value);
0070 }
0071
0072 template<class Source, class Target>
0073 inline void lcast_set_precision(std::ios_base& stream, Source*, Target*)
0074 {
0075     std::streamsize const s = lcast_precision<Source>::value;
0076     std::streamsize const t = lcast_precision<Target*>::value;
0077     stream.precision(s > t ? s : t);
0078 }
0079
0080 }}
0081
0082 #endif //  BOOST_DETAIL_LCAST_PRECISION_HPP_INCLUDED
0083