Re: [boost] Spurious warning in lexical_cast.hpp

From: Paul A Bristow (pbristow_at_[hidden])
Date: 2006-08-03 07:29:34

|  -----Original Message-----
|  From: boost-bounces_at_[hidden] 
|  [mailto:boost-bounces_at_[hidden]] On Behalf Of Gennaro Prota
|  Sent: 02 August 2006 22:10
|  To: boost_at_[hidden]
|  Subject: Re: [boost] Spurious warning in lexical_cast.hpp
|  
|  On Wed, 2 Aug 2006 20:35:19 +0100, "Paul A Bristow"
|  <pbristow_at_[hidden]> wrote:
|    
|  >|  But...: are you serializing floating point values by means of
|  >|  lexical_cast?
|  >
|  >No - I meant 'stringizing'
|  
|  Ok. It seems to me we need something along these lines (off 
|  the top of
|  my head):
|  
|  #include "boost/limits.hpp"
|  
|  #define BOOST_is_ieee754(t)                        \
|      typedef char required [                        \
|          std::numeric_limits<t>::is_iec559? 1 : -1  \
|      ]                                           /**/
|  
|  
|  
|  template <typename T>
|  struct digits2
|  {
|      enum { value =
|          std::numeric_limits<T>::digits };
|  };
|  
|  
|  template <typename T>
|  struct min_digits_10
|  {
|      BOOST_is_ieee754(T);
|  
|      enum { value =
|          30103 * (digits2<T>::value - 1) / 100000 };
|  };
|  
|  template <typename T>
|  struct max_digits_10
|  {
|      BOOST_is_ieee754(T);
|  
|      enum { value = 2 +
|          30103 * digits2<T>::value / 100000 
|      };
|  }
|  
|  #undef BOOST_is_ieee754

This seems overcomplicated to me.

the guaranteed 'minimum' digits are already defined by
numeric_limits<>::digits10()  (for float 6)

and the next version of C++ will provide numeric_limits<>::max_digits10()
(for float 9)

so all we need to do is to use the

stream.precision(2 + std::numeric_limits<Source>::digits * 301/1000);

for the time being.

If the type is not specialized, then it is not clear how many digits to use,
so do nothing - will be the default 6 decimal digits so ...

Moral - if you are providing a User defined Type, always specialize it for
numeric_limits and provide a suitable value for digits, digits10 (and in
future max_digits10).

Paul

---
Paul A Bristow
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