Boost logo

Boost :

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
Prizet Farmhouse, Kendal, Cumbria UK LA8 8AB
+44 1539561830 & SMS, Mobile +44 7714 330204 & SMS
pbristow_at_[hidden]
 

Boost list run by bdawes at acm.org, gregod at cs.rpi.edu, cpdaniel at pacbell.net, john at johnmaddock.co.uk