Re: [boost] [lexical_cast] performace patch

From: Paul A Bristow (pbristow_at_[hidden])
Date: 2006-07-12 12:08:01

|  -----Original Message-----
|  From: boost-bounces_at_[hidden] 
|  [mailto:boost-bounces_at_[hidden]] On Behalf Of Alexander Nasonov
|  Sent: 12 July 2006 16:27
|  To: boost_at_[hidden]
|  Subject: Re: [boost] [lexical_cast] performace patch
|  
|  Alexander Nasonov wrote:
|  >I googled for lexical_cast on www.open-std.org recently but 
|  found no proposal. Now I see it:
|  >http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n1973.html
|  
|  BTW,
|  
|  1. unsetf(std::ios::skipws) is not mentioned in the doc
|  2. loopback tests may fail
|  
|  N1973: "If std::numeric_limits<Target>::is_specialized, the 
|  underlying stream precision is set according to 
|  std::numeric_limits<Target>::digits10 + 1, otherwise if 
|  std::numeric_limits<Source>::is_specialized, the underlying 
|  stream precision is set according to 
|  std::numeric_limits<Source>::digits10 + 1."
|  
|  See http://article.gmane.org/gmane.comp.lib.boost.devel/145097

As I said before - it is the wrong formula :-((

It gives the wrong number of digits for float and will probably also fail
for user defined types.

Since it is for TR2, it should be specified in terms of max_digits10, now
accepted for TR2.

I suggest you use the right formula from digits not digits10 for now.

If you've serialized a lot of stuff, you might be cross not to get back as
near as possible what you've saved.
And you'd probably waste a lot of time trying to find out why not.

(This is a separate issue from the double loopback failures - even in MSVC,
float loopback IS correct for ALL float values - there are few enough of
them for an exhaustive test ;-)

Paul

---
Paul A Bristow
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