Re: [boost] lexical_cast "Future directions"

From: Thomas Witt (witt_at_[hidden])
Date: 2002-12-29 13:24:36

Hi,

On Sunday 29 December 2002 01:29, Terje Slettebø wrote:
> > there...)  Anyway, the surprising result of this was that the space in
> > "Hello there" caused the interpreter.eof() check in lexical_cast to fail.
> > So if that check were not present, dest would have been assigned the
> > value "Hello".  Fortunately, the check was there, and instead I got a
> > bad_lexical_cast exception.
>
> This issue comes up at regular intervals, since this is one of the known
> problems of lexical_cast, but it hasn't yet been fixed. See e.g. this
> posting (http://aspn.activestate.com/ASPN/Mail/Message/1454894).
>
> A proposition has been made to fix this and other things
> (http://groups.yahoo.com/group/boost/files/lexical_cast_proposition/), and
> I'll get to update it properly, and write the docs for it, soon. It should
> work correctly as it is, as it's been tested in an extensive unit test
> (also found at the same place).
>
> > Is this the way lexical_cast is intended to work?
>
> No, Kevlin has acknowledged that this is a known problem with the current
> version of lexical_cast, the handling of whitespace in strings and
> characters.

I am still uncertain whether this is a problem with lexical_cast and whether 
it should be fixed.

The stated purpose of lexical_cast is type conversion through string 
representation. I think this is a simple but powerful concept.

To me the actual problem is not in lexical_cast but in the std::basic_strings 
stream operator semantics. Basically you cannot read strings containing 
whitespace as a whole. I.e. the integrity of a string containing whitespace 
is lost once you streamed it.

This is a fundamental if at  times undesirable property of std::basic_string 
and char const* for that matter. I don't know whether it is a good idea for 
lexical_cast to try to fix it.

Just my 2c

Thomas

-- 
Dipl.-Ing. Thomas Witt
Institut fuer Verkehrswesen, Eisenbahnbau und -betrieb, Universitaet Hannover
voice: [phone] - 4273, fax: [phone]
http://www.ive.uni-hannover.de