Re: [Boost-bugs] [Boost C++ Libraries] #8124: No more exception when casting from negative number string to unsigned

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #8124: No more exception when casting from negative number string to unsigned
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2013-02-24 07:28:33

#8124: No more exception when casting from negative number string to unsigned
------------------------------------------------+---------------------------
  Reporter:  Bernd Raum <braum@…>               |       Owner:  apolukhin            
      Type:  Bugs                               |      Status:  closed               
 Milestone:  To Be Determined                   |   Component:  lexical_cast         
   Version:  Boost 1.51.0                       |    Severity:  Problem              
Resolution:  invalid                            |    Keywords:  lexical_cast unsigned
------------------------------------------------+---------------------------
Changes (by apolukhin):

  * status:  new => closed
  * resolution:  => invalid

Comment:

 I'm afraid that this is not an error.

 boost::lexical_cast has the behavior of std::stringstream, which uses
 num_get functions of std::locale to convert numbers. If we look at the
 Programming languages — C++, we'll see, that num_get uses the rules of
 scanf for conversions. And in the C99 standard for unsigned input value
 minus sign is optional, so if a negative number is read, no errors will
 arise and the result will be the two's complement.

 I do not like such behaviour, but nothing can be done.

 Some code to demonstrate that lexical_cast works exactly like
 std::stringstream

 {{{
 #include <boost/lexical_cast.hpp>
 #include <sstream>
 #include <assert.h>

 int main() {
     std::stringstream ss("-1");
     unsigned int i;
     ss >> i;
     assert(boost::lexical_cast<unsigned int>("-1") == i);
 }
 }}}

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/8124#comment:1>
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