No more exception when casting from negative number string to unsigned

[Bernd Raum <braum@…>]

With gcc 4.2.4, a lexical_cast from negative number strings to an unsigned causes a bad_lexical_cast exception. With gcc 4.7.1 and 4.7.2, the value interpreted as positive integer is assigned instead. This bug occurs with Boost 1.33 and 1.51 on Open Suse and Solaris 10.

[Antony Polukhin]

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);
}